Edit File - File Manager

Edit File: datastructures.html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 
 <title>5. Data Structures &mdash; Python v2.6.6 documentation</title>
 <link rel="stylesheet" href="../_static/default.css" type="text/css" />
 <link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
 <script type="text/javascript">
 var DOCUMENTATION_OPTIONS = {
 URL_ROOT: '../',
 VERSION: '2.6.6',
 COLLAPSE_MODINDEX: false,
 FILE_SUFFIX: '.html',
 HAS_SOURCE: true
 };
 </script>
 <script type="text/javascript" src="../_static/jquery.js"></script>
 <script type="text/javascript" src="../_static/doctools.js"></script>
 <link rel="search" type="application/opensearchdescription+xml"
 title="Search within Python v2.6.6 documentation"
 href="../_static/opensearch.xml"/>
 <link rel="author" title="About these documents" href="../about.html" />
 <link rel="copyright" title="Copyright" href="../copyright.html" />
 <link rel="top" title="Python v2.6.6 documentation" href="../index.html" />
 <link rel="up" title="The Python Tutorial" href="index.html" />
 <link rel="next" title="6. Modules" href="modules.html" />
 <link rel="prev" title="4. More Control Flow Tools" href="controlflow.html" />
 <link rel="shortcut icon" type="image/png" href="../_static/py.png" />

</head>
 <body>
 <div class="related">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 accesskey="I">index</a></li>
 <li class="right" >
 <a href="../modindex.html" title="Global Module Index"
 accesskey="M">modules</a> |</li>
 <li class="right" >
 <a href="modules.html" title="6. Modules"
 accesskey="N">next</a> |</li>
 <li class="right" >
 <a href="controlflow.html" title="4. More Control Flow Tools"
 accesskey="P">previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="../index.html">Python v2.6.6 documentation</a> &raquo;</li>

<li><a href="index.html" accesskey="U">The Python Tutorial</a> &raquo;</li> 
 </ul>
 </div>

<div class="document">
 <div class="documentwrapper">
 <div class="bodywrapper">
 <div class="body">
 
 <div class="section" id="data-structures">
<h1>5. Data Structures<a class="headerlink" href="#data-structures" title="Permalink to this headline">¶</a></h1>
This chapter describes some things you&#8217;ve learned about already in more detail,
and adds some new things as well.
<div class="section" id="more-on-lists">
<h2>5.1. More on Lists<a class="headerlink" href="#more-on-lists" title="Permalink to this headline">¶</a></h2>
The list data type has some more methods. Here are all of the methods of list
objects:
<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">append</tt><big>(</big>x<big>)</big></dt>
<dd>Add an item to the end of the list; equivalent to <tt class="docutils literal">a[len(a):] = [x]</tt>.</dd></dl>

<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">extend</tt><big>(</big>L<big>)</big></dt>
<dd>Extend the list by appending all the items in the given list; equivalent to
<tt class="docutils literal">a[len(a):] = L</tt>.</dd></dl>

<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">insert</tt><big>(</big>i, x<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">a.insert(0, x)</tt> inserts at the front of
the list, and <tt class="docutils literal">a.insert(len(a), x)</tt> is equivalent to <tt class="docutils literal">a.append(x)</tt>.</dd></dl>

<dl class="method">
<dt>
<tt class="descclassname">list.</tt><tt class="descname">remove</tt><big>(</big>x<big>)</big></dt>
<dd>Remove the first item from the list whose value is x. 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>[i]<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">a.pop()</tt> removes and returns the last item in the list. (The
square brackets around the i 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>x<big>)</big></dt>
<dd>Return the index in the list of the first item whose value is x. 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>x<big>)</big></dt>
<dd>Return the number of times x 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>

An example that uses most of the list methods:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a = [66.25, 333, 333, 1, 1234.5]
&gt;&gt;&gt; print a.count(333), a.count(66.25), a.count(&#39;x&#39;)
2 1 0
&gt;&gt;&gt; a.insert(2, -1)
&gt;&gt;&gt; a.append(333)
&gt;&gt;&gt; a
[66.25, 333, -1, 333, 1, 1234.5, 333]
&gt;&gt;&gt; a.index(333)
1
&gt;&gt;&gt; a.remove(333)
&gt;&gt;&gt; a
[66.25, -1, 333, 1, 1234.5, 333]
&gt;&gt;&gt; a.reverse()
&gt;&gt;&gt; a
[333, 1234.5, 1, 333, -1, 66.25]
&gt;&gt;&gt; a.sort()
&gt;&gt;&gt; a
[-1, 1, 66.25, 333, 333, 1234.5]
</pre></div>
</div>
<div class="section" id="using-lists-as-stacks">
<h3>5.1.1. Using Lists as Stacks<a class="headerlink" href="#using-lists-as-stacks" title="Permalink to this headline">¶</a></h3>
The list methods make it very easy to use a list as a stack, where the last
element added is the first element retrieved (&#8220;last-in, first-out&#8221;). To add an
item to the top of the stack, use <tt class="xref docutils literal">append()</tt>. To retrieve an item from the
top of the stack, use <tt class="xref docutils literal">pop()</tt> without an explicit index. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; stack = [3, 4, 5]
&gt;&gt;&gt; stack.append(6)
&gt;&gt;&gt; stack.append(7)
&gt;&gt;&gt; stack
[3, 4, 5, 6, 7]
&gt;&gt;&gt; stack.pop()
7
&gt;&gt;&gt; stack
[3, 4, 5, 6]
&gt;&gt;&gt; stack.pop()
6
&gt;&gt;&gt; stack.pop()
5
&gt;&gt;&gt; stack
[3, 4]
</pre></div>
</div>
</div>
<div class="section" id="using-lists-as-queues">
<h3>5.1.2. Using Lists as Queues<a class="headerlink" href="#using-lists-as-queues" title="Permalink to this headline">¶</a></h3>
It is also possible to use a list as a queue, where the first element added is
the first element retrieved (&#8220;first-in, first-out&#8221;); 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).
To implement a queue, use <a title="collections.deque" class="reference external" href="../library/collections.html#collections.deque"><tt class="xref docutils literal">collections.deque</tt></a> which was designed to
have fast appends and pops from both ends. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; from collections import deque
&gt;&gt;&gt; queue = deque([&quot;Eric&quot;, &quot;John&quot;, &quot;Michael&quot;])
&gt;&gt;&gt; queue.append(&quot;Terry&quot;) # Terry arrives
&gt;&gt;&gt; queue.append(&quot;Graham&quot;) # Graham arrives
&gt;&gt;&gt; queue.popleft() # The first to arrive now leaves
&#39;Eric&#39;
&gt;&gt;&gt; queue.popleft() # The second to arrive now leaves
&#39;John&#39;
&gt;&gt;&gt; queue # Remaining queue in order of arrival
deque([&#39;Michael&#39;, &#39;Terry&#39;, &#39;Graham&#39;])
</pre></div>
</div>
</div>
<div class="section" id="functional-programming-tools">
<h3>5.1.3. Functional Programming Tools<a class="headerlink" href="#functional-programming-tools" title="Permalink to this headline">¶</a></h3>
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">filter()</tt></a>, <a title="map" class="reference external" href="../library/functions.html#map"><tt class="xref docutils literal">map()</tt></a>, and <a title="reduce" class="reference external" href="../library/functions.html#reduce"><tt class="xref docutils literal">reduce()</tt></a>.
<tt class="docutils literal">filter(function, sequence)</tt> returns a sequence consisting of those items from
the sequence for which <tt class="docutils literal">function(item)</tt> is true. If sequence is a
<tt class="xref docutils literal">string</tt> or <a title="tuple" class="reference external" href="../library/functions.html#tuple"><tt class="xref docutils literal">tuple</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">list</tt></a>. For example, to compute some primes:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def f(x): return x % 2 != 0 and x % 3 != 0
...
&gt;&gt;&gt; filter(f, range(2, 25))
[5, 7, 11, 13, 17, 19, 23]
</pre></div>
</div>
<tt class="docutils literal">map(function, sequence)</tt> calls <tt class="docutils literal">function(item)</tt> for each of the sequence&#8217;s
items and returns a list of the return values. For example, to compute some
cubes:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def cube(x): return x*x*x
...
&gt;&gt;&gt; map(cube, range(1, 11))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
</pre></div>
</div>
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">None</tt> if some sequence is shorter than another). For
example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; seq = range(8)
&gt;&gt;&gt; def add(x, y): return x+y
...
&gt;&gt;&gt; map(add, seq, seq)
[0, 2, 4, 6, 8, 10, 12, 14]
</pre></div>
</div>
<tt class="docutils literal">reduce(function, sequence)</tt> returns a single value constructed by calling the
binary function function 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def add(x,y): return x+y
...
&gt;&gt;&gt; reduce(add, range(1, 11))
55
</pre></div>
</div>
If there&#8217;s only one item in the sequence, its value is returned; if the sequence
is empty, an exception is raised.
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,
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def sum(seq):
... def add(x,y): return x+y
... return reduce(add, seq, 0)
...
&gt;&gt;&gt; sum(range(1, 11))
55
&gt;&gt;&gt; sum([])
0
</pre></div>
</div>
Don&#8217;t use this example&#8217;s definition of <a title="sum" class="reference external" href="../library/functions.html#sum"><tt class="xref docutils literal">sum()</tt></a>: since summing numbers is
such a common need, a built-in function <tt class="docutils literal">sum(sequence)</tt> is already provided,
and works exactly like this.

New in version 2.3.
</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>
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">map()</tt></a>, <a title="filter" class="reference external" href="../library/functions.html#filter"><tt class="xref docutils literal">filter()</tt></a> and/or <a class="reference external" href="../reference/expressions.html#lambda"><tt class="xref docutils literal">lambda</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">for</tt></a>
clause, then zero or more <a class="reference external" href="../reference/compound_stmts.html#for"><tt class="xref docutils literal">for</tt></a> or <a class="reference external" href="../reference/compound_stmts.html#if"><tt class="xref docutils literal">if</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">for</tt></a> and <a class="reference external" href="../reference/compound_stmts.html#if"><tt class="xref docutils literal">if</tt></a> clauses which follow it. If the expression
would evaluate to a tuple, it must be parenthesized.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; freshfruit = [&#39; banana&#39;, &#39; loganberry &#39;, &#39;passion fruit &#39;]
&gt;&gt;&gt; [weapon.strip() for weapon in freshfruit]
[&#39;banana&#39;, &#39;loganberry&#39;, &#39;passion fruit&#39;]
&gt;&gt;&gt; vec = [2, 4, 6]
&gt;&gt;&gt; [3*x for x in vec]
[6, 12, 18]
&gt;&gt;&gt; [3*x for x in vec if x &gt; 3]
[12, 18]
&gt;&gt;&gt; [3*x for x in vec if x &lt; 2]
[]
&gt;&gt;&gt; [[x,x**2] for x in vec]
[[2, 4], [4, 16], [6, 36]]
&gt;&gt;&gt; [x, x**2 for x in vec] # error - parens required for tuples
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
 [x, x**2 for x in vec]
 ^
SyntaxError: invalid syntax
&gt;&gt;&gt; [(x, x**2) for x in vec]
[(2, 4), (4, 16), (6, 36)]
&gt;&gt;&gt; vec1 = [2, 4, 6]
&gt;&gt;&gt; vec2 = [4, 3, -9]
&gt;&gt;&gt; [x*y for x in vec1 for y in vec2]
[8, 6, -18, 16, 12, -36, 24, 18, -54]
&gt;&gt;&gt; [x+y for x in vec1 for y in vec2]
[6, 5, -7, 8, 7, -5, 10, 9, -3]
&gt;&gt;&gt; [vec1[i]*vec2[i] for i in range(len(vec1))]
[8, 12, -54]
</pre></div>
</div>
List comprehensions are much more flexible than <a title="map" class="reference external" href="../library/functions.html#map"><tt class="xref docutils literal">map()</tt></a> and can be applied
to complex expressions and nested functions:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; [str(round(355/113.0, i)) for i in range(1,6)]
[&#39;3.1&#39;, &#39;3.14&#39;, &#39;3.142&#39;, &#39;3.1416&#39;, &#39;3.14159&#39;]
</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>
If you&#8217;ve got the stomach for it, list comprehensions can be nested. They are a
powerful tool but &#8211; like all powerful tools &#8211; they need to be used carefully,
if at all.
Consider the following example of a 3x3 matrix held as a list containing three
lists, one list per row:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; mat = [
... [1, 2, 3],
... [4, 5, 6],
... [7, 8, 9],
... ]
</pre></div>
</div>
Now, if you wanted to swap rows and columns, you could use a list
comprehension:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; print [[row[i] for row in mat] for i in [0, 1, 2]]
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]
</pre></div>
</div>
Special care has to be taken for the nested list comprehension:
<blockquote>
To avoid apprehension when nesting list comprehensions, read from right to
left.</blockquote>
A more verbose version of this snippet shows the flow explicitly:
<div class="highlight-python"><div class="highlight"><pre>for i in [0, 1, 2]:
 for row in mat:
 print row[i],
 print
</pre></div>
</div>
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">zip()</tt></a> function would do a great job for this use case:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; zip(*mat)
[(1, 4, 7), (2, 5, 8), (3, 6, 9)]
</pre></div>
</div>
See <a class="reference external" href="controlflow.html#tut-unpacking-arguments">Unpacking Argument Lists</a> for details on the asterisk in this line.
</div>
</div>
<div class="section" id="the-del-statement">
<h2>5.2. The <a class="reference external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal">del</tt></a> statement<a class="headerlink" href="#the-del-statement" title="Permalink to this headline">¶</a></h2>
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">del</tt></a> statement. This differs from the <tt class="xref docutils literal">pop()</tt> method
which returns a value. The <a class="reference external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal">del</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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a = [-1, 1, 66.25, 333, 333, 1234.5]
&gt;&gt;&gt; del a[0]
&gt;&gt;&gt; a
[1, 66.25, 333, 333, 1234.5]
&gt;&gt;&gt; del a[2:4]
&gt;&gt;&gt; a
[1, 66.25, 1234.5]
&gt;&gt;&gt; del a[:]
&gt;&gt;&gt; a
[]
</pre></div>
</div>
<a class="reference external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal">del</tt></a> can also be used to delete entire variables:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; del a
</pre></div>
</div>
Referencing the name <tt class="docutils literal">a</tt> hereafter is an error (at least until another value
is assigned to it). We&#8217;ll find other uses for <a class="reference external" href="../reference/simple_stmts.html#del"><tt class="xref docutils literal">del</tt></a> later.
</div>
<div class="section" id="tuples-and-sequences">
<h2>5.3. Tuples and Sequences<a class="headerlink" href="#tuples-and-sequences" title="Permalink to this headline">¶</a></h2>
We saw that lists and strings have many common properties, such as indexing and
slicing operations. They are two examples of sequence data types (see
<a class="reference external" href="../library/stdtypes.html#typesseq">Sequence Types &#8212; str, unicode, list, tuple, buffer, xrange</a>). Since Python is an evolving language, other sequence data
types may be added. There is also another standard sequence data type: the
tuple.
A tuple consists of a number of values separated by commas, for instance:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; t = 12345, 54321, &#39;hello!&#39;
&gt;&gt;&gt; t[0]
12345
&gt;&gt;&gt; t
(12345, 54321, &#39;hello!&#39;)
&gt;&gt;&gt; # Tuples may be nested:
... u = t, (1, 2, 3, 4, 5)
&gt;&gt;&gt; u
((12345, 54321, &#39;hello!&#39;), (1, 2, 3, 4, 5))
</pre></div>
</div>
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).
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.
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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; empty = ()
&gt;&gt;&gt; singleton = &#39;hello&#39;, # &lt;-- note trailing comma
&gt;&gt;&gt; len(empty)
0
&gt;&gt;&gt; len(singleton)
1
&gt;&gt;&gt; singleton
(&#39;hello&#39;,)
</pre></div>
</div>
The statement <tt class="docutils literal">t = 12345, 54321, 'hello!'</tt> is an example of tuple packing:
the values <tt class="docutils literal">12345</tt>, <tt class="docutils literal">54321</tt> and <tt class="docutils literal">'hello!'</tt> are packed together in a tuple.
The reverse operation is also possible:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; x, y, z = t
</pre></div>
</div>
This is called, appropriately enough, sequence unpacking 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.
</div>
<div class="section" id="sets">
<h2>5.4. Sets<a class="headerlink" href="#sets" title="Permalink to this headline">¶</a></h2>
Python also includes a data type for sets. 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.
Here is a brief demonstration:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; basket = [&#39;apple&#39;, &#39;orange&#39;, &#39;apple&#39;, &#39;pear&#39;, &#39;orange&#39;, &#39;banana&#39;]
&gt;&gt;&gt; fruit = set(basket) # create a set without duplicates
&gt;&gt;&gt; fruit
set([&#39;orange&#39;, &#39;pear&#39;, &#39;apple&#39;, &#39;banana&#39;])
&gt;&gt;&gt; &#39;orange&#39; in fruit # fast membership testing
True
&gt;&gt;&gt; &#39;crabgrass&#39; in fruit
False

&gt;&gt;&gt; # Demonstrate set operations on unique letters from two words
...
&gt;&gt;&gt; a = set(&#39;abracadabra&#39;)
&gt;&gt;&gt; b = set(&#39;alacazam&#39;)
&gt;&gt;&gt; a # unique letters in a
set([&#39;a&#39;, &#39;r&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;])
&gt;&gt;&gt; a - b # letters in a but not in b
set([&#39;r&#39;, &#39;d&#39;, &#39;b&#39;])
&gt;&gt;&gt; a | b # letters in either a or b
set([&#39;a&#39;, &#39;c&#39;, &#39;r&#39;, &#39;d&#39;, &#39;b&#39;, &#39;m&#39;, &#39;z&#39;, &#39;l&#39;])
&gt;&gt;&gt; a &amp; b # letters in both a and b
set([&#39;a&#39;, &#39;c&#39;])
&gt;&gt;&gt; a ^ b # letters in a or b but not both
set([&#39;r&#39;, &#39;d&#39;, &#39;b&#39;, &#39;m&#39;, &#39;z&#39;, &#39;l&#39;])
</pre></div>
</div>
</div>
<div class="section" id="dictionaries">
<h2>5.5. Dictionaries<a class="headerlink" href="#dictionaries" title="Permalink to this headline">¶</a></h2>
Another useful data type built into Python is the dictionary (see
<a class="reference external" href="../library/stdtypes.html#typesmapping">Mapping Types &#8212; dict</a>). Dictionaries are sometimes found in other languages as
&#8220;associative memories&#8221; or &#8220;associative arrays&#8221;. Unlike sequences, which are
indexed by a range of numbers, dictionaries are indexed by keys, 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&#8217;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">append()</tt> and
<tt class="xref docutils literal">extend()</tt>.
It is best to think of a dictionary as an unordered set of key: value pairs,
with the requirement that the keys are unique (within one dictionary). A pair of
braces creates an empty dictionary: <tt class="docutils literal">{}</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.
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">del</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.
The <tt class="xref docutils literal">keys()</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">sort()</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">in</tt></a> keyword.
Here is a small example using a dictionary:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; tel = {&#39;jack&#39;: 4098, &#39;sape&#39;: 4139}
&gt;&gt;&gt; tel[&#39;guido&#39;] = 4127
&gt;&gt;&gt; tel
{&#39;sape&#39;: 4139, &#39;guido&#39;: 4127, &#39;jack&#39;: 4098}
&gt;&gt;&gt; tel[&#39;jack&#39;]
4098
&gt;&gt;&gt; del tel[&#39;sape&#39;]
&gt;&gt;&gt; tel[&#39;irv&#39;] = 4127
&gt;&gt;&gt; tel
{&#39;guido&#39;: 4127, &#39;irv&#39;: 4127, &#39;jack&#39;: 4098}
&gt;&gt;&gt; tel.keys()
[&#39;guido&#39;, &#39;irv&#39;, &#39;jack&#39;]
&gt;&gt;&gt; &#39;guido&#39; in tel
True
</pre></div>
</div>
The <a title="dict" class="reference external" href="../library/stdtypes.html#dict"><tt class="xref docutils literal">dict()</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.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; dict([(&#39;sape&#39;, 4139), (&#39;guido&#39;, 4127), (&#39;jack&#39;, 4098)])
{&#39;sape&#39;: 4139, &#39;jack&#39;: 4098, &#39;guido&#39;: 4127}
&gt;&gt;&gt; dict([(x, x**2) for x in (2, 4, 6)]) # use a list comprehension
{2: 4, 4: 16, 6: 36}
</pre></div>
</div>
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">dict()</tt></a>
constructor.
When the keys are simple strings, it is sometimes easier to specify pairs using
keyword arguments:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; dict(sape=4139, guido=4127, jack=4098)
{&#39;sape&#39;: 4139, &#39;jack&#39;: 4098, &#39;guido&#39;: 4127}
</pre></div>
</div>
</div>
<div class="section" id="looping-techniques">
<h2>5.6. Looping Techniques<a class="headerlink" href="#looping-techniques" title="Permalink to this headline">¶</a></h2>
When looping through dictionaries, the key and corresponding value can be
retrieved at the same time using the <tt class="xref docutils literal">iteritems()</tt> method.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; knights = {&#39;gallahad&#39;: &#39;the pure&#39;, &#39;robin&#39;: &#39;the brave&#39;}
&gt;&gt;&gt; for k, v in knights.iteritems():
... print k, v
...
gallahad the pure
robin the brave
</pre></div>
</div>
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">enumerate()</tt></a> function.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for i, v in enumerate([&#39;tic&#39;, &#39;tac&#39;, &#39;toe&#39;]):
... print i, v
...
0 tic
1 tac
2 toe
</pre></div>
</div>
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">zip()</tt></a> function.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; questions = [&#39;name&#39;, &#39;quest&#39;, &#39;favorite color&#39;]
&gt;&gt;&gt; answers = [&#39;lancelot&#39;, &#39;the holy grail&#39;, &#39;blue&#39;]
&gt;&gt;&gt; for q, a in zip(questions, answers):
... print &#39;What is your {0}? It is {1}.&#39;.format(q, a)
...
What is your name? It is lancelot.
What is your quest? It is the holy grail.
What is your favorite color? It is blue.
</pre></div>
</div>
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">reversed()</tt></a> function.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for i in reversed(xrange(1,10,2)):
... print i
...
9
7
5
3
1
</pre></div>
</div>
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">sorted()</tt></a> function which
returns a new sorted list while leaving the source unaltered.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; basket = [&#39;apple&#39;, &#39;orange&#39;, &#39;apple&#39;, &#39;pear&#39;, &#39;orange&#39;, &#39;banana&#39;]
&gt;&gt;&gt; for f in sorted(set(basket)):
... print f
...
apple
banana
orange
pear
</pre></div>
</div>
</div>
<div class="section" id="more-on-conditions">
<h2>5.7. More on Conditions<a class="headerlink" href="#more-on-conditions" title="Permalink to this headline">¶</a></h2>
The conditions used in <tt class="docutils literal">while</tt> and <tt class="docutils literal">if</tt> statements can contain any
operators, not just comparisons.
The comparison operators <tt class="docutils literal">in</tt> and <tt class="docutils literal">not in</tt> check whether a value occurs
(does not occur) in a sequence. The operators <tt class="docutils literal">is</tt> and <tt class="docutils literal">is not</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.
Comparisons can be chained. For example, <tt class="docutils literal">a &lt; b == c</tt> tests whether <tt class="docutils literal">a</tt> is
less than <tt class="docutils literal">b</tt> and moreover <tt class="docutils literal">b</tt> equals <tt class="docutils literal">c</tt>.
Comparisons may be combined using the Boolean operators <tt class="docutils literal">and</tt> and <tt class="docutils literal">or</tt>, and
the outcome of a comparison (or of any other Boolean expression) may be negated
with <tt class="docutils literal">not</tt>. These have lower priorities than comparison operators; between
them, <tt class="docutils literal">not</tt> has the highest priority and <tt class="docutils literal">or</tt> the lowest, so that <tt class="docutils literal">A and
not B or C</tt> is equivalent to <tt class="docutils literal">(A and (not B)) or C</tt>. As always, parentheses
can be used to express the desired composition.
The Boolean operators <tt class="docutils literal">and</tt> and <tt class="docutils literal">or</tt> are so-called short-circuit
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">A</tt> and <tt class="docutils literal">C</tt> are
true but <tt class="docutils literal">B</tt> is false, <tt class="docutils literal">A and B and C</tt> does not evaluate the expression
<tt class="docutils literal">C</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.
It is possible to assign the result of a comparison or other Boolean expression
to a variable. For example,
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; string1, string2, string3 = &#39;&#39;, &#39;Trondheim&#39;, &#39;Hammer Dance&#39;
&gt;&gt;&gt; non_null = string1 or string2 or string3
&gt;&gt;&gt; non_null
&#39;Trondheim&#39;
</pre></div>
</div>
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">=</tt> in an expression when <tt class="docutils literal">==</tt> was
intended.
</div>
<div class="section" id="comparing-sequences-and-other-types">
<h2>5.8. Comparing Sequences and Other Types<a class="headerlink" href="#comparing-sequences-and-other-types" title="Permalink to this headline">¶</a></h2>
Sequence objects may be compared to other objects with the same sequence type.
The comparison uses lexicographical 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:
<div class="highlight-python"><div class="highlight"><pre>(1, 2, 3) &lt; (1, 2, 4)
[1, 2, 3] &lt; [1, 2, 4]
&#39;ABC&#39; &lt; &#39;C&#39; &lt; &#39;Pascal&#39; &lt; &#39;Python&#39;
(1, 2, 3, 4) &lt; (1, 2, 4)
(1, 2) &lt; (1, 2, -1)
(1, 2, 3) == (1.0, 2.0, 3.0)
(1, 2, (&#39;aa&#39;, &#39;ab&#39;)) &lt; (1, 2, (&#39;abc&#39;, &#39;a&#39;), 4)
</pre></div>
</div>
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.
Footnotes
<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>
 <div class="sphinxsidebar">
 <div class="sphinxsidebarwrapper">
 <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">del</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>

<h4>Previous topic</h4>
 <a href="controlflow.html"
 title="previous chapter">4. More Control Flow Tools</a>
 <h4>Next topic</h4>
 <a href="modules.html"
 title="next chapter">6. Modules</a>
<h3>This Page</h3>
<ul class="this-page-menu">
 <li><a href="../bugs.html">Report a Bug</a></li>
 <li><a href="../_sources/tutorial/datastructures.txt"
 rel="nofollow">Show Source</a></li>
</ul>

<div id="searchbox" style="display: none">
 <h3>Quick search</h3>
 <form class="search" action="../search.html" method="get">
 <input type="text" name="q" size="18" />
 <input type="submit" value="Go" />
 <input type="hidden" name="check_keywords" value="yes" />
 <input type="hidden" name="area" value="default" />
 </form>
 
 Enter search terms or a module, class or function name.
 
 </div>
 <script type="text/javascript">$('#searchbox').show(0);</script>
 </div>
 </div>
 <div class="clearer"></div>
 </div>
 <div class="related">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 >index</a></li>
 <li class="right" >
 <a href="../modindex.html" title="Global Module Index"
 >modules</a> |</li>
 <li class="right" >
 <a href="modules.html" title="6. Modules"
 >next</a> |</li>
 <li class="right" >
 <a href="controlflow.html" title="4. More Control Flow Tools"
 >previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="../index.html">Python v2.6.6 documentation</a> &raquo;</li>

<li><a href="index.html" >The Python Tutorial</a> &raquo;</li> 
 </ul>
 </div>
 <div class="footer">
 &copy; <a href="../copyright.html">Copyright</a> 1990-2011, Python Software Foundation.
 
 The Python Software Foundation is a non-profit corporation. 
 <a href="http://www.python.org/psf/donations/">Please donate.</a>
 
 Last updated on Jul 20, 2011.
 <a href="../bugs.html">Found a bug</a>?
 
 Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 0.6.6.
 </div>

</body>
</html>

Back to File Manager