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<H2><A NAME="SECTION004370000000000000000"></A><A NAME="types-set"></A>
<BR>
2.3.7 Set Types --
            <tt class="class">set</tt>, <tt class="class">frozenset</tt>

</H2>
<a id='l2h-226' xml:id='l2h-226'></a>
<P>
A <i class="dfn">set</i> object is an unordered collection of immutable values.
Common uses include membership testing, removing duplicates from a sequence,
and computing mathematical operations such as intersection, union, difference,
and symmetric difference.

<span class="versionnote">New in version 2.4.</span>

<P>
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.

<P>
There are currently two builtin set types, <tt class="class">set</tt> and <tt class="class">frozenset</tt>.
The <tt class="class">set</tt> type is mutable -- the contents can be changed using methods
like <tt class="method">add()</tt> and <tt class="method">remove()</tt>.  Since it is mutable, it has no
hash value and cannot be used as either a dictionary key or as an element of
another set.  The <tt class="class">frozenset</tt> type is immutable and hashable -- its
contents cannot be altered after is created; however, it can be used as
a dictionary key or as an element of another set.

<P>
Instances of <tt class="class">set</tt> and <tt class="class">frozenset</tt> provide the following operations:

<P>
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="center">Operation</th>
      <th class="center">Equivalent</th>
      <th class="left"  >Result</th>
      </tr>
    </thead>
  <tbody>
    <tr><td class="center" valign="baseline"><code>len(<var>s</var>)</code></td>
        <td class="center"></td>
        <td class="left"  >cardinality of set <var>s</var></td></tr><P>

    <tr><td class="center" valign="baseline"><code><var>x</var> in <var>s</var></code></td>
        <td class="center"></td>
        <td class="left"  >test <var>x</var> for membership in <var>s</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>x</var> not in <var>s</var></code></td>
        <td class="center"></td>
        <td class="left"  >test <var>x</var> for non-membership in <var>s</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.issubset(<var>t</var>)</code></td>
        <td class="center"><code><var>s</var> &lt;= <var>t</var></code></td>
        <td class="left"  >test whether every element in <var>s</var> is in <var>t</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.issuperset(<var>t</var>)</code></td>
        <td class="center"><code><var>s</var> &gt;= <var>t</var></code></td>
        <td class="left"  >test whether every element in <var>t</var> is in <var>s</var></td></tr><P>

    <tr><td class="center" valign="baseline"><code><var>s</var>.union(<var>t</var>)</code></td>
        <td class="center"><var>s</var> | <var>t</var></td>
        <td class="left"  >new set with elements from both <var>s</var> and <var>t</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.intersection(<var>t</var>)</code></td>
        <td class="center"><var>s</var> &amp; <var>t</var></td>
        <td class="left"  >new set with elements common to <var>s</var> and <var>t</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.difference(<var>t</var>)</code></td>
        <td class="center"><var>s</var> - <var>t</var></td>
        <td class="left"  >new set with elements in <var>s</var> but not in <var>t</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.symmetric_difference(<var>t</var>)</code></td>
        <td class="center"><var>s</var> ^ <var>t</var></td>
        <td class="left"  >new set with elements in either <var>s</var> or <var>t</var> but not both</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.copy()</code></td>
        <td class="center"></td>
        <td class="left"  >new set with a shallow copy of <var>s</var></td></tr></tbody>
</table></div>

<P>
Note, the non-operator versions of <tt class="method">union()</tt>, <tt class="method">intersection()</tt>,
<tt class="method">difference()</tt>, and <tt class="method">symmetric_difference()</tt>,
<tt class="method">issubset()</tt>, and <tt class="method">issuperset()</tt> methods will accept any
iterable as an argument.  In contrast, their operator based counterparts
require their arguments to be sets.  This precludes error-prone constructions
like <code>set('abc') &amp; 'cbs'</code> in favor of the more readable
<code>set('abc').intersection('cbs')</code>.

<P>
Both <tt class="class">set</tt> and <tt class="class">frozenset</tt> support set to set comparisons.
Two sets are equal if and only if every element of each set is contained in
the other (each is a subset of the other).
A set is less than another set if and only if the first set is a proper
subset of the second set (is a subset, but is not equal).
A set is greater than another set if and only if the first set is a proper
superset of the second set (is a superset, but is not equal).

<P>
Instances of <tt class="class">set</tt> are compared to instances of <tt class="class">frozenset</tt> based
on their members.  For example, "<tt class="samp">set('abc') == frozenset('abc')</tt>" returns
<code>True</code>.

<P>
The subset and equality comparisons do not generalize to a complete
ordering function.  For example, any two disjoint sets are not equal and
are not subsets of each other, so <em>all</em> of the following return
<code>False</code>:  <code><var>a</var>&lt;<var>b</var></code>, <code><var>a</var>==<var>b</var></code>, or
<code><var>a</var>&gt;<var>b</var></code>.
Accordingly, sets do not implement the <tt class="method">__cmp__</tt> method.

<P>
Since sets only define partial ordering (subset relationships), the output
of the <tt class="method">list.sort()</tt> method is undefined for lists of sets.

<P>
Set elements are like dictionary keys; they need to define both
<tt class="method">__hash__</tt> and <tt class="method">__eq__</tt> methods.

<P>
Binary operations that mix <tt class="class">set</tt> instances with <tt class="class">frozenset</tt>
return the type of the first operand.  For example:
"<tt class="samp">frozenset('ab') | set('bc')</tt>" returns an instance of <tt class="class">frozenset</tt>.

<P>
The following table lists operations available for <tt class="class">set</tt>
that do not apply to immutable instances of <tt class="class">frozenset</tt>:

<P>
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="center">Operation</th>
      <th class="center">Equivalent</th>
      <th class="left"  >Result</th>
      </tr>
    </thead>
  <tbody>
    <tr><td class="center" valign="baseline"><code><var>s</var>.update(<var>t</var>)</code></td>
        <td class="center"><var>s</var> |= <var>t</var></td>
        <td class="left"  >return set <var>s</var> with elements added from <var>t</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.intersection_update(<var>t</var>)</code></td>
        <td class="center"><var>s</var> &amp;= <var>t</var></td>
        <td class="left"  >return set <var>s</var> keeping only elements also found in <var>t</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.difference_update(<var>t</var>)</code></td>
        <td class="center"><var>s</var> -= <var>t</var></td>
        <td class="left"  >return set <var>s</var> after removing elements found in <var>t</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.symmetric_difference_update(<var>t</var>)</code></td>
        <td class="center"><var>s</var> ^= <var>t</var></td>
        <td class="left"  >return set <var>s</var> with elements from <var>s</var> or <var>t</var>
          but not both</td></tr><P>

    <tr><td class="center" valign="baseline"><code><var>s</var>.add(<var>x</var>)</code></td>
        <td class="center"></td>
        <td class="left"  >add element <var>x</var> to set <var>s</var></td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.remove(<var>x</var>)</code></td>
        <td class="center"></td>
        <td class="left"  >remove <var>x</var> from set <var>s</var>; raises KeyError if not present</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.discard(<var>x</var>)</code></td>
        <td class="center"></td>
        <td class="left"  >removes <var>x</var> from set <var>s</var> if present</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.pop()</code></td>
        <td class="center"></td>
        <td class="left"  >remove and return an arbitrary element from <var>s</var>; raises
          <tt class="exception">KeyError</tt> if empty</td></tr>
    <tr><td class="center" valign="baseline"><code><var>s</var>.clear()</code></td>
        <td class="center"></td>
        <td class="left"  >remove all elements from set <var>s</var></td></tr></tbody>
</table></div>

<P>
Note, the non-operator versions of the <tt class="method">update()</tt>,
<tt class="method">intersection_update()</tt>, <tt class="method">difference_update()</tt>, and
<tt class="method">symmetric_difference_update()</tt> methods will accept any iterable
as an argument.

<P>
The design of the set types was based on lessons learned from the
<tt class="module">sets</tt> module.

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

  <dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-comparison-to-builtin-set.html">sets</a></tt>:</b>
    <dd>Differences between
     the <tt class="module">sets</tt> module and the built-in set types.
  </dl>
</div>

<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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