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<H1><A NAME="SECTION000200000000000000000">
1 PEP 218: Built-In Set Objects</A>
</H1>

<P>
Python 2.3 introduced the <tt class="module">sets</tt> module.  C implementations of
set data types have now been added to the Python core as two new
built-in types, <tt class="function">set(<var>iterable</var>)</tt> and
<tt class="function">frozenset(<var>iterable</var>)</tt>.  They provide high speed
operations for membership testing, for eliminating duplicates from
sequences, and for mathematical operations like unions, intersections,
differences, and symmetric differences.

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; a = set('abracadabra')              # form a set from a string
&gt;&gt;&gt; 'z' in a                            # fast membership testing
False
&gt;&gt;&gt; a                                   # unique letters in a
set(['a', 'r', 'b', 'c', 'd'])
&gt;&gt;&gt; ''.join(a)                          # convert back into a string
'arbcd'

&gt;&gt;&gt; b = set('alacazam')                 # form a second set
&gt;&gt;&gt; a - b                               # letters in a but not in b
set(['r', 'd', 'b'])
&gt;&gt;&gt; a | b                               # letters in either a or b
set(['a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'])
&gt;&gt;&gt; a &amp; b                               # letters in both a and b
set(['a', 'c'])
&gt;&gt;&gt; a ^ b                               # letters in a or b but not both
set(['r', 'd', 'b', 'm', 'z', 'l'])

&gt;&gt;&gt; a.add('z')                          # add a new element
&gt;&gt;&gt; a.update('wxy')                     # add multiple new elements
&gt;&gt;&gt; a
set(['a', 'c', 'b', 'd', 'r', 'w', 'y', 'x', 'z'])
&gt;&gt;&gt; a.remove('x')                       # take one element out
&gt;&gt;&gt; a
set(['a', 'c', 'b', 'd', 'r', 'w', 'y', 'z'])
</pre></div>

<P>
The <tt class="function">frozenset</tt> type is an immutable version of <tt class="function">set</tt>.
Since it is immutable and hashable, it may be used as a dictionary key or
as a member of another set.

<P>
The <tt class="module">sets</tt> module remains in the standard library, and may be
useful if you wish to subclass the <tt class="class">Set</tt> or <tt class="class">ImmutableSet</tt>
classes.  There are currently no plans to deprecate the module.

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

<dl compact="compact" class="seerfc">
    <dt><a href="http://www.python.org/peps/pep-0218.html"
        title="Adding a Built-In Set Object Type"
        >PEP 218, <em>Adding a Built-In Set Object Type</em></a>
    <dd>Originally proposed by
Greg Wilson and ultimately implemented by Raymond Hettinger.
  </dl>
</div>

<P>

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