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<h1 class="page-title">PEP 351 – The freeze protocol</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Barry Warsaw <barry at python.org></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Formally declined and will not be accepted">Rejected</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">14-Apr-2005</dd>
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<dt class="field-odd">Post-History<span class="colon">:</span></dt>
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<dd class="field-odd"><p></p></dd>
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#rejection-notice">Rejection Notice</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#proposal">Proposal</a></li>
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<li><a class="reference internal" href="#sample-implementations">Sample implementations</a></li>
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<li><a class="reference internal" href="#reference-implementation">Reference implementation</a></li>
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<li><a class="reference internal" href="#open-issues">Open issues</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
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<p>This PEP describes a simple protocol for requesting a frozen,
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immutable copy of a mutable object. It also defines a new built-in
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function which uses this protocol to provide an immutable copy on any
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cooperating object.</p>
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</section>
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<section id="rejection-notice">
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<h2><a class="toc-backref" href="#rejection-notice" role="doc-backlink">Rejection Notice</a></h2>
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<p>This PEP was rejected. For a rationale, see <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2006-February/060793.html">this thread on python-dev</a>.</p>
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</section>
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<section id="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<p>Built-in objects such dictionaries and sets accept only immutable
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objects as keys. This means that mutable objects like lists cannot be
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used as keys to a dictionary. However, a Python programmer can
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convert a list to a tuple; the two objects are similar, but the latter
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is immutable, and can be used as a dictionary key.</p>
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<p>It is conceivable that third party objects also have similar mutable
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and immutable counterparts, and it would be useful to have a standard
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protocol for conversion of such objects.</p>
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<p>sets.Set objects expose a “protocol for automatic conversion to
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immutable” so that you can create sets.Sets of sets.Sets. <a class="pep reference internal" href="../pep-0218/" title="PEP 218 – Adding a Built-In Set Object Type">PEP 218</a>
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deliberately dropped this feature from built-in sets. This PEP
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advances that the feature is still useful and proposes a standard
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mechanism for its support.</p>
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</section>
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<section id="proposal">
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<h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2>
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<p>It is proposed that a new built-in function called freeze() is added.</p>
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<p>If freeze() is passed an immutable object, as determined by hash() on
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that object not raising a TypeError, then the object is returned
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directly.</p>
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<p>If freeze() is passed a mutable object (i.e. hash() of that object
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raises a TypeError), then freeze() will call that object’s
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__freeze__() method to get an immutable copy. If the object does not
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have a __freeze__() method, then a TypeError is raised.</p>
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</section>
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<section id="sample-implementations">
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<h2><a class="toc-backref" href="#sample-implementations" role="doc-backlink">Sample implementations</a></h2>
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<p>Here is a Python implementation of the freeze() built-in:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>def freeze(obj):
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try:
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hash(obj)
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return obj
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except TypeError:
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freezer = getattr(obj, '__freeze__', None)
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if freezer:
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return freezer()
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raise TypeError('object is not freezable')``
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</pre></div>
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</div>
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<p>Here are some code samples which show the intended semantics:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>class xset(set):
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def __freeze__(self):
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return frozenset(self)
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class xlist(list):
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def __freeze__(self):
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return tuple(self)
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class imdict(dict):
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def __hash__(self):
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return id(self)
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def _immutable(self, *args, **kws):
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raise TypeError('object is immutable')
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__setitem__ = _immutable
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__delitem__ = _immutable
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clear = _immutable
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update = _immutable
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setdefault = _immutable
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pop = _immutable
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popitem = _immutable
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class xdict(dict):
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def __freeze__(self):
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return imdict(self)
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>>> s = set([1, 2, 3])
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>>> {s: 4}
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Traceback (most recent call last):
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File "<stdin>", line 1, in ?
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TypeError: set objects are unhashable
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>>> t = freeze(s)
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Traceback (most recent call last):
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File "<stdin>", line 1, in ?
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File "/usr/tmp/python-lWCjBK.py", line 9, in freeze
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TypeError: object is not freezable
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>>> t = xset(s)
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>>> u = freeze(t)
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>>> {u: 4}
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{frozenset([1, 2, 3]): 4}
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>>> x = 'hello'
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>>> freeze(x) is x
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True
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>>> d = xdict(a=7, b=8, c=9)
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>>> hash(d)
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Traceback (most recent call last):
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File "<stdin>", line 1, in ?
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TypeError: dict objects are unhashable
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>>> hash(freeze(d))
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-1210776116
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>>> {d: 4}
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Traceback (most recent call last):
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||
File "<stdin>", line 1, in ?
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TypeError: dict objects are unhashable
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>>> {freeze(d): 4}
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{{'a': 7, 'c': 9, 'b': 8}: 4}
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</pre></div>
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</div>
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</section>
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<section id="reference-implementation">
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<h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference implementation</a></h2>
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<p>Patch <a class="reference external" href="http://sourceforge.net/tracker/index.php?func=detail&aid=1335812&group_id=5470&atid=305470">1335812</a> provides the C implementation of this feature. It adds the
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freeze() built-in, along with implementations of the __freeze__()
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method for lists and sets. Dictionaries are not easily freezable in
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current Python, so an implementation of dict.__freeze__() is not
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provided yet.</p>
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</section>
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<section id="open-issues">
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<h2><a class="toc-backref" href="#open-issues" role="doc-backlink">Open issues</a></h2>
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<ul class="simple">
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<li>Should we define a similar protocol for thawing frozen objects?</li>
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<li>Should dicts and sets automatically freeze their mutable keys?</li>
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<li>Should we support “temporary freezing” (perhaps with a method called
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__congeal__()) a la __as_temporarily_immutable__() in sets.Set?</li>
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<li>For backward compatibility with sets.Set, should we support
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__as_immutable__()? Or should __freeze__() just be renamed to
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__as_immutable__()?</li>
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</ul>
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</section>
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<section id="copyright">
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<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
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<p>This document has been placed in the public domain.</p>
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</section>
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</section>
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<hr class="docutils" />
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<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0351.rst">https://github.com/python/peps/blob/main/peps/pep-0351.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0351.rst">2023-09-09 17:39:29 GMT</a></p>
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<h2>Contents</h2>
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<ul>
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#rejection-notice">Rejection Notice</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#proposal">Proposal</a></li>
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<li><a class="reference internal" href="#sample-implementations">Sample implementations</a></li>
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<li><a class="reference internal" href="#reference-implementation">Reference implementation</a></li>
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<li><a class="reference internal" href="#open-issues">Open issues</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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</ul>
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