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<div class="section" id="data-structures">
<span id="tut-structures"></span><h1>5. Data Structures<a class="headerlink" href="#data-structures" title="Permalink to this headline">¶</a></h1>
<p>This chapter describes some things you’ve learned about already in more detail,
and adds some new things as well.</p>
<div class="section" id="more-on-lists">
<span id="tut-morelists"></span><h2>5.1. More on Lists<a class="headerlink" href="#more-on-lists" title="Permalink to this headline">¶</a></h2>
<p>The list data type has some more methods. Here are all of the methods of list
objects:</p>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">append</tt><big>(</big><em>x</em><big>)</big></dt>
<dd>Add an item to the end of the list; equivalent to <tt class="docutils literal"><span class="pre">a[len(a):]</span> <span class="pre">=</span> <span class="pre">[x]</span></tt>.</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">extend</tt><big>(</big><em>L</em><big>)</big></dt>
<dd>Extend the list by appending all the items in the given list; equivalent to
<tt class="docutils literal"><span class="pre">a[len(a):]</span> <span class="pre">=</span> <span class="pre">L</span></tt>.</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">insert</tt><big>(</big><em>i</em>, <em>x</em><big>)</big></dt>
<dd>Insert an item at a given position. The first argument is the index of the
element before which to insert, so <tt class="docutils literal"><span class="pre">a.insert(0,</span> <span class="pre">x)</span></tt> inserts at the front of
the list, and <tt class="docutils literal"><span class="pre">a.insert(len(a),</span> <span class="pre">x)</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">a.append(x)</span></tt>.</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">remove</tt><big>(</big><em>x</em><big>)</big></dt>
<dd>Remove the first item from the list whose value is <em>x</em>. It is an error if there
is no such item.</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">pop</tt><big>(</big><span class="optional">[</span><em>i</em><span class="optional">]</span><big>)</big></dt>
<dd>Remove the item at the given position in the list, and return it. If no index
is specified, <tt class="docutils literal"><span class="pre">a.pop()</span></tt> removes and returns the last item in the list. (The
square brackets around the <em>i</em> in the method signature denote that the parameter
is optional, not that you should type square brackets at that position. You
will see this notation frequently in the Python Library Reference.)</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">index</tt><big>(</big><em>x</em><big>)</big></dt>
<dd>Return the index in the list of the first item whose value is <em>x</em>. It is an
error if there is no such item.</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">count</tt><big>(</big><em>x</em><big>)</big></dt>
<dd>Return the number of times <em>x</em> appears in the list.</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">sort</tt><big>(</big><big>)</big></dt>
<dd>Sort the items of the list, in place.</dd></dl>
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">reverse</tt><big>(</big><big>)</big></dt>
<dd>Reverse the elements of the list, in place.</dd></dl>
<p>An example that uses most of the list methods:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</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="mi">1</span><span class="p">,</span> <span class="mf">1234.5</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">print</span> <span class="n">a</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="mi">333</span><span class="p">),</span> <span class="n">a</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="mf">66.25</span><span class="p">),</span> <span class="n">a</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s">'x'</span><span class="p">)</span>
<span class="go">2 1 0</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">insert</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="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">333</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">a</span>
<span class="go">[66.25, 333, -1, 333, 1, 1234.5, 333]</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="mi">333</span><span class="p">)</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="mi">333</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">a</span>
<span class="go">[66.25, -1, 333, 1, 1234.5, 333]</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">reverse</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">a</span>
<span class="go">[333, 1234.5, 1, 333, -1, 66.25]</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">a</span>
<span class="go">[-1, 1, 66.25, 333, 333, 1234.5]</span>
</pre></div>
</div>
<div class="section" id="using-lists-as-stacks">
<span id="tut-lists-as-stacks"></span><h3>5.1.1. Using Lists as Stacks<a class="headerlink" href="#using-lists-as-stacks" title="Permalink to this headline">¶</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 <tt class="xref docutils literal"><span class="pre">append()</span></tt>. To retrieve an item from the
top of the stack, use <tt class="xref docutils literal"><span class="pre">pop()</span></tt> without an explicit index. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><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>
</div>
<div class="section" id="using-lists-as-queues">
<span id="tut-lists-as-queues"></span><h3>5.1.2. Using Lists as Queues<a class="headerlink" href="#using-lists-as-queues" title="Permalink to this headline">¶</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 title="collections.deque" class="reference external" href="../library/collections.html#collections.deque"><tt class="xref docutils literal"><span class="pre">collections.deque</span></tt></a> which was designed to
have fast appends and pops from both ends. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">collections</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="s">"Eric"</span><span class="p">,</span> <span class="s">"John"</span><span class="p">,</span> <span class="s">"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="s">"Terry"</span><span class="p">)</span> <span class="c"># 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="s">"Graham"</span><span class="p">)</span> <span class="c"># 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="c"># 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="c"># The second to arrive now leaves</span>
<span class="go">'John'</span>
<span class="gp">>>> </span><span class="n">queue</span> <span class="c"># Remaining queue in order of arrival</span>
<span class="go">deque(['Michael', 'Terry', 'Graham'])</span>
</pre></div>
</div>
</div>
<div class="section" id="functional-programming-tools">
<span id="tut-functional"></span><h3>5.1.3. Functional Programming Tools<a class="headerlink" href="#functional-programming-tools" title="Permalink to this headline">¶</a></h3>
<p>There are three built-in functions that are very useful when used with lists:
<a title="filter" class="reference external" href="../library/functions.html#filter"><tt class="xref docutils literal"><span class="pre">filter()</span></tt></a>, <a title="map" class="reference external" href="../library/functions.html#map"><tt class="xref docutils literal"><span class="pre">map()</span></tt></a>, and <a title="reduce" class="reference external" href="../library/functions.html#reduce"><tt class="xref docutils literal"><span class="pre">reduce()</span></tt></a>.</p>
<p><tt class="docutils literal"><span class="pre">filter(function,</span> <span class="pre">sequence)</span></tt> returns a sequence consisting of those items from
the sequence for which <tt class="docutils literal"><span class="pre">function(item)</span></tt> is true. If <em>sequence</em> is a
<tt class="xref docutils literal"><span class="pre">string</span></tt> or <a title="tuple" class="reference external" href="../library/functions.html#tuple"><tt class="xref docutils literal"><span class="pre">tuple</span></tt></a>, the result will be of the same type;
otherwise, it is always a <a title="list" class="reference external" href="../library/functions.html#list"><tt class="xref docutils literal"><span class="pre">list</span></tt></a>. For example, to compute some primes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span> <span class="k">return</span> <span class="n">x</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">!=</span> <span class="mi">0</span> <span class="ow">and</span> <span class="n">x</span> <span class="o">%</span> <span class="mi">3</span> <span class="o">!=</span> <span class="mi">0</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nb">filter</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">25</span><span class="p">))</span>
<span class="go">[5, 7, 11, 13, 17, 19, 23]</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">map(function,</span> <span class="pre">sequence)</span></tt> calls <tt class="docutils literal"><span class="pre">function(item)</span></tt> for each of the sequence’s
items and returns a list of the return values. For example, to compute some
cubes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">def</span> <span class="nf">cube</span><span class="p">(</span><span class="n">x</span><span class="p">):</span> <span class="k">return</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="o">*</span><span class="n">x</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nb">map</span><span class="p">(</span><span class="n">cube</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">11</span><span class="p">))</span>
<span class="go">[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]</span>
</pre></div>
</div>
<p>More than one sequence may be passed; the function must then have as many
arguments as there are sequences and is called with the corresponding item from
each sequence (or <tt class="xref docutils literal"><span class="pre">None</span></tt> if some sequence is shorter than another). For
example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">seq</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="mi">8</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">add</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">return</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nb">map</span><span class="p">(</span><span class="n">add</span><span class="p">,</span> <span class="n">seq</span><span class="p">,</span> <span class="n">seq</span><span class="p">)</span>
<span class="go">[0, 2, 4, 6, 8, 10, 12, 14]</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">reduce(function,</span> <span class="pre">sequence)</span></tt> returns a single value constructed by calling the
binary function <em>function</em> on the first two items of the sequence, then on the
result and the next item, and so on. For example, to compute the sum of the
numbers 1 through 10:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">def</span> <span class="nf">add</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">return</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nb">reduce</span><span class="p">(</span><span class="n">add</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">11</span><span class="p">))</span>
<span class="go">55</span>
</pre></div>
</div>
<p>If there’s only one item in the sequence, its value is returned; if the sequence
is empty, an exception is raised.</p>
<p>A third argument can be passed to indicate the starting value. In this case the
starting value is returned for an empty sequence, and the function is first
applied to the starting value and the first sequence item, then to the result
and the next item, and so on. For example,</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">def</span> <span class="nf">sum</span><span class="p">(</span><span class="n">seq</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">add</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">return</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span>
<span class="gp">... </span> <span class="k">return</span> <span class="nb">reduce</span><span class="p">(</span><span class="n">add</span><span class="p">,</span> <span class="n">seq</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nb">sum</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">11</span><span class="p">))</span>
<span class="go">55</span>
<span class="gp">>>> </span><span class="nb">sum</span><span class="p">([])</span>
<span class="go">0</span>
</pre></div>
</div>
<p>Don’t use this example’s definition of <a title="sum" class="reference external" href="../library/functions.html#sum"><tt class="xref docutils literal"><span class="pre">sum()</span></tt></a>: since summing numbers is
such a common need, a built-in function <tt class="docutils literal"><span class="pre">sum(sequence)</span></tt> is already provided,
and works exactly like this.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.3.</span></p>
</div>
<div class="section" id="list-comprehensions">
<h3>5.1.4. List Comprehensions<a class="headerlink" href="#list-comprehensions" title="Permalink to this headline">¶</a></h3>
<p>List comprehensions provide a concise way to create lists without resorting to
use of <a title="map" class="reference external" href="../library/functions.html#map"><tt class="xref docutils literal"><span class="pre">map()</span></tt></a>, <a title="filter" class="reference external" href="../library/functions.html#filter"><tt class="xref docutils literal"><span class="pre">filter()</span></tt></a> and/or <a class="reference external" href="../reference/expressions.html#lambda"><tt class="xref docutils literal"><span class="pre">lambda</span></tt></a>. The resulting list
definition tends often to be clearer than lists built using those constructs.
Each list comprehension consists of an expression followed by a <a class="reference external" href="../reference/compound_stmts.html#for"><tt class="xref docutils literal"><span class="pre">for</span></tt></a>
clause, then zero or more <a class="reference external" href="../reference/compound_stmts.html#for"><tt class="xref docutils literal"><span class="pre">for</span></tt></a> or <a class="reference external" href="../reference/compound_stmts.html#if"><tt class="xref docutils literal"><span class="pre">if</span></tt></a> clauses. The result
will be a list resulting from evaluating the expression in the context of the
<a class="reference external" href="../reference/compound_stmts.html#for"><tt class="xref docutils literal"><span class="pre">for</span></tt></a> and <a class="reference external" href="../reference/compound_stmts.html#if"><tt class="xref docutils literal"><span class="pre">if</span></tt></a> clauses which follow it. If the expression
would evaluate to a tuple, it must be parenthesized.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">freshfruit</span> <span class="o">=</span> <span class="p">[</span><span class="s">' banana'</span><span class="p">,</span> <span class="s">' loganberry '</span><span class="p">,</span> <span class="s">'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="n">vec</span> <span class="o">=</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="gp">>>> </span><span class="p">[</span><span class="mi">3</span><span class="o">*</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="p">]</span>
<span class="go">[6, 12, 18]</span>
<span class="gp">>>> </span><span class="p">[</span><span class="mi">3</span><span class="o">*</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">3</span><span class="p">]</span>
<span class="go">[12, 18]</span>
<span class="gp">>>> </span><span class="p">[</span><span class="mi">3</span><span class="o">*</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">2</span><span class="p">]</span>
<span class="go">[]</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="n">vec</span><span class="p">]</span>
<span class="go">[[2, 4], [4, 16], [6, 36]]</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="n">vec</span><span class="p">]</span> <span class="c"># error - parens required for tuples</span>
<span class="go"> File "<stdin>", line 1, in ?</span>
<span class="go"> [x, x**2 for x in vec]</span>
<span class="go"> ^</span>
<span class="go">SyntaxError: invalid syntax</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="n">vec</span><span class="p">]</span>
<span class="go">[(2, 4), (4, 16), (6, 36)]</span>
<span class="gp">>>> </span><span class="n">vec1</span> <span class="o">=</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="gp">>>> </span><span class="n">vec2</span> <span class="o">=</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="o">-</span><span class="mi">9</span><span class="p">]</span>
<span class="gp">>>> </span><span class="p">[</span><span class="n">x</span><span class="o">*</span><span class="n">y</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec1</span> <span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="n">vec2</span><span class="p">]</span>
<span class="go">[8, 6, -18, 16, 12, -36, 24, 18, -54]</span>
<span class="gp">>>> </span><span class="p">[</span><span class="n">x</span><span class="o">+</span><span class="n">y</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec1</span> <span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="n">vec2</span><span class="p">]</span>
<span class="go">[6, 5, -7, 8, 7, -5, 10, 9, -3]</span>
<span class="gp">>>> </span><span class="p">[</span><span class="n">vec1</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="o">*</span><span class="n">vec2</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="nb">len</span><span class="p">(</span><span class="n">vec1</span><span class="p">))]</span>
<span class="go">[8, 12, -54]</span>
</pre></div>
</div>
<p>List comprehensions are much more flexible than <a title="map" class="reference external" href="../library/functions.html#map"><tt class="xref docutils literal"><span class="pre">map()</span></tt></a> and can be applied
to complex expressions and nested functions:</p>
<div class="highlight-python"><div class="highlight"><pre><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="mi">355</span><span class="o">/</span><span class="mf">113.0</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>
</div>
<div class="section" id="nested-list-comprehensions">
<h3>5.1.5. Nested List Comprehensions<a class="headerlink" href="#nested-list-comprehensions" title="Permalink to this headline">¶</a></h3>
<p>If you’ve got the stomach for it, list comprehensions can be nested. They are a
powerful tool but – like all powerful tools – they need to be used carefully,
if at all.</p>
<p>Consider the following example of a 3x3 matrix held as a list containing three
lists, one list per row:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">mat</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="gp">... </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="gp">... </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>
</pre></div>
</div>
<p>Now, if you wanted to swap rows and columns, you could use a list
comprehension:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">print</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">mat</span><span class="p">]</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">0</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="go">[[1, 4, 7], [2, 5, 8], [3, 6, 9]]</span>
</pre></div>
</div>
<p>Special care has to be taken for the <em>nested</em> list comprehension:</p>
<blockquote>
To avoid apprehension when nesting list comprehensions, read from right to
left.</blockquote>
<p>A more verbose version of this snippet shows the flow explicitly:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">0</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="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">mat</span><span class="p">:</span>
<span class="k">print</span> <span class="n">row</span><span class="p">[</span><span class="n">i</span><span class="p">],</span>
<span class="k">print</span>
</pre></div>
</div>
<p>In real world, you should prefer built-in functions to complex flow statements.
The <a title="zip" class="reference external" href="../library/functions.html#zip"><tt class="xref docutils literal"><span class="pre">zip()</span></tt></a> function would do a great job for this use case:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="nb">zip</span><span class="p">(</span><span class="o">*</span><span class="n">mat</span><span class="p">)</span>
<span class="go">[(1, 4, 7), (2, 5, 8), (3, 6, 9)]</span>
</pre></div>
</div>
<p>See <a class="reference external" href="controlflow.html#tut-unpacking-arguments"><em>Unpacking Argument Lists</em></a> for details on the asterisk in this line.</p>
</div>
</div>
<div class="section" id="the-del-statement">
<span id="tut-del"></span><h2>5.2. The <a class="reference external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal"><span class="pre">del</span></tt></a> statement<a class="headerlink" href="#the-del-statement" title="Permalink to this headline">¶</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 external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal"><span class="pre">del</span></tt></a> statement. This differs from the <tt class="xref docutils literal"><span class="pre">pop()</span></tt> method
which returns a value. The <a class="reference external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal"><span class="pre">del</span></tt></a> 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-python"><div class="highlight"><pre><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 external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal"><span class="pre">del</span></tt></a> can also be used to delete entire variables:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span>
</pre></div>
</div>
<p>Referencing the name <tt class="docutils literal"><span class="pre">a</span></tt> hereafter is an error (at least until another value
is assigned to it). We’ll find other uses for <a class="reference external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal"><span class="pre">del</span></tt></a> later.</p>
</div>
<div class="section" id="tuples-and-sequences">
<span id="tut-tuples"></span><h2>5.3. Tuples and Sequences<a class="headerlink" href="#tuples-and-sequences" title="Permalink to this headline">¶</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 external" href="../library/stdtypes.html#typesseq"><em>Sequence Types — str, unicode, list, tuple, buffer, xrange</em></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-python"><div class="highlight"><pre><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="s">'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="c"># 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>
</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).</p>
<p>Tuples have many uses. For example: (x, y) coordinate pairs, employee records
from a database, etc. Tuples, like strings, are immutable: it is not possible
to assign to the individual items of a tuple (you can simulate much of the same
effect with slicing and concatenation, though). It is also possible to create
tuples which contain mutable objects, such as lists.</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-python"><div class="highlight"><pre><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="s">'hello'</span><span class="p">,</span> <span class="c"># <-- 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 <tt class="docutils literal"><span class="pre">t</span> <span class="pre">=</span> <span class="pre">12345,</span> <span class="pre">54321,</span> <span class="pre">'hello!'</span></tt> is an example of <em>tuple packing</em>:
the values <tt class="docutils literal"><span class="pre">12345</span></tt>, <tt class="docutils literal"><span class="pre">54321</span></tt> and <tt class="docutils literal"><span class="pre">'hello!'</span></tt> are packed together in a tuple.
The reverse operation is also possible:</p>
<div class="highlight-python"><div class="highlight"><pre><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 the list of
variables on the left to have the same number of elements as the length of the
sequence. Note that multiple assignment is really just a combination of tuple
packing and sequence unpacking.</p>
</div>
<div class="section" id="sets">
<span id="tut-sets"></span><h2>5.4. Sets<a class="headerlink" href="#sets" title="Permalink to this headline">¶</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>Here is a brief demonstration:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">basket</span> <span class="o">=</span> <span class="p">[</span><span class="s">'apple'</span><span class="p">,</span> <span class="s">'orange'</span><span class="p">,</span> <span class="s">'apple'</span><span class="p">,</span> <span class="s">'pear'</span><span class="p">,</span> <span class="s">'orange'</span><span class="p">,</span> <span class="s">'banana'</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">fruit</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="n">basket</span><span class="p">)</span> <span class="c"># create a set without duplicates</span>
<span class="gp">>>> </span><span class="n">fruit</span>
<span class="go">set(['orange', 'pear', 'apple', 'banana'])</span>
<span class="gp">>>> </span><span class="s">'orange'</span> <span class="ow">in</span> <span class="n">fruit</span> <span class="c"># fast membership testing</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="s">'crabgrass'</span> <span class="ow">in</span> <span class="n">fruit</span>
<span class="go">False</span>
<span class="gp">>>> </span><span class="c"># 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="s">'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="s">'alacazam'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">a</span> <span class="c"># unique letters in a</span>
<span class="go">set(['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="c"># letters in a but not in b</span>
<span class="go">set(['r', 'd', 'b'])</span>
<span class="gp">>>> </span><span class="n">a</span> <span class="o">|</span> <span class="n">b</span> <span class="c"># letters in either a or b</span>
<span class="go">set(['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="c"># letters in both a and b</span>
<span class="go">set(['a', 'c'])</span>
<span class="gp">>>> </span><span class="n">a</span> <span class="o">^</span> <span class="n">b</span> <span class="c"># letters in a or b but not both</span>
<span class="go">set(['r', 'd', 'b', 'm', 'z', 'l'])</span>
</pre></div>
</div>
</div>
<div class="section" id="dictionaries">
<span id="tut-dictionaries"></span><h2>5.5. Dictionaries<a class="headerlink" href="#dictionaries" title="Permalink to this headline">¶</a></h2>
<p>Another useful data type built into Python is the <em>dictionary</em> (see
<a class="reference external" href="../library/stdtypes.html#typesmapping"><em>Mapping Types — dict</em></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 <tt class="xref docutils literal"><span class="pre">append()</span></tt> and
<tt class="xref docutils literal"><span class="pre">extend()</span></tt>.</p>
<p>It is best to think of a dictionary as an unordered 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: <tt class="docutils literal"><span class="pre">{}</span></tt>. 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 <tt class="docutils literal"><span class="pre">del</span></tt>. 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>The <tt class="xref docutils literal"><span class="pre">keys()</span></tt> method of a dictionary object returns a list of all the keys
used in the dictionary, in arbitrary order (if you want it sorted, just apply
the <tt class="xref docutils literal"><span class="pre">sort()</span></tt> method to the list of keys). To check whether a single key is
in the dictionary, use the <a class="reference external" href="../reference/expressions.html#in"><tt class="xref docutils literal"><span class="pre">in</span></tt></a> keyword.</p>
<p>Here is a small example using a dictionary:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">tel</span> <span class="o">=</span> <span class="p">{</span><span class="s">'jack'</span><span class="p">:</span> <span class="mi">4098</span><span class="p">,</span> <span class="s">'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="s">'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">{'sape': 4139, 'guido': 4127, 'jack': 4098}</span>
<span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s">'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="s">'sape'</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s">'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">{'guido': 4127, 'irv': 4127, 'jack': 4098}</span>
<span class="gp">>>> </span><span class="n">tel</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="go">['guido', 'irv', 'jack']</span>
<span class="gp">>>> </span><span class="s">'guido'</span> <span class="ow">in</span> <span class="n">tel</span>
<span class="go">True</span>
</pre></div>
</div>
<p>The <a title="dict" class="reference external" href="../library/stdtypes.html#dict"><tt class="xref docutils literal"><span class="pre">dict()</span></tt></a> constructor builds dictionaries directly from lists of
key-value pairs stored as tuples. When the pairs form a pattern, list
comprehensions can compactly specify the key-value list.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="nb">dict</span><span class="p">([(</span><span class="s">'sape'</span><span class="p">,</span> <span class="mi">4139</span><span class="p">),</span> <span class="p">(</span><span class="s">'guido'</span><span class="p">,</span> <span class="mi">4127</span><span class="p">),</span> <span class="p">(</span><span class="s">'jack'</span><span class="p">,</span> <span class="mi">4098</span><span class="p">)])</span>
<span class="go">{'sape': 4139, 'jack': 4098, 'guido': 4127}</span>
<span class="gp">>>> </span><span class="nb">dict</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="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="c"># use a list comprehension</span>
<span class="go">{2: 4, 4: 16, 6: 36}</span>
</pre></div>
</div>
<p>Later in the tutorial, we will learn about Generator Expressions which are even
better suited for the task of supplying key-values pairs to the <a title="dict" class="reference external" href="../library/stdtypes.html#dict"><tt class="xref docutils literal"><span class="pre">dict()</span></tt></a>
constructor.</p>
<p>When the keys are simple strings, it is sometimes easier to specify pairs using
keyword arguments:</p>
<div class="highlight-python"><div class="highlight"><pre><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, 'jack': 4098, 'guido': 4127}</span>
</pre></div>
</div>
</div>
<div class="section" id="looping-techniques">
<span id="tut-loopidioms"></span><h2>5.6. Looping Techniques<a class="headerlink" href="#looping-techniques" title="Permalink to this headline">¶</a></h2>
<p>When looping through dictionaries, the key and corresponding value can be
retrieved at the same time using the <tt class="xref docutils literal"><span class="pre">iteritems()</span></tt> method.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">knights</span> <span class="o">=</span> <span class="p">{</span><span class="s">'gallahad'</span><span class="p">:</span> <span class="s">'the pure'</span><span class="p">,</span> <span class="s">'robin'</span><span class="p">:</span> <span class="s">'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">iteritems</span><span class="p">():</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</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 title="enumerate" class="reference external" href="../library/functions.html#enumerate"><tt class="xref docutils literal"><span class="pre">enumerate()</span></tt></a> function.</p>
<div class="highlight-python"><div class="highlight"><pre><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="s">'tic'</span><span class="p">,</span> <span class="s">'tac'</span><span class="p">,</span> <span class="s">'toe'</span><span class="p">]):</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">i</span><span class="p">,</span> <span class="n">v</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 title="zip" class="reference external" href="../library/functions.html#zip"><tt class="xref docutils literal"><span class="pre">zip()</span></tt></a> function.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">questions</span> <span class="o">=</span> <span class="p">[</span><span class="s">'name'</span><span class="p">,</span> <span class="s">'quest'</span><span class="p">,</span> <span class="s">'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="s">'lancelot'</span><span class="p">,</span> <span class="s">'the holy grail'</span><span class="p">,</span> <span class="s">'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="k">print</span> <span class="s">'What is your {0}? It is {1}.'</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 title="reversed" class="reference external" href="../library/functions.html#reversed"><tt class="xref docutils literal"><span class="pre">reversed()</span></tt></a> function.</p>
<div class="highlight-python"><div class="highlight"><pre><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">xrange</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="k">print</span> <span class="n">i</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 title="sorted" class="reference external" href="../library/functions.html#sorted"><tt class="xref docutils literal"><span class="pre">sorted()</span></tt></a> function which
returns a new sorted list while leaving the source unaltered.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">basket</span> <span class="o">=</span> <span class="p">[</span><span class="s">'apple'</span><span class="p">,</span> <span class="s">'orange'</span><span class="p">,</span> <span class="s">'apple'</span><span class="p">,</span> <span class="s">'pear'</span><span class="p">,</span> <span class="s">'orange'</span><span class="p">,</span> <span class="s">'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="k">print</span> <span class="n">f</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>
</div>
<div class="section" id="more-on-conditions">
<span id="tut-conditions"></span><h2>5.7. More on Conditions<a class="headerlink" href="#more-on-conditions" title="Permalink to this headline">¶</a></h2>
<p>The conditions used in <tt class="docutils literal"><span class="pre">while</span></tt> and <tt class="docutils literal"><span class="pre">if</span></tt> statements can contain any
operators, not just comparisons.</p>
<p>The comparison operators <tt class="docutils literal"><span class="pre">in</span></tt> and <tt class="docutils literal"><span class="pre">not</span> <span class="pre">in</span></tt> check whether a value occurs
(does not occur) in a sequence. The operators <tt class="docutils literal"><span class="pre">is</span></tt> and <tt class="docutils literal"><span class="pre">is</span> <span class="pre">not</span></tt> compare
whether two objects are really the same object; this only matters for mutable
objects like lists. All comparison operators have the same priority, which is
lower than that of all numerical operators.</p>
<p>Comparisons can be chained. For example, <tt class="docutils literal"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span> <span class="pre">==</span> <span class="pre">c</span></tt> tests whether <tt class="docutils literal"><span class="pre">a</span></tt> is
less than <tt class="docutils literal"><span class="pre">b</span></tt> and moreover <tt class="docutils literal"><span class="pre">b</span></tt> equals <tt class="docutils literal"><span class="pre">c</span></tt>.</p>
<p>Comparisons may be combined using the Boolean operators <tt class="docutils literal"><span class="pre">and</span></tt> and <tt class="docutils literal"><span class="pre">or</span></tt>, and
the outcome of a comparison (or of any other Boolean expression) may be negated
with <tt class="docutils literal"><span class="pre">not</span></tt>. These have lower priorities than comparison operators; between
them, <tt class="docutils literal"><span class="pre">not</span></tt> has the highest priority and <tt class="docutils literal"><span class="pre">or</span></tt> the lowest, so that <tt class="docutils literal"><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></tt> is equivalent to <tt class="docutils literal"><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></tt>. As always, parentheses
can be used to express the desired composition.</p>
<p>The Boolean operators <tt class="docutils literal"><span class="pre">and</span></tt> and <tt class="docutils literal"><span class="pre">or</span></tt> 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 <tt class="docutils literal"><span class="pre">A</span></tt> and <tt class="docutils literal"><span class="pre">C</span></tt> are
true but <tt class="docutils literal"><span class="pre">B</span></tt> is false, <tt class="docutils literal"><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></tt> does not evaluate the expression
<tt class="docutils literal"><span class="pre">C</span></tt>. 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-python"><div class="highlight"><pre><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="s">''</span><span class="p">,</span> <span class="s">'Trondheim'</span><span class="p">,</span> <span class="s">'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 cannot occur inside expressions. C
programmers may grumble about this, but it avoids a common class of problems
encountered in C programs: typing <tt class="docutils literal"><span class="pre">=</span></tt> in an expression when <tt class="docutils literal"><span class="pre">==</span></tt> was
intended.</p>
</div>
<div class="section" id="comparing-sequences-and-other-types">
<span id="tut-comparing"></span><h2>5.8. Comparing Sequences and Other Types<a class="headerlink" href="#comparing-sequences-and-other-types" title="Permalink to this headline">¶</a></h2>
<p>Sequence objects 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 ASCII
ordering for individual characters. Some examples of comparisons between
sequences of the same type:</p>
<div class="highlight-python"><div class="highlight"><pre><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="s">'ABC'</span> <span class="o"><</span> <span class="s">'C'</span> <span class="o"><</span> <span class="s">'Pascal'</span> <span class="o"><</span> <span class="s">'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="s">'aa'</span><span class="p">,</span> <span class="s">'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="s">'abc'</span><span class="p">,</span> <span class="s">'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 is legal. The outcome is
deterministic but arbitrary: the types are ordered by their name. Thus, a list
is always smaller than a string, a string is always smaller than a tuple, etc.
<a class="footnote-reference" href="#id2" id="id1">[1]</a> Mixed numeric types are compared according to their numeric value, so 0
equals 0.0, etc.</p>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>The rules for comparing objects of different types should not be relied upon;
they may change in a future version of the language.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="#">5. Data Structures</a><ul>
<li><a class="reference external" href="#more-on-lists">5.1. More on Lists</a><ul>
<li><a class="reference external" href="#using-lists-as-stacks">5.1.1. Using Lists as Stacks</a></li>
<li><a class="reference external" href="#using-lists-as-queues">5.1.2. Using Lists as Queues</a></li>
<li><a class="reference external" href="#functional-programming-tools">5.1.3. Functional Programming Tools</a></li>
<li><a class="reference external" href="#list-comprehensions">5.1.4. List Comprehensions</a></li>
<li><a class="reference external" href="#nested-list-comprehensions">5.1.5. Nested List Comprehensions</a></li>
</ul>
</li>
<li><a class="reference external" href="#the-del-statement">5.2. The <tt class="docutils literal"><span class="pre">del</span></tt> statement</a></li>
<li><a class="reference external" href="#tuples-and-sequences">5.3. Tuples and Sequences</a></li>
<li><a class="reference external" href="#sets">5.4. Sets</a></li>
<li><a class="reference external" href="#dictionaries">5.5. Dictionaries</a></li>
<li><a class="reference external" href="#looping-techniques">5.6. Looping Techniques</a></li>
<li><a class="reference external" href="#more-on-conditions">5.7. More on Conditions</a></li>
<li><a class="reference external" href="#comparing-sequences-and-other-types">5.8. Comparing Sequences and Other Types</a></li>
</ul>
</li>
</ul>
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