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<li><a class="reference internal" href="#">5. Data Structures</a><ul>
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<li><a class="reference internal" href="#more-on-lists">5.1. More on Lists</a><ul>
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<li><a class="reference internal" href="#using-lists-as-stacks">5.1.1. Using Lists as Stacks</a></li>
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<li><a class="reference internal" href="#the-del-statement">5.2. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code> statement</a></li>
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<li><a class="reference internal" href="#tuples-and-sequences">5.3. Tuples and Sequences</a></li>
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<li><a class="reference internal" href="#sets">5.4. Sets</a></li>
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<section id="data-structures">
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<span id="tut-structures"></span><h1><span class="section-number">5. </span>Data Structures<a class="headerlink" href="#data-structures" title="Link to this heading">¶</a></h1>
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<p>This chapter describes some things you’ve learned about already in more detail,
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and adds some new things as well.</p>
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<section id="more-on-lists">
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<span id="tut-morelists"></span><h2><span class="section-number">5.1. </span>More on Lists<a class="headerlink" href="#more-on-lists" title="Link to this heading">¶</a></h2>
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<p>The list data type has some more methods. Here are all of the methods of list
|
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objects:</p>
|
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<dl class="py method">
|
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<dt class="sig sig-object py">
|
||
<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">append</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">x</span></span></em><span class="sig-paren">)</span></dt>
|
||
<dd><p>Add an item to the end of the list. Similar to <code class="docutils literal notranslate"><span class="pre">a[len(a):]</span> <span class="pre">=</span> <span class="pre">[x]</span></code>.</p>
|
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</dd></dl>
|
||
|
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<dl class="py method">
|
||
<dt class="sig sig-object py">
|
||
<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">extend</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">iterable</span></span></em><span class="sig-paren">)</span></dt>
|
||
<dd><p>Extend the list by appending all the items from the iterable. Similar to
|
||
<code class="docutils literal notranslate"><span class="pre">a[len(a):]</span> <span class="pre">=</span> <span class="pre">iterable</span></code>.</p>
|
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</dd></dl>
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|
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<dl class="py method">
|
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<dt class="sig sig-object py">
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<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">insert</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">i</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">x</span></span></em><span class="sig-paren">)</span></dt>
|
||
<dd><p>Insert an item at a given position. The first argument is the index of the
|
||
element before which to insert, so <code class="docutils literal notranslate"><span class="pre">a.insert(0,</span> <span class="pre">x)</span></code> inserts at the front of
|
||
the list, and <code class="docutils literal notranslate"><span class="pre">a.insert(len(a),</span> <span class="pre">x)</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">a.append(x)</span></code>.</p>
|
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</dd></dl>
|
||
|
||
<dl class="py method">
|
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<dt class="sig sig-object py">
|
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<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">remove</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">x</span></span></em><span class="sig-paren">)</span></dt>
|
||
<dd><p>Remove the first item from the list whose value is equal to <em>x</em>. It raises a
|
||
<a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if there is no such item.</p>
|
||
</dd></dl>
|
||
|
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<dl class="py method">
|
||
<dt class="sig sig-object py">
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<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">pop</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">i</span></span></em><span class="optional">]</span><span class="sig-paren">)</span></dt>
|
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<dd><p>Remove the item at the given position in the list, and return it. If no index
|
||
is specified, <code class="docutils literal notranslate"><span class="pre">a.pop()</span></code> removes and returns the last item in the list.
|
||
It raises an <a class="reference internal" href="../library/exceptions.html#IndexError" title="IndexError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IndexError</span></code></a> if the list is empty or the index is
|
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outside the list range.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py">
|
||
<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">clear</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span></dt>
|
||
<dd><p>Remove all items from the list. Similar to <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">a[:]</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py">
|
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<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">index</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">x</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">start</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">end</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span></dt>
|
||
<dd><p>Return zero-based index in the list of the first item whose value is equal to <em>x</em>.
|
||
Raises a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if there is no such item.</p>
|
||
<p>The optional arguments <em>start</em> and <em>end</em> are interpreted as in the slice
|
||
notation and are used to limit the search to a particular subsequence of
|
||
the list. The returned index is computed relative to the beginning of the full
|
||
sequence rather than the <em>start</em> argument.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py">
|
||
<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">count</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">x</span></span></em><span class="sig-paren">)</span></dt>
|
||
<dd><p>Return the number of times <em>x</em> appears in the list.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py">
|
||
<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">sort</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="keyword-only-separator o"><abbr title="Keyword-only parameters separator (PEP 3102)"><span class="pre">*</span></abbr></span></em>, <em class="sig-param"><span class="n"><span class="pre">key</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">reverse</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">False</span></span></em><span class="sig-paren">)</span></dt>
|
||
<dd><p>Sort the items of the list in place (the arguments can be used for sort
|
||
customization, see <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> for their explanation).</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py">
|
||
<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">reverse</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span></dt>
|
||
<dd><p>Reverse the elements of the list in place.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py">
|
||
<span class="sig-prename descclassname"><span class="pre">list.</span></span><span class="sig-name descname"><span class="pre">copy</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span></dt>
|
||
<dd><p>Return a shallow copy of the list. Similar to <code class="docutils literal notranslate"><span class="pre">a[:]</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<p>An example that uses most of the list methods:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fruits</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'pear'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">,</span> <span class="s1">'kiwi'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'apple'</span><span class="p">)</span>
|
||
<span class="go">2</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'tangerine'</span><span class="p">)</span>
|
||
<span class="go">0</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s1">'banana'</span><span class="p">)</span>
|
||
<span class="go">3</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s1">'banana'</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span> <span class="c1"># Find next banana starting at position 4</span>
|
||
<span class="go">6</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">reverse</span><span class="p">()</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span>
|
||
<span class="go">['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange']</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s1">'grape'</span><span class="p">)</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span>
|
||
<span class="go">['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange', 'grape']</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span>
|
||
<span class="go">['apple', 'apple', 'banana', 'banana', 'grape', 'kiwi', 'orange', 'pear']</span>
|
||
<span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
|
||
<span class="go">'pear'</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>You might have noticed that methods like <code class="docutils literal notranslate"><span class="pre">insert</span></code>, <code class="docutils literal notranslate"><span class="pre">remove</span></code> or <code class="docutils literal notranslate"><span class="pre">sort</span></code> that
|
||
only modify the list have no return value printed – they return the default
|
||
<code class="docutils literal notranslate"><span class="pre">None</span></code>. <a class="footnote-reference brackets" href="#id2" id="id1" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a> This is a design principle for all mutable data structures in
|
||
Python.</p>
|
||
<p>Another thing you might notice is that not all data can be sorted or
|
||
compared. For instance, <code class="docutils literal notranslate"><span class="pre">[None,</span> <span class="pre">'hello',</span> <span class="pre">10]</span></code> doesn’t sort because
|
||
integers can’t be compared to strings and <code class="docutils literal notranslate"><span class="pre">None</span></code> can’t be compared to
|
||
other types. Also, there are some types that don’t have a defined
|
||
ordering relation. For example, <code class="docutils literal notranslate"><span class="pre">3+4j</span> <span class="pre"><</span> <span class="pre">5+7j</span></code> isn’t a valid
|
||
comparison.</p>
|
||
<section id="using-lists-as-stacks">
|
||
<span id="tut-lists-as-stacks"></span><h3><span class="section-number">5.1.1. </span>Using Lists as Stacks<a class="headerlink" href="#using-lists-as-stacks" title="Link to this heading">¶</a></h3>
|
||
<p>The list methods make it very easy to use a list as a stack, where the last
|
||
element added is the first element retrieved (“last-in, first-out”). To add an
|
||
item to the top of the stack, use <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code>. To retrieve an item from the
|
||
top of the stack, use <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code> without an explicit index. For example:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">stack</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">6</span><span class="p">)</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span>
|
||
<span class="go">[3, 4, 5, 6, 7]</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
|
||
<span class="go">7</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span>
|
||
<span class="go">[3, 4, 5, 6]</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
|
||
<span class="go">6</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
|
||
<span class="go">5</span>
|
||
<span class="gp">>>> </span><span class="n">stack</span>
|
||
<span class="go">[3, 4]</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="using-lists-as-queues">
|
||
<span id="tut-lists-as-queues"></span><h3><span class="section-number">5.1.2. </span>Using Lists as Queues<a class="headerlink" href="#using-lists-as-queues" title="Link to this heading">¶</a></h3>
|
||
<p>It is also possible to use a list as a queue, where the first element added is
|
||
the first element retrieved (“first-in, first-out”); however, lists are not
|
||
efficient for this purpose. While appends and pops from the end of list are
|
||
fast, doing inserts or pops from the beginning of a list is slow (because all
|
||
of the other elements have to be shifted by one).</p>
|
||
<p>To implement a queue, use <a class="reference internal" href="../library/collections.html#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate"><span class="pre">collections.deque</span></code></a> which was designed to
|
||
have fast appends and pops from both ends. For example:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span><span class="w"> </span><span class="nn">collections</span><span class="w"> </span><span class="kn">import</span> <span class="n">deque</span>
|
||
<span class="gp">>>> </span><span class="n">queue</span> <span class="o">=</span> <span class="n">deque</span><span class="p">([</span><span class="s2">"Eric"</span><span class="p">,</span> <span class="s2">"John"</span><span class="p">,</span> <span class="s2">"Michael"</span><span class="p">])</span>
|
||
<span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s2">"Terry"</span><span class="p">)</span> <span class="c1"># Terry arrives</span>
|
||
<span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s2">"Graham"</span><span class="p">)</span> <span class="c1"># Graham arrives</span>
|
||
<span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">popleft</span><span class="p">()</span> <span class="c1"># The first to arrive now leaves</span>
|
||
<span class="go">'Eric'</span>
|
||
<span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">popleft</span><span class="p">()</span> <span class="c1"># The second to arrive now leaves</span>
|
||
<span class="go">'John'</span>
|
||
<span class="gp">>>> </span><span class="n">queue</span> <span class="c1"># Remaining queue in order of arrival</span>
|
||
<span class="go">deque(['Michael', 'Terry', 'Graham'])</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="list-comprehensions">
|
||
<span id="tut-listcomps"></span><h3><span class="section-number">5.1.3. </span>List Comprehensions<a class="headerlink" href="#list-comprehensions" title="Link to this heading">¶</a></h3>
|
||
<p>List comprehensions provide a concise way to create lists.
|
||
Common applications are to make new lists where each element is the result of
|
||
some operations applied to each member of another sequence or iterable, or to
|
||
create a subsequence of those elements that satisfy a certain condition.</p>
|
||
<p>For example, assume we want to create a list of squares, like:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="n">squares</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">squares</span>
|
||
<span class="go">[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Note that this creates (or overwrites) a variable named <code class="docutils literal notranslate"><span class="pre">x</span></code> that still exists
|
||
after the loop completes. We can calculate the list of squares without any
|
||
side effects using:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">squares</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)))</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>or, equivalently:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">squares</span> <span class="o">=</span> <span class="p">[</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>which is more concise and readable.</p>
|
||
<p>A list comprehension consists of brackets containing an expression followed
|
||
by a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clause, then zero or more <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> or <code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code>
|
||
clauses. The result will be a new list resulting from evaluating the expression
|
||
in the context of the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> and <code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code> clauses which follow it.
|
||
For example, this listcomp combines the elements of two lists if they are not
|
||
equal:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">[(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]</span> <span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">4</span><span class="p">]</span> <span class="k">if</span> <span class="n">x</span> <span class="o">!=</span> <span class="n">y</span><span class="p">]</span>
|
||
<span class="go">[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>and it’s equivalent to:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">combs</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]:</span>
|
||
<span class="gp">... </span> <span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">4</span><span class="p">]:</span>
|
||
<span class="gp">... </span> <span class="k">if</span> <span class="n">x</span> <span class="o">!=</span> <span class="n">y</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">combs</span><span class="o">.</span><span class="n">append</span><span class="p">((</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">))</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">combs</span>
|
||
<span class="go">[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Note how the order of the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> and <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statements is the
|
||
same in both these snippets.</p>
|
||
<p>If the expression is a tuple (e.g. the <code class="docutils literal notranslate"><span class="pre">(x,</span> <span class="pre">y)</span></code> in the previous example),
|
||
it must be parenthesized.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">vec</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mi">4</span><span class="p">,</span> <span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="c1"># create a new list with the values doubled</span>
|
||
<span class="gp">>>> </span><span class="p">[</span><span class="n">x</span><span class="o">*</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec</span><span class="p">]</span>
|
||
<span class="go">[-8, -4, 0, 4, 8]</span>
|
||
<span class="gp">>>> </span><span class="c1"># filter the list to exclude negative numbers</span>
|
||
<span class="gp">>>> </span><span class="p">[</span><span class="n">x</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec</span> <span class="k">if</span> <span class="n">x</span> <span class="o">>=</span> <span class="mi">0</span><span class="p">]</span>
|
||
<span class="go">[0, 2, 4]</span>
|
||
<span class="gp">>>> </span><span class="c1"># apply a function to all the elements</span>
|
||
<span class="gp">>>> </span><span class="p">[</span><span class="nb">abs</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec</span><span class="p">]</span>
|
||
<span class="go">[4, 2, 0, 2, 4]</span>
|
||
<span class="gp">>>> </span><span class="c1"># call a method on each element</span>
|
||
<span class="gp">>>> </span><span class="n">freshfruit</span> <span class="o">=</span> <span class="p">[</span><span class="s1">' banana'</span><span class="p">,</span> <span class="s1">' loganberry '</span><span class="p">,</span> <span class="s1">'passion fruit '</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="p">[</span><span class="n">weapon</span><span class="o">.</span><span class="n">strip</span><span class="p">()</span> <span class="k">for</span> <span class="n">weapon</span> <span class="ow">in</span> <span class="n">freshfruit</span><span class="p">]</span>
|
||
<span class="go">['banana', 'loganberry', 'passion fruit']</span>
|
||
<span class="gp">>>> </span><span class="c1"># create a list of 2-tuples like (number, square)</span>
|
||
<span class="gp">>>> </span><span class="p">[(</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">6</span><span class="p">)]</span>
|
||
<span class="go">[(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]</span>
|
||
<span class="gp">>>> </span><span class="c1"># the tuple must be parenthesized, otherwise an error is raised</span>
|
||
<span class="gp">>>> </span><span class="p">[</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">6</span><span class="p">)]</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>
|
||
<span class="w"> </span><span class="p">[</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">6</span><span class="p">)]</span>
|
||
<span class="w"> </span><span class="pm">^^^^^^^</span>
|
||
<span class="gr">SyntaxError</span>: <span class="n">did you forget parentheses around the comprehension target?</span>
|
||
<span class="gp">>>> </span><span class="c1"># flatten a list using a listcomp with two 'for'</span>
|
||
<span class="gp">>>> </span><span class="n">vec</span> <span class="o">=</span> <span class="p">[[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">],</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">6</span><span class="p">],</span> <span class="p">[</span><span class="mi">7</span><span class="p">,</span><span class="mi">8</span><span class="p">,</span><span class="mi">9</span><span class="p">]]</span>
|
||
<span class="gp">>>> </span><span class="p">[</span><span class="n">num</span> <span class="k">for</span> <span class="n">elem</span> <span class="ow">in</span> <span class="n">vec</span> <span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="n">elem</span><span class="p">]</span>
|
||
<span class="go">[1, 2, 3, 4, 5, 6, 7, 8, 9]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>List comprehensions can contain complex expressions and nested functions:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span><span class="w"> </span><span class="nn">math</span><span class="w"> </span><span class="kn">import</span> <span class="n">pi</span>
|
||
<span class="gp">>>> </span><span class="p">[</span><span class="nb">str</span><span class="p">(</span><span class="nb">round</span><span class="p">(</span><span class="n">pi</span><span class="p">,</span> <span class="n">i</span><span class="p">))</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">6</span><span class="p">)]</span>
|
||
<span class="go">['3.1', '3.14', '3.142', '3.1416', '3.14159']</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="nested-list-comprehensions">
|
||
<h3><span class="section-number">5.1.4. </span>Nested List Comprehensions<a class="headerlink" href="#nested-list-comprehensions" title="Link to this heading">¶</a></h3>
|
||
<p>The initial expression in a list comprehension can be any arbitrary expression,
|
||
including another list comprehension.</p>
|
||
<p>Consider the following example of a 3x4 matrix implemented as a list of
|
||
3 lists of length 4:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">matrix</span> <span class="o">=</span> <span class="p">[</span>
|
||
<span class="gp">... </span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">],</span>
|
||
<span class="gp">... </span> <span class="p">[</span><span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">],</span>
|
||
<span class="gp">... </span> <span class="p">[</span><span class="mi">9</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">11</span><span class="p">,</span> <span class="mi">12</span><span class="p">],</span>
|
||
<span class="gp">... </span><span class="p">]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The following list comprehension will transpose rows and columns:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">[[</span><span class="n">row</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">matrix</span><span class="p">]</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)]</span>
|
||
<span class="go">[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>As we saw in the previous section, the inner list comprehension is evaluated in
|
||
the context of the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> that follows it, so this example is
|
||
equivalent to:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">transposed</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="n">transposed</span><span class="o">.</span><span class="n">append</span><span class="p">([</span><span class="n">row</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">matrix</span><span class="p">])</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">transposed</span>
|
||
<span class="go">[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>which, in turn, is the same as:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">transposed</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="c1"># the following 3 lines implement the nested listcomp</span>
|
||
<span class="gp">... </span> <span class="n">transposed_row</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">... </span> <span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">matrix</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">transposed_row</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">row</span><span class="p">[</span><span class="n">i</span><span class="p">])</span>
|
||
<span class="gp">... </span> <span class="n">transposed</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">transposed_row</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">transposed</span>
|
||
<span class="go">[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>In the real world, you should prefer built-in functions to complex flow statements.
|
||
The <a class="reference internal" href="../library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal notranslate"><span class="pre">zip()</span></code></a> function would do a great job for this use case:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">zip</span><span class="p">(</span><span class="o">*</span><span class="n">matrix</span><span class="p">))</span>
|
||
<span class="go">[(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>See <a class="reference internal" href="controlflow.html#tut-unpacking-arguments"><span class="std std-ref">Unpacking Argument Lists</span></a> for details on the asterisk in this line.</p>
|
||
</section>
|
||
</section>
|
||
<section id="the-del-statement">
|
||
<span id="tut-del"></span><h2><span class="section-number">5.2. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code> statement<a class="headerlink" href="#the-del-statement" title="Link to this heading">¶</a></h2>
|
||
<p>There is a way to remove an item from a list given its index instead of its
|
||
value: the <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement. This differs from the <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code> method
|
||
which returns a value. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code> statement can also be used to remove
|
||
slices from a list or clear the entire list (which we did earlier by assignment
|
||
of an empty list to the slice). For example:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mf">66.25</span><span class="p">,</span> <span class="mi">333</span><span class="p">,</span> <span class="mi">333</span><span class="p">,</span> <span class="mf">1234.5</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="n">a</span>
|
||
<span class="go">[1, 66.25, 333, 333, 1234.5]</span>
|
||
<span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">4</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="n">a</span>
|
||
<span class="go">[1, 66.25, 1234.5]</span>
|
||
<span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span><span class="p">[:]</span>
|
||
<span class="gp">>>> </span><span class="n">a</span>
|
||
<span class="go">[]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p><a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> can also be used to delete entire variables:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Referencing the name <code class="docutils literal notranslate"><span class="pre">a</span></code> hereafter is an error (at least until another value
|
||
is assigned to it). We’ll find other uses for <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> later.</p>
|
||
</section>
|
||
<section id="tuples-and-sequences">
|
||
<span id="tut-tuples"></span><h2><span class="section-number">5.3. </span>Tuples and Sequences<a class="headerlink" href="#tuples-and-sequences" title="Link to this heading">¶</a></h2>
|
||
<p>We saw that lists and strings have many common properties, such as indexing and
|
||
slicing operations. They are two examples of <em>sequence</em> data types (see
|
||
<a class="reference internal" href="../library/stdtypes.html#typesseq"><span class="std std-ref">Sequence Types — list, tuple, range</span></a>). Since Python is an evolving language, other sequence data
|
||
types may be added. There is also another standard sequence data type: the
|
||
<em>tuple</em>.</p>
|
||
<p>A tuple consists of a number of values separated by commas, for instance:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">t</span> <span class="o">=</span> <span class="mi">12345</span><span class="p">,</span> <span class="mi">54321</span><span class="p">,</span> <span class="s1">'hello!'</span>
|
||
<span class="gp">>>> </span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="go">12345</span>
|
||
<span class="gp">>>> </span><span class="n">t</span>
|
||
<span class="go">(12345, 54321, 'hello!')</span>
|
||
<span class="gp">>>> </span><span class="c1"># Tuples may be nested:</span>
|
||
<span class="gp">>>> </span><span class="n">u</span> <span class="o">=</span> <span class="n">t</span><span class="p">,</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
|
||
<span class="gp">>>> </span><span class="n">u</span>
|
||
<span class="go">((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))</span>
|
||
<span class="gp">>>> </span><span class="c1"># Tuples are immutable:</span>
|
||
<span class="gp">>>> </span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">88888</span>
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
|
||
<span class="gr">TypeError</span>: <span class="n">'tuple' object does not support item assignment</span>
|
||
<span class="gp">>>> </span><span class="c1"># but they can contain mutable objects:</span>
|
||
<span class="gp">>>> </span><span class="n">v</span> <span class="o">=</span> <span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">])</span>
|
||
<span class="gp">>>> </span><span class="n">v</span>
|
||
<span class="go">([1, 2, 3], [3, 2, 1])</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>As you see, on output tuples are always enclosed in parentheses, so that nested
|
||
tuples are interpreted correctly; they may be input with or without surrounding
|
||
parentheses, although often parentheses are necessary anyway (if the tuple is
|
||
part of a larger expression). It is not possible to assign to the individual
|
||
items of a tuple, however it is possible to create tuples which contain mutable
|
||
objects, such as lists.</p>
|
||
<p>Though tuples may seem similar to lists, they are often used in different
|
||
situations and for different purposes.
|
||
Tuples are <a class="reference internal" href="../glossary.html#term-immutable"><span class="xref std std-term">immutable</span></a>, and usually contain a heterogeneous sequence of
|
||
elements that are accessed via unpacking (see later in this section) or indexing
|
||
(or even by attribute in the case of <a class="reference internal" href="../library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate"><span class="pre">namedtuples</span></code></a>).
|
||
Lists are <a class="reference internal" href="../glossary.html#term-mutable"><span class="xref std std-term">mutable</span></a>, and their elements are usually homogeneous and are
|
||
accessed by iterating over the list.</p>
|
||
<p>A special problem is the construction of tuples containing 0 or 1 items: the
|
||
syntax has some extra quirks to accommodate these. Empty tuples are constructed
|
||
by an empty pair of parentheses; a tuple with one item is constructed by
|
||
following a value with a comma (it is not sufficient to enclose a single value
|
||
in parentheses). Ugly, but effective. For example:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">empty</span> <span class="o">=</span> <span class="p">()</span>
|
||
<span class="gp">>>> </span><span class="n">singleton</span> <span class="o">=</span> <span class="s1">'hello'</span><span class="p">,</span> <span class="c1"># <-- note trailing comma</span>
|
||
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">empty</span><span class="p">)</span>
|
||
<span class="go">0</span>
|
||
<span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">singleton</span><span class="p">)</span>
|
||
<span class="go">1</span>
|
||
<span class="gp">>>> </span><span class="n">singleton</span>
|
||
<span class="go">('hello',)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The statement <code class="docutils literal notranslate"><span class="pre">t</span> <span class="pre">=</span> <span class="pre">12345,</span> <span class="pre">54321,</span> <span class="pre">'hello!'</span></code> is an example of <em>tuple packing</em>:
|
||
the values <code class="docutils literal notranslate"><span class="pre">12345</span></code>, <code class="docutils literal notranslate"><span class="pre">54321</span></code> and <code class="docutils literal notranslate"><span class="pre">'hello!'</span></code> are packed together in a tuple.
|
||
The reverse operation is also possible:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span> <span class="o">=</span> <span class="n">t</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>This is called, appropriately enough, <em>sequence unpacking</em> and works for any
|
||
sequence on the right-hand side. Sequence unpacking requires that there are as
|
||
many variables on the left side of the equals sign as there are elements in the
|
||
sequence. Note that multiple assignment is really just a combination of tuple
|
||
packing and sequence unpacking.</p>
|
||
</section>
|
||
<section id="sets">
|
||
<span id="tut-sets"></span><h2><span class="section-number">5.4. </span>Sets<a class="headerlink" href="#sets" title="Link to this heading">¶</a></h2>
|
||
<p>Python also includes a data type for <em>sets</em>. A set is an unordered collection
|
||
with no duplicate elements. Basic uses include membership testing and
|
||
eliminating duplicate entries. Set objects also support mathematical operations
|
||
like union, intersection, difference, and symmetric difference.</p>
|
||
<p>Curly braces or the <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-func docutils literal notranslate"><span class="pre">set()</span></code></a> function can be used to create sets. Note: to
|
||
create an empty set you have to use <code class="docutils literal notranslate"><span class="pre">set()</span></code>, not <code class="docutils literal notranslate"><span class="pre">{}</span></code>; the latter creates an
|
||
empty dictionary, a data structure that we discuss in the next section.</p>
|
||
<p>Here is a brief demonstration:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">basket</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'pear'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">}</span>
|
||
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">basket</span><span class="p">)</span> <span class="c1"># show that duplicates have been removed</span>
|
||
<span class="go">{'orange', 'banana', 'pear', 'apple'}</span>
|
||
<span class="gp">>>> </span><span class="s1">'orange'</span> <span class="ow">in</span> <span class="n">basket</span> <span class="c1"># fast membership testing</span>
|
||
<span class="go">True</span>
|
||
<span class="gp">>>> </span><span class="s1">'crabgrass'</span> <span class="ow">in</span> <span class="n">basket</span>
|
||
<span class="go">False</span>
|
||
|
||
<span class="gp">>>> </span><span class="c1"># Demonstrate set operations on unique letters from two words</span>
|
||
<span class="gp">>>></span>
|
||
<span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="s1">'abracadabra'</span><span class="p">)</span>
|
||
<span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="s1">'alacazam'</span><span class="p">)</span>
|
||
<span class="gp">>>> </span><span class="n">a</span> <span class="c1"># unique letters in a</span>
|
||
<span class="go">{'a', 'r', 'b', 'c', 'd'}</span>
|
||
<span class="gp">>>> </span><span class="n">a</span> <span class="o">-</span> <span class="n">b</span> <span class="c1"># letters in a but not in b</span>
|
||
<span class="go">{'r', 'd', 'b'}</span>
|
||
<span class="gp">>>> </span><span class="n">a</span> <span class="o">|</span> <span class="n">b</span> <span class="c1"># letters in a or b or both</span>
|
||
<span class="go">{'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}</span>
|
||
<span class="gp">>>> </span><span class="n">a</span> <span class="o">&</span> <span class="n">b</span> <span class="c1"># letters in both a and b</span>
|
||
<span class="go">{'a', 'c'}</span>
|
||
<span class="gp">>>> </span><span class="n">a</span> <span class="o">^</span> <span class="n">b</span> <span class="c1"># letters in a or b but not both</span>
|
||
<span class="go">{'r', 'd', 'b', 'm', 'z', 'l'}</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Similarly to <a class="reference internal" href="#tut-listcomps"><span class="std std-ref">list comprehensions</span></a>, set comprehensions
|
||
are also supported:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">{</span><span class="n">x</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="s1">'abracadabra'</span> <span class="k">if</span> <span class="n">x</span> <span class="ow">not</span> <span class="ow">in</span> <span class="s1">'abc'</span><span class="p">}</span>
|
||
<span class="gp">>>> </span><span class="n">a</span>
|
||
<span class="go">{'r', 'd'}</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="dictionaries">
|
||
<span id="tut-dictionaries"></span><h2><span class="section-number">5.5. </span>Dictionaries<a class="headerlink" href="#dictionaries" title="Link to this heading">¶</a></h2>
|
||
<p>Another useful data type built into Python is the <em>dictionary</em> (see
|
||
<a class="reference internal" href="../library/stdtypes.html#typesmapping"><span class="std std-ref">Mapping Types — dict</span></a>). Dictionaries are sometimes found in other languages as
|
||
“associative memories” or “associative arrays”. Unlike sequences, which are
|
||
indexed by a range of numbers, dictionaries are indexed by <em>keys</em>, which can be
|
||
any immutable type; strings and numbers can always be keys. Tuples can be used
|
||
as keys if they contain only strings, numbers, or tuples; if a tuple contains
|
||
any mutable object either directly or indirectly, it cannot be used as a key.
|
||
You can’t use lists as keys, since lists can be modified in place using index
|
||
assignments, slice assignments, or methods like <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> and
|
||
<code class="xref py py-meth docutils literal notranslate"><span class="pre">extend()</span></code>.</p>
|
||
<p>It is best to think of a dictionary as a set of <em>key: value</em> pairs,
|
||
with the requirement that the keys are unique (within one dictionary). A pair of
|
||
braces creates an empty dictionary: <code class="docutils literal notranslate"><span class="pre">{}</span></code>. Placing a comma-separated list of
|
||
key:value pairs within the braces adds initial key:value pairs to the
|
||
dictionary; this is also the way dictionaries are written on output.</p>
|
||
<p>The main operations on a dictionary are storing a value with some key and
|
||
extracting the value given the key. It is also possible to delete a key:value
|
||
pair with <code class="docutils literal notranslate"><span class="pre">del</span></code>. If you store using a key that is already in use, the old
|
||
value associated with that key is forgotten. It is an error to extract a value
|
||
using a non-existent key.</p>
|
||
<p>Performing <code class="docutils literal notranslate"><span class="pre">list(d)</span></code> on a dictionary returns a list of all the keys
|
||
used in the dictionary, in insertion order (if you want it sorted, just use
|
||
<code class="docutils literal notranslate"><span class="pre">sorted(d)</span></code> instead). To check whether a single key is in the
|
||
dictionary, use the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> keyword.</p>
|
||
<p>Here is a small example using a dictionary:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">tel</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'jack'</span><span class="p">:</span> <span class="mi">4098</span><span class="p">,</span> <span class="s1">'sape'</span><span class="p">:</span> <span class="mi">4139</span><span class="p">}</span>
|
||
<span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s1">'guido'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">4127</span>
|
||
<span class="gp">>>> </span><span class="n">tel</span>
|
||
<span class="go">{'jack': 4098, 'sape': 4139, 'guido': 4127}</span>
|
||
<span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s1">'jack'</span><span class="p">]</span>
|
||
<span class="go">4098</span>
|
||
<span class="gp">>>> </span><span class="k">del</span> <span class="n">tel</span><span class="p">[</span><span class="s1">'sape'</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s1">'irv'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">4127</span>
|
||
<span class="gp">>>> </span><span class="n">tel</span>
|
||
<span class="go">{'jack': 4098, 'guido': 4127, 'irv': 4127}</span>
|
||
<span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="n">tel</span><span class="p">)</span>
|
||
<span class="go">['jack', 'guido', 'irv']</span>
|
||
<span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">tel</span><span class="p">)</span>
|
||
<span class="go">['guido', 'irv', 'jack']</span>
|
||
<span class="gp">>>> </span><span class="s1">'guido'</span> <span class="ow">in</span> <span class="n">tel</span>
|
||
<span class="go">True</span>
|
||
<span class="gp">>>> </span><span class="s1">'jack'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">tel</span>
|
||
<span class="go">False</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-func docutils literal notranslate"><span class="pre">dict()</span></code></a> constructor builds dictionaries directly from sequences of
|
||
key-value pairs:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">dict</span><span class="p">([(</span><span class="s1">'sape'</span><span class="p">,</span> <span class="mi">4139</span><span class="p">),</span> <span class="p">(</span><span class="s1">'guido'</span><span class="p">,</span> <span class="mi">4127</span><span class="p">),</span> <span class="p">(</span><span class="s1">'jack'</span><span class="p">,</span> <span class="mi">4098</span><span class="p">)])</span>
|
||
<span class="go">{'sape': 4139, 'guido': 4127, 'jack': 4098}</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>In addition, dict comprehensions can be used to create dictionaries from
|
||
arbitrary key and value expressions:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">{</span><span class="n">x</span><span class="p">:</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">6</span><span class="p">)}</span>
|
||
<span class="go">{2: 4, 4: 16, 6: 36}</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>When the keys are simple strings, it is sometimes easier to specify pairs using
|
||
keyword arguments:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">dict</span><span class="p">(</span><span class="n">sape</span><span class="o">=</span><span class="mi">4139</span><span class="p">,</span> <span class="n">guido</span><span class="o">=</span><span class="mi">4127</span><span class="p">,</span> <span class="n">jack</span><span class="o">=</span><span class="mi">4098</span><span class="p">)</span>
|
||
<span class="go">{'sape': 4139, 'guido': 4127, 'jack': 4098}</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="looping-techniques">
|
||
<span id="tut-loopidioms"></span><h2><span class="section-number">5.6. </span>Looping Techniques<a class="headerlink" href="#looping-techniques" title="Link to this heading">¶</a></h2>
|
||
<p>When looping through dictionaries, the key and corresponding value can be
|
||
retrieved at the same time using the <a class="reference internal" href="../library/stdtypes.html#dict.items" title="dict.items"><code class="xref py py-meth docutils literal notranslate"><span class="pre">items()</span></code></a> method.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">knights</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'gallahad'</span><span class="p">:</span> <span class="s1">'the pure'</span><span class="p">,</span> <span class="s1">'robin'</span><span class="p">:</span> <span class="s1">'the brave'</span><span class="p">}</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">knights</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">k</span><span class="p">,</span> <span class="n">v</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">gallahad the pure</span>
|
||
<span class="go">robin the brave</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>When looping through a sequence, the position index and corresponding value can
|
||
be retrieved at the same time using the <a class="reference internal" href="../library/functions.html#enumerate" title="enumerate"><code class="xref py py-func docutils literal notranslate"><span class="pre">enumerate()</span></code></a> function.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">([</span><span class="s1">'tic'</span><span class="p">,</span> <span class="s1">'tac'</span><span class="p">,</span> <span class="s1">'toe'</span><span class="p">]):</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">v</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">0 tic</span>
|
||
<span class="go">1 tac</span>
|
||
<span class="go">2 toe</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>To loop over two or more sequences at the same time, the entries can be paired
|
||
with the <a class="reference internal" href="../library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal notranslate"><span class="pre">zip()</span></code></a> function.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">questions</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'name'</span><span class="p">,</span> <span class="s1">'quest'</span><span class="p">,</span> <span class="s1">'favorite color'</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="n">answers</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'lancelot'</span><span class="p">,</span> <span class="s1">'the holy grail'</span><span class="p">,</span> <span class="s1">'blue'</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">q</span><span class="p">,</span> <span class="n">a</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">questions</span><span class="p">,</span> <span class="n">answers</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'What is your </span><span class="si">{0}</span><span class="s1">? It is </span><span class="si">{1}</span><span class="s1">.'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">a</span><span class="p">))</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">What is your name? It is lancelot.</span>
|
||
<span class="go">What is your quest? It is the holy grail.</span>
|
||
<span class="go">What is your favorite color? It is blue.</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>To loop over a sequence in reverse, first specify the sequence in a forward
|
||
direction and then call the <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a> function.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">reversed</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">2</span><span class="p">)):</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">9</span>
|
||
<span class="go">7</span>
|
||
<span class="go">5</span>
|
||
<span class="go">3</span>
|
||
<span class="go">1</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>To loop over a sequence in sorted order, use the <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> function which
|
||
returns a new sorted list while leaving the source unaltered.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">basket</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'pear'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">sorted</span><span class="p">(</span><span class="n">basket</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">apple</span>
|
||
<span class="go">apple</span>
|
||
<span class="go">banana</span>
|
||
<span class="go">orange</span>
|
||
<span class="go">orange</span>
|
||
<span class="go">pear</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Using <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-func docutils literal notranslate"><span class="pre">set()</span></code></a> on a sequence eliminates duplicate elements. The use of
|
||
<a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> in combination with <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-func docutils literal notranslate"><span class="pre">set()</span></code></a> over a sequence is an idiomatic
|
||
way to loop over unique elements of the sequence in sorted order.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">basket</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'pear'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="nb">sorted</span><span class="p">(</span><span class="nb">set</span><span class="p">(</span><span class="n">basket</span><span class="p">)):</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">apple</span>
|
||
<span class="go">banana</span>
|
||
<span class="go">orange</span>
|
||
<span class="go">pear</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>It is sometimes tempting to change a list while you are looping over it;
|
||
however, it is often simpler and safer to create a new list instead.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span><span class="w"> </span><span class="nn">math</span>
|
||
<span class="gp">>>> </span><span class="n">raw_data</span> <span class="o">=</span> <span class="p">[</span><span class="mf">56.2</span><span class="p">,</span> <span class="nb">float</span><span class="p">(</span><span class="s1">'NaN'</span><span class="p">),</span> <span class="mf">51.7</span><span class="p">,</span> <span class="mf">55.3</span><span class="p">,</span> <span class="mf">52.5</span><span class="p">,</span> <span class="nb">float</span><span class="p">(</span><span class="s1">'NaN'</span><span class="p">),</span> <span class="mf">47.8</span><span class="p">]</span>
|
||
<span class="gp">>>> </span><span class="n">filtered_data</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">>>> </span><span class="k">for</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">raw_data</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">if</span> <span class="ow">not</span> <span class="n">math</span><span class="o">.</span><span class="n">isnan</span><span class="p">(</span><span class="n">value</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="n">filtered_data</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">filtered_data</span>
|
||
<span class="go">[56.2, 51.7, 55.3, 52.5, 47.8]</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="more-on-conditions">
|
||
<span id="tut-conditions"></span><h2><span class="section-number">5.7. </span>More on Conditions<a class="headerlink" href="#more-on-conditions" title="Link to this heading">¶</a></h2>
|
||
<p>The conditions used in <code class="docutils literal notranslate"><span class="pre">while</span></code> and <code class="docutils literal notranslate"><span class="pre">if</span></code> statements can contain any
|
||
operators, not just comparisons.</p>
|
||
<p>The comparison operators <code class="docutils literal notranslate"><span class="pre">in</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code> are membership tests that
|
||
determine whether a value is in (or not in) a container. The operators <code class="docutils literal notranslate"><span class="pre">is</span></code>
|
||
and <code class="docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code> compare whether two objects are really the same object. All
|
||
comparison operators have the same priority, which is lower than that of all
|
||
numerical operators.</p>
|
||
<p>Comparisons can be chained. For example, <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span> <span class="pre">==</span> <span class="pre">c</span></code> tests whether <code class="docutils literal notranslate"><span class="pre">a</span></code> is
|
||
less than <code class="docutils literal notranslate"><span class="pre">b</span></code> and moreover <code class="docutils literal notranslate"><span class="pre">b</span></code> equals <code class="docutils literal notranslate"><span class="pre">c</span></code>.</p>
|
||
<p>Comparisons may be combined using the Boolean operators <code class="docutils literal notranslate"><span class="pre">and</span></code> and <code class="docutils literal notranslate"><span class="pre">or</span></code>, and
|
||
the outcome of a comparison (or of any other Boolean expression) may be negated
|
||
with <code class="docutils literal notranslate"><span class="pre">not</span></code>. These have lower priorities than comparison operators; between
|
||
them, <code class="docutils literal notranslate"><span class="pre">not</span></code> has the highest priority and <code class="docutils literal notranslate"><span class="pre">or</span></code> the lowest, so that <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">and</span>
|
||
<span class="pre">not</span> <span class="pre">B</span> <span class="pre">or</span> <span class="pre">C</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">(A</span> <span class="pre">and</span> <span class="pre">(not</span> <span class="pre">B))</span> <span class="pre">or</span> <span class="pre">C</span></code>. As always, parentheses
|
||
can be used to express the desired composition.</p>
|
||
<p>The Boolean operators <code class="docutils literal notranslate"><span class="pre">and</span></code> and <code class="docutils literal notranslate"><span class="pre">or</span></code> are so-called <em>short-circuit</em>
|
||
operators: their arguments are evaluated from left to right, and evaluation
|
||
stops as soon as the outcome is determined. For example, if <code class="docutils literal notranslate"><span class="pre">A</span></code> and <code class="docutils literal notranslate"><span class="pre">C</span></code> are
|
||
true but <code class="docutils literal notranslate"><span class="pre">B</span></code> is false, <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">and</span> <span class="pre">B</span> <span class="pre">and</span> <span class="pre">C</span></code> does not evaluate the expression
|
||
<code class="docutils literal notranslate"><span class="pre">C</span></code>. When used as a general value and not as a Boolean, the return value of a
|
||
short-circuit operator is the last evaluated argument.</p>
|
||
<p>It is possible to assign the result of a comparison or other Boolean expression
|
||
to a variable. For example,</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">string1</span><span class="p">,</span> <span class="n">string2</span><span class="p">,</span> <span class="n">string3</span> <span class="o">=</span> <span class="s1">''</span><span class="p">,</span> <span class="s1">'Trondheim'</span><span class="p">,</span> <span class="s1">'Hammer Dance'</span>
|
||
<span class="gp">>>> </span><span class="n">non_null</span> <span class="o">=</span> <span class="n">string1</span> <span class="ow">or</span> <span class="n">string2</span> <span class="ow">or</span> <span class="n">string3</span>
|
||
<span class="gp">>>> </span><span class="n">non_null</span>
|
||
<span class="go">'Trondheim'</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Note that in Python, unlike C, assignment inside expressions must be done
|
||
explicitly with the
|
||
<a class="reference internal" href="../faq/design.html#why-can-t-i-use-an-assignment-in-an-expression"><span class="std std-ref">walrus operator</span></a> <code class="docutils literal notranslate"><span class="pre">:=</span></code>.
|
||
This avoids a common class of problems encountered in C programs: typing <code class="docutils literal notranslate"><span class="pre">=</span></code>
|
||
in an expression when <code class="docutils literal notranslate"><span class="pre">==</span></code> was intended.</p>
|
||
</section>
|
||
<section id="comparing-sequences-and-other-types">
|
||
<span id="tut-comparing"></span><h2><span class="section-number">5.8. </span>Comparing Sequences and Other Types<a class="headerlink" href="#comparing-sequences-and-other-types" title="Link to this heading">¶</a></h2>
|
||
<p>Sequence objects typically may be compared to other objects with the same sequence
|
||
type. The comparison uses <em>lexicographical</em> ordering: first the first two
|
||
items are compared, and if they differ this determines the outcome of the
|
||
comparison; if they are equal, the next two items are compared, and so on, until
|
||
either sequence is exhausted. If two items to be compared are themselves
|
||
sequences of the same type, the lexicographical comparison is carried out
|
||
recursively. If all items of two sequences compare equal, the sequences are
|
||
considered equal. If one sequence is an initial sub-sequence of the other, the
|
||
shorter sequence is the smaller (lesser) one. Lexicographical ordering for
|
||
strings uses the Unicode code point number to order individual characters.
|
||
Some examples of comparisons between sequences of the same type:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span> <span class="o"><</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>
|
||
<span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span> <span class="o"><</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">]</span>
|
||
<span class="s1">'ABC'</span> <span class="o"><</span> <span class="s1">'C'</span> <span class="o"><</span> <span class="s1">'Pascal'</span> <span class="o"><</span> <span class="s1">'Python'</span>
|
||
<span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span> <span class="o"><</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>
|
||
<span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> <span class="o"><</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span>
|
||
<span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span> <span class="o">==</span> <span class="p">(</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">2.0</span><span class="p">,</span> <span class="mf">3.0</span><span class="p">)</span>
|
||
<span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="p">(</span><span class="s1">'aa'</span><span class="p">,</span> <span class="s1">'ab'</span><span class="p">))</span> <span class="o"><</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="p">(</span><span class="s1">'abc'</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">),</span> <span class="mi">4</span><span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Note that comparing objects of different types with <code class="docutils literal notranslate"><span class="pre"><</span></code> or <code class="docutils literal notranslate"><span class="pre">></span></code> is legal
|
||
provided that the objects have appropriate comparison methods. For example,
|
||
mixed numeric types are compared according to their numeric value, so 0 equals
|
||
0.0, etc. Otherwise, rather than providing an arbitrary ordering, the
|
||
interpreter will raise a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception.</p>
|
||
<p class="rubric">Footnotes</p>
|
||
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|
||
<aside class="footnote brackets" id="id2" role="doc-footnote">
|
||
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id1">1</a><span class="fn-bracket">]</span></span>
|
||
<p>Other languages may return the mutated object, which allows method
|
||
chaining, such as <code class="docutils literal notranslate"><span class="pre">d->insert("a")->remove("b")->sort();</span></code>.</p>
|
||
</aside>
|
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<h3><a href="../contents.html">Table of Contents</a></h3>
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<ul>
|
||
<li><a class="reference internal" href="#">5. Data Structures</a><ul>
|
||
<li><a class="reference internal" href="#more-on-lists">5.1. More on Lists</a><ul>
|
||
<li><a class="reference internal" href="#using-lists-as-stacks">5.1.1. Using Lists as Stacks</a></li>
|
||
<li><a class="reference internal" href="#using-lists-as-queues">5.1.2. Using Lists as Queues</a></li>
|
||
<li><a class="reference internal" href="#list-comprehensions">5.1.3. List Comprehensions</a></li>
|
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<li><a class="reference internal" href="#nested-list-comprehensions">5.1.4. Nested List Comprehensions</a></li>
|
||
</ul>
|
||
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|
||
<li><a class="reference internal" href="#the-del-statement">5.2. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code> statement</a></li>
|
||
<li><a class="reference internal" href="#tuples-and-sequences">5.3. Tuples and Sequences</a></li>
|
||
<li><a class="reference internal" href="#sets">5.4. Sets</a></li>
|
||
<li><a class="reference internal" href="#dictionaries">5.5. Dictionaries</a></li>
|
||
<li><a class="reference internal" href="#looping-techniques">5.6. Looping Techniques</a></li>
|
||
<li><a class="reference internal" href="#more-on-conditions">5.7. More on Conditions</a></li>
|
||
<li><a class="reference internal" href="#comparing-sequences-and-other-types">5.8. Comparing Sequences and Other Types</a></li>
|
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