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<!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>8.7. sets — Unordered collections of unique elements — Python 2.7.5 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: '2.7.5', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.5 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 2.7.5 documentation" href="../index.html" /> <link rel="up" title="8. 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Data Types</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-sets"> <span id="sets-unordered-collections-of-unique-elements"></span><h1>8.7. <a class="reference internal" href="#module-sets" title="sets: Implementation of sets of unique elements. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">sets</span></tt></a> — Unordered collections of unique elements<a class="headerlink" href="#module-sets" title="Permalink to this headline">¶</a></h1> <p class="versionadded"> <span class="versionmodified">New in version 2.3.</span></p> <p class="deprecated"> <span class="versionmodified">Deprecated since version 2.6: </span>The built-in <a class="reference internal" href="stdtypes.html#set" title="set"><tt class="xref py py-class docutils literal"><span class="pre">set</span></tt></a>/<a class="reference internal" href="stdtypes.html#frozenset" title="frozenset"><tt class="xref py py-class docutils literal"><span class="pre">frozenset</span></tt></a> types replace this module.</p> <p>The <a class="reference internal" href="#module-sets" title="sets: Implementation of sets of unique elements. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">sets</span></tt></a> 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 difference.</p> <p>Like other collections, sets support <tt class="docutils literal"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">set</span></tt>, <tt class="docutils literal"><span class="pre">len(set)</span></tt>, and <tt class="docutils literal"><span class="pre">for</span> <span class="pre">x</span> <span class="pre">in</span> <span class="pre">set</span></tt>. 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> <p>Most set applications use the <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> class which provides every set method except for <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><tt class="xref py py-meth docutils literal"><span class="pre">__hash__()</span></tt></a>. For advanced applications requiring a hash method, the <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> class adds a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><tt class="xref py py-meth docutils literal"><span class="pre">__hash__()</span></tt></a> method but omits methods which alter the contents of the set. Both <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> and <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> derive from <tt class="xref py py-class docutils literal"><span class="pre">BaseSet</span></tt>, an abstract class useful for determining whether something is a set: <tt class="docutils literal"><span class="pre">isinstance(obj,</span> <span class="pre">BaseSet)</span></tt>.</p> <p>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 <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><tt class="xref py py-meth docutils literal"><span class="pre">__eq__()</span></tt></a> and <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><tt class="xref py py-meth docutils literal"><span class="pre">__hash__()</span></tt></a>. As a result, sets cannot contain mutable elements such as lists or dictionaries. However, they can contain immutable collections such as tuples or instances of <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a>. For convenience in implementing sets of sets, inner sets are automatically converted to immutable form, for example, <tt class="docutils literal"><span class="pre">Set([Set(['dog'])])</span></tt> is transformed to <tt class="docutils literal"><span class="pre">Set([ImmutableSet(['dog'])])</span></tt>.</p> <dl class="class"> <dt id="sets.Set"> <em class="property">class </em><tt class="descclassname">sets.</tt><tt class="descname">Set</tt><big>(</big><span class="optional">[</span><em>iterable</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#sets.Set" title="Permalink to this definition">¶</a></dt> <dd><p>Constructs a new empty <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> object. If the optional <em>iterable</em> parameter is supplied, updates the set with elements obtained from iteration. All of the elements in <em>iterable</em> should be immutable or be transformable to an immutable using the protocol described in section <a class="reference internal" href="#immutable-transforms"><em>Protocol for automatic conversion to immutable</em></a>.</p> </dd></dl> <dl class="class"> <dt id="sets.ImmutableSet"> <em class="property">class </em><tt class="descclassname">sets.</tt><tt class="descname">ImmutableSet</tt><big>(</big><span class="optional">[</span><em>iterable</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#sets.ImmutableSet" title="Permalink to this definition">¶</a></dt> <dd><p>Constructs a new empty <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> object. If the optional <em>iterable</em> parameter is supplied, updates the set with elements obtained from iteration. All of the elements in <em>iterable</em> should be immutable or be transformable to an immutable using the protocol described in section <a class="reference internal" href="#immutable-transforms"><em>Protocol for automatic conversion to immutable</em></a>.</p> <p>Because <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> objects provide a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><tt class="xref py py-meth docutils literal"><span class="pre">__hash__()</span></tt></a> method, they can be used as set elements or as dictionary keys. <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> objects do not have methods for adding or removing elements, so all of the elements must be known when the constructor is called.</p> </dd></dl> <div class="section" id="set-objects"> <span id="id1"></span><h2>8.7.1. Set Objects<a class="headerlink" href="#set-objects" title="Permalink to this headline">¶</a></h2> <p>Instances of <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> and <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> both provide the following operations:</p> <table border="1" class="docutils"> <colgroup> <col width="41%" /> <col width="16%" /> <col width="43%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Operation</th> <th class="head">Equivalent</th> <th class="head">Result</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">len(s)</span></tt></td> <td> </td> <td>cardinality of set <em>s</em></td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">s</span></tt></td> <td> </td> <td>test <em>x</em> for membership in <em>s</em></td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">x</span> <span class="pre">not</span> <span class="pre">in</span> <span class="pre">s</span></tt></td> <td> </td> <td>test <em>x</em> for non-membership in <em>s</em></td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.issubset(t)</span></tt></td> <td><tt class="docutils literal"><span class="pre">s</span> <span class="pre"><=</span> <span class="pre">t</span></tt></td> <td>test whether every element in <em>s</em> is in <em>t</em></td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.issuperset(t)</span></tt></td> <td><tt class="docutils literal"><span class="pre">s</span> <span class="pre">>=</span> <span class="pre">t</span></tt></td> <td>test whether every element in <em>t</em> is in <em>s</em></td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.union(t)</span></tt></td> <td><tt class="docutils literal"><span class="pre">s</span> <span class="pre">|</span> <span class="pre">t</span></tt></td> <td>new set with elements from both <em>s</em> and <em>t</em></td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.intersection(t)</span></tt></td> <td><tt class="docutils literal"><span class="pre">s</span> <span class="pre">&</span> <span class="pre">t</span></tt></td> <td>new set with elements common to <em>s</em> and <em>t</em></td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.difference(t)</span></tt></td> <td><tt class="docutils literal"><span class="pre">s</span> <span class="pre">-</span> <span class="pre">t</span></tt></td> <td>new set with elements in <em>s</em> but not in <em>t</em></td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.symmetric_difference(t)</span></tt></td> <td><tt class="docutils literal"><span class="pre">s</span> <span class="pre">^</span> <span class="pre">t</span></tt></td> <td>new set with elements in either <em>s</em> or <em>t</em> but not both</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.copy()</span></tt></td> <td> </td> <td>new set with a shallow copy of <em>s</em></td> </tr> </tbody> </table> <p>Note, the non-operator versions of <tt class="xref py py-meth docutils literal"><span class="pre">union()</span></tt>, <tt class="xref py py-meth docutils literal"><span class="pre">intersection()</span></tt>, <tt class="xref py py-meth docutils literal"><span class="pre">difference()</span></tt>, and <tt class="xref py py-meth docutils literal"><span class="pre">symmetric_difference()</span></tt> 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 <tt class="docutils literal"><span class="pre">Set('abc')</span> <span class="pre">&</span> <span class="pre">'cbs'</span></tt> in favor of the more readable <tt class="docutils literal"><span class="pre">Set('abc').intersection('cbs')</span></tt>.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.3.1: </span>Formerly all arguments were required to be sets.</p> <p>In addition, both <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> and <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> 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> <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 <tt class="docutils literal"><span class="pre">False</span></tt>: <tt class="docutils literal"><span class="pre">a<b</span></tt>, <tt class="docutils literal"><span class="pre">a==b</span></tt>, or <tt class="docutils literal"><span class="pre">a>b</span></tt>. Accordingly, sets do not implement the <a class="reference internal" href="../reference/datamodel.html#object.__cmp__" title="object.__cmp__"><tt class="xref py py-meth docutils literal"><span class="pre">__cmp__()</span></tt></a> method.</p> <p>Since sets only define partial ordering (subset relationships), the output of the <tt class="xref py py-meth docutils literal"><span class="pre">list.sort()</span></tt> method is undefined for lists of sets.</p> <p>The following table lists operations available in <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> but not found in <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a>:</p> <table border="1" class="docutils"> <colgroup> <col width="30%" /> <col width="70%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Operation</th> <th class="head">Result</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">hash(s)</span></tt></td> <td>returns a hash value for <em>s</em></td> </tr> </tbody> </table> <p>The following table lists operations available in <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> but not found in <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a>:</p> <table border="1" class="docutils"> <colgroup> <col width="45%" /> <col width="15%" /> <col width="39%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Operation</th> <th class="head">Equivalent</th> <th class="head">Result</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.update(t)</span></tt></td> <td><em>s</em> |= <em>t</em></td> <td>return set <em>s</em> with elements added from <em>t</em></td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.intersection_update(t)</span></tt></td> <td><em>s</em> &= <em>t</em></td> <td>return set <em>s</em> keeping only elements also found in <em>t</em></td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.difference_update(t)</span></tt></td> <td><em>s</em> -= <em>t</em></td> <td>return set <em>s</em> after removing elements found in <em>t</em></td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.symmetric_difference_update(t)</span></tt></td> <td><em>s</em> ^= <em>t</em></td> <td>return set <em>s</em> with elements from <em>s</em> or <em>t</em> but not both</td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.add(x)</span></tt></td> <td> </td> <td>add element <em>x</em> to set <em>s</em></td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.remove(x)</span></tt></td> <td> </td> <td>remove <em>x</em> from set <em>s</em>; raises <a class="reference internal" href="exceptions.html#exceptions.KeyError" title="exceptions.KeyError"><tt class="xref py py-exc docutils literal"><span class="pre">KeyError</span></tt></a> if not present</td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.discard(x)</span></tt></td> <td> </td> <td>removes <em>x</em> from set <em>s</em> if present</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">s.pop()</span></tt></td> <td> </td> <td>remove and return an arbitrary element from <em>s</em>; raises <a class="reference internal" href="exceptions.html#exceptions.KeyError" title="exceptions.KeyError"><tt class="xref py py-exc docutils literal"><span class="pre">KeyError</span></tt></a> if empty</td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">s.clear()</span></tt></td> <td> </td> <td>remove all elements from set <em>s</em></td> </tr> </tbody> </table> <p>Note, the non-operator versions of <tt class="xref py py-meth docutils literal"><span class="pre">update()</span></tt>, <tt class="xref py py-meth docutils literal"><span class="pre">intersection_update()</span></tt>, <tt class="xref py py-meth docutils literal"><span class="pre">difference_update()</span></tt>, and <tt class="xref py py-meth docutils literal"><span class="pre">symmetric_difference_update()</span></tt> will accept any iterable as an argument.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.3.1: </span>Formerly all arguments were required to be sets.</p> <p>Also note, the module also includes a <tt class="xref py py-meth docutils literal"><span class="pre">union_update()</span></tt> method which is an alias for <tt class="xref py py-meth docutils literal"><span class="pre">update()</span></tt>. The method is included for backwards compatibility. Programmers should prefer the <tt class="xref py py-meth docutils literal"><span class="pre">update()</span></tt> method because it is supported by the built-in <a class="reference internal" href="stdtypes.html#set" title="set"><tt class="xref py py-class docutils literal"><span class="pre">set()</span></tt></a> and <a class="reference internal" href="stdtypes.html#frozenset" title="frozenset"><tt class="xref py py-class docutils literal"><span class="pre">frozenset()</span></tt></a> types.</p> </div> <div class="section" id="example"> <span id="set-example"></span><h2>8.7.2. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2> <div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">sets</span> <span class="kn">import</span> <span class="n">Set</span> <span class="gp">>>> </span><span class="n">engineers</span> <span class="o">=</span> <span class="n">Set</span><span class="p">([</span><span class="s">'John'</span><span class="p">,</span> <span class="s">'Jane'</span><span class="p">,</span> <span class="s">'Jack'</span><span class="p">,</span> <span class="s">'Janice'</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">programmers</span> <span class="o">=</span> <span class="n">Set</span><span class="p">([</span><span class="s">'Jack'</span><span class="p">,</span> <span class="s">'Sam'</span><span class="p">,</span> <span class="s">'Susan'</span><span class="p">,</span> <span class="s">'Janice'</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">managers</span> <span class="o">=</span> <span class="n">Set</span><span class="p">([</span><span class="s">'Jane'</span><span class="p">,</span> <span class="s">'Jack'</span><span class="p">,</span> <span class="s">'Susan'</span><span class="p">,</span> <span class="s">'Zack'</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">employees</span> <span class="o">=</span> <span class="n">engineers</span> <span class="o">|</span> <span class="n">programmers</span> <span class="o">|</span> <span class="n">managers</span> <span class="c"># union</span> <span class="gp">>>> </span><span class="n">engineering_management</span> <span class="o">=</span> <span class="n">engineers</span> <span class="o">&</span> <span class="n">managers</span> <span class="c"># intersection</span> <span class="gp">>>> </span><span class="n">fulltime_management</span> <span class="o">=</span> <span class="n">managers</span> <span class="o">-</span> <span class="n">engineers</span> <span class="o">-</span> <span class="n">programmers</span> <span class="c"># difference</span> <span class="gp">>>> </span><span class="n">engineers</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="s">'Marvin'</span><span class="p">)</span> <span class="c"># add element</span> <span class="gp">>>> </span><span class="k">print</span> <span class="n">engineers</span> <span class="go">Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])</span> <span class="gp">>>> </span><span class="n">employees</span><span class="o">.</span><span class="n">issuperset</span><span class="p">(</span><span class="n">engineers</span><span class="p">)</span> <span class="c"># superset test</span> <span class="go">False</span> <span class="gp">>>> </span><span class="n">employees</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">engineers</span><span class="p">)</span> <span class="c"># update from another set</span> <span class="gp">>>> </span><span class="n">employees</span><span class="o">.</span><span class="n">issuperset</span><span class="p">(</span><span class="n">engineers</span><span class="p">)</span> <span class="go">True</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">group</span> <span class="ow">in</span> <span class="p">[</span><span class="n">engineers</span><span class="p">,</span> <span class="n">programmers</span><span class="p">,</span> <span class="n">managers</span><span class="p">,</span> <span class="n">employees</span><span class="p">]:</span> <span class="gp">... </span> <span class="n">group</span><span class="o">.</span><span class="n">discard</span><span class="p">(</span><span class="s">'Susan'</span><span class="p">)</span> <span class="c"># unconditionally remove element</span> <span class="gp">... </span> <span class="k">print</span> <span class="n">group</span> <span class="gp">...</span> <span class="go">Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])</span> <span class="go">Set(['Janice', 'Jack', 'Sam'])</span> <span class="go">Set(['Jane', 'Zack', 'Jack'])</span> <span class="go">Set(['Jack', 'Sam', 'Jane', 'Marvin', 'Janice', 'John', 'Zack'])</span> </pre></div> </div> </div> <div class="section" id="protocol-for-automatic-conversion-to-immutable"> <span id="immutable-transforms"></span><h2>8.7.3. Protocol for automatic conversion to immutable<a class="headerlink" href="#protocol-for-automatic-conversion-to-immutable" title="Permalink to this headline">¶</a></h2> <p>Sets can only contain immutable elements. For convenience, mutable <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> objects are automatically copied to an <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> before being added as a set element.</p> <p>The mechanism is to always add a <a class="reference internal" href="../glossary.html#term-hashable"><em class="xref std std-term">hashable</em></a> element, or if it is not hashable, the element is checked to see if it has an <tt class="xref py py-meth docutils literal"><span class="pre">__as_immutable__()</span></tt> method which returns an immutable equivalent.</p> <p>Since <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> objects have a <tt class="xref py py-meth docutils literal"><span class="pre">__as_immutable__()</span></tt> method returning an instance of <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a>, it is possible to construct sets of sets.</p> <p>A similar mechanism is needed by the <a class="reference internal" href="../reference/datamodel.html#object.__contains__" title="object.__contains__"><tt class="xref py py-meth docutils literal"><span class="pre">__contains__()</span></tt></a> and <tt class="xref py py-meth docutils literal"><span class="pre">remove()</span></tt> methods which need to hash an element to check for membership in a set. Those methods check an element for hashability and, if not, check for a <tt class="xref py py-meth docutils literal"><span class="pre">__as_temporarily_immutable__()</span></tt> method which returns the element wrapped by a class that provides temporary methods for <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><tt class="xref py py-meth docutils literal"><span class="pre">__hash__()</span></tt></a>, <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><tt class="xref py py-meth docutils literal"><span class="pre">__eq__()</span></tt></a>, and <a class="reference internal" href="../reference/datamodel.html#object.__ne__" title="object.__ne__"><tt class="xref py py-meth docutils literal"><span class="pre">__ne__()</span></tt></a>.</p> <p>The alternate mechanism spares the need to build a separate copy of the original mutable object.</p> <p><a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> objects implement the <tt class="xref py py-meth docutils literal"><span class="pre">__as_temporarily_immutable__()</span></tt> method which returns the <a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> object wrapped by a new class <tt class="xref py py-class docutils literal"><span class="pre">_TemporarilyImmutableSet</span></tt>.</p> <p>The two mechanisms for adding hashability are normally invisible to the user; however, a conflict can arise in a multi-threaded environment where one thread is updating a set while another has temporarily wrapped it in <tt class="xref py py-class docutils literal"><span class="pre">_TemporarilyImmutableSet</span></tt>. In other words, sets of mutable sets are not thread-safe.</p> </div> <div class="section" id="comparison-to-the-built-in-set-types"> <span id="comparison-to-builtin-set"></span><h2>8.7.4. Comparison to the built-in <a class="reference internal" href="stdtypes.html#set" title="set"><tt class="xref py py-class docutils literal"><span class="pre">set</span></tt></a> types<a class="headerlink" href="#comparison-to-the-built-in-set-types" title="Permalink to this headline">¶</a></h2> <p>The built-in <a class="reference internal" href="stdtypes.html#set" title="set"><tt class="xref py py-class docutils literal"><span class="pre">set</span></tt></a> and <a class="reference internal" href="stdtypes.html#frozenset" title="frozenset"><tt class="xref py py-class docutils literal"><span class="pre">frozenset</span></tt></a> types were designed based on lessons learned from the <a class="reference internal" href="#module-sets" title="sets: Implementation of sets of unique elements. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">sets</span></tt></a> module. The key differences are:</p> <ul class="simple"> <li><a class="reference internal" href="#sets.Set" title="sets.Set"><tt class="xref py py-class docutils literal"><span class="pre">Set</span></tt></a> and <a class="reference internal" href="#sets.ImmutableSet" title="sets.ImmutableSet"><tt class="xref py py-class docutils literal"><span class="pre">ImmutableSet</span></tt></a> were renamed to <a class="reference internal" href="stdtypes.html#set" title="set"><tt class="xref py py-class docutils literal"><span class="pre">set</span></tt></a> and <a class="reference internal" href="stdtypes.html#frozenset" title="frozenset"><tt class="xref py py-class docutils literal"><span class="pre">frozenset</span></tt></a>.</li> <li>There is no equivalent to <tt class="xref py py-class docutils literal"><span class="pre">BaseSet</span></tt>. Instead, use <tt class="docutils literal"><span class="pre">isinstance(x,</span> <span class="pre">(set,</span> <span class="pre">frozenset))</span></tt>.</li> <li>The hash algorithm for the built-ins performs significantly better (fewer collisions) for most datasets.</li> <li>The built-in versions have more space efficient pickles.</li> <li>The built-in versions do not have a <tt class="xref py py-meth docutils literal"><span class="pre">union_update()</span></tt> method. Instead, use the <tt class="xref py py-meth docutils literal"><span class="pre">update()</span></tt> method which is equivalent.</li> <li>The built-in versions do not have a <tt class="docutils literal"><span class="pre">_repr(sorted=True)</span></tt> method. Instead, use the built-in <a class="reference internal" href="repr.html#module-repr" title="repr: Alternate repr() implementation with size limits."><tt class="xref py py-func docutils literal"><span class="pre">repr()</span></tt></a> and <a class="reference internal" href="functions.html#sorted" title="sorted"><tt class="xref py py-func docutils literal"><span class="pre">sorted()</span></tt></a> functions: <tt class="docutils literal"><span class="pre">repr(sorted(s))</span></tt>.</li> <li>The built-in version does not have a protocol for automatic conversion to immutable. Many found this feature to be confusing and no one in the community reported having found real uses for it.</li> </ul> </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 internal" href="#">8.7. <tt class="docutils literal"><span class="pre">sets</span></tt> — Unordered collections of unique elements</a><ul> <li><a class="reference internal" href="#set-objects">8.7.1. Set Objects</a></li> <li><a class="reference internal" href="#example">8.7.2. Example</a></li> <li><a class="reference internal" href="#protocol-for-automatic-conversion-to-immutable">8.7.3. Protocol for automatic conversion to immutable</a></li> <li><a class="reference internal" href="#comparison-to-the-built-in-set-types">8.7.4. Comparison to the built-in <tt class="docutils literal"><span class="pre">set</span></tt> types</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="array.html" title="previous chapter">8.6. <tt class="docutils literal"><span class="pre">array</span></tt> — Efficient arrays of numeric values</a></p> <h4>Next topic</h4> <p class="topless"><a href="sched.html" title="next chapter">8.8. <tt class="docutils literal"><span class="pre">sched</span></tt> — Event scheduler</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/sets.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" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </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="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="sched.html" title="8.8. sched — Event scheduler" >next</a> |</li> <li class="right" > <a href="array.html" title="8.6. array — Efficient arrays of numeric values" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="datatypes.html" >8. 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