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<title>5.15 sets -- Unordered collections of unique elements
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<H1><A NAME=
"SECTION0071500000000000000000">
5.15 <tt class=
"module">sets
</tt> --
Unordered collections of unique elements
</A>
<A NAME=
"module-sets"></A>
<span class=
"versionnote">New in version
2.3.
</span>
The
<tt class=
"module">sets
</tt> module provides classes for constructing and manipulating
unordered collections of unique elements. Common uses include membership
testing, removing duplicates from a sequence, and computing standard math
operations on sets such as intersection, union, difference, and symmetric
Like other collections, sets support
<code><var>x
</var> in
<var>set
</var></code>,
<code>len(
<var>set
</var>)
</code>, and
<code>for
<var>x
</var> in
<var>set
</var></code>. Being an
unordered collection, sets do not record element position or order of
insertion. Accordingly, sets do not support indexing, slicing, or
other sequence-like behavior.
Most set applications use the
<tt class=
"class">Set
</tt> class which provides every set
method except for
<tt class=
"method">__hash__()
</tt>. For advanced applications requiring
a hash method, the
<tt class=
"class">ImmutableSet
</tt> class adds a
<tt class=
"method">__hash__()
</tt>
method but omits methods which alter the contents of the set. Both
<tt class=
"class">Set
</tt> and
<tt class=
"class">ImmutableSet
</tt> derive from
<tt class=
"class">BaseSet
</tt>, an
abstract class useful for determining whether something is a set:
<code>isinstance(
<var>obj
</var>, BaseSet)
</code>.
The set classes are implemented using dictionaries. Accordingly, the
requirements for set elements are the same as those for dictionary keys;
namely, that the element defines both
<tt class=
"method">__eq__
</tt> and
<tt class=
"method">__hash__
</tt>.
cannot contain mutable elements such as lists or dictionaries.
However, they can contain immutable collections such as tuples or
instances of
<tt class=
"class">ImmutableSet
</tt>. For convenience in implementing
sets of sets, inner sets are automatically converted to immutable
form, for example,
<code>Set([Set(['dog'])])
</code> is transformed to
<code>Set([ImmutableSet(['dog'])])
</code>.
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <tt id='l2h-
1380' xml:id='l2h-
1380'
class=
"class">Set
</tt></b>(
</nobr></td>
<td><var></var><big>[
</big><var>iterable
</var><big>]
</big><var></var>)
</td></tr></table></dt>
Constructs a new empty
<tt class=
"class">Set
</tt> object. If the optional
<var>iterable
</var>
parameter is supplied, updates the set with elements obtained from iteration.
All of the elements in
<var>iterable
</var> should be immutable or be transformable
to an immutable using the protocol described in
section
<A href=
"immutable-transforms.html#immutable-transforms">5.15.3</A>.
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <tt id='l2h-
1381' xml:id='l2h-
1381'
class=
"class">ImmutableSet
</tt></b>(
</nobr></td>
<td><var></var><big>[
</big><var>iterable
</var><big>]
</big><var></var>)
</td></tr></table></dt>
Constructs a new empty
<tt class=
"class">ImmutableSet
</tt> object. If the optional
<var>iterable
</var> parameter is supplied, updates the set with elements obtained
from iteration. All of the elements in
<var>iterable
</var> should be immutable or
be transformable to an immutable using the protocol described in
section
<A href=
"immutable-transforms.html#immutable-transforms">5.15.3</A>.
Because
<tt class=
"class">ImmutableSet
</tt> objects provide a
<tt class=
"method">__hash__()
</tt> method,
they can be used as set elements or as dictionary keys.
<tt class=
"class">ImmutableSet
</tt>
objects do not have methods for adding or removing elements, so all of the
elements must be known when the constructor is called.
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<!--Table of Child-Links-->
<A NAME=
"CHILD_LINKS"><STRONG>Subsections
</STRONG></a>
<LI><A href=
"set-objects.html">5.15.1 Set Objects
</a>
<LI><A href=
"set-example.html">5.15.2 Example
</a>
<LI><A href=
"immutable-transforms.html">5.15.3 Protocol for automatic conversion to immutable
</a>
<LI><A href=
"comparison-to-builtin-set.html">5.15.4 Comparison to the built-in
<tt class=
"class">set
</tt> types
</a>
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<span class=
"release-info">Release
2.4.2, documentation updated on
28 September
2005.
</span>
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