PEP 351 – The freeze protocol

<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 351 – The freeze protocol | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0351/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 351 – The freeze protocol | peps.python.org'> <meta property="og:description" content="This PEP describes a simple protocol for requesting a frozen, immutable copy of a mutable object. 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It also defines a new built-in function which uses this protocol to provide an immutable copy on any cooperating object.</p> </section> <section id="rejection-notice"> <h2><a class="toc-backref" href="#rejection-notice" role="doc-backlink">Rejection Notice</a></h2> <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> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>Built-in objects such dictionaries and sets accept only immutable objects as keys. This means that mutable objects like lists cannot be used as keys to a dictionary. However, a Python programmer can convert a list to a tuple; the two objects are similar, but the latter is immutable, and can be used as a dictionary key.</p> <p>It is conceivable that third party objects also have similar mutable and immutable counterparts, and it would be useful to have a standard protocol for conversion of such objects.</p> <p>sets.Set objects expose a “protocol for automatic conversion to 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> deliberately dropped this feature from built-in sets. This PEP advances that the feature is still useful and proposes a standard mechanism for its support.</p> </section> <section id="proposal"> <h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2> <p>It is proposed that a new built-in function called freeze() is added.</p> <p>If freeze() is passed an immutable object, as determined by hash() on that object not raising a TypeError, then the object is returned directly.</p> <p>If freeze() is passed a mutable object (i.e. hash() of that object raises a TypeError), then freeze() will call that object’s __freeze__() method to get an immutable copy. If the object does not have a __freeze__() method, then a TypeError is raised.</p> </section> <section id="sample-implementations"> <h2><a class="toc-backref" href="#sample-implementations" role="doc-backlink">Sample implementations</a></h2> <p>Here is a Python implementation of the freeze() built-in:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>def freeze(obj): try: hash(obj) return obj except TypeError: freezer = getattr(obj, '__freeze__', None) if freezer: return freezer() raise TypeError('object is not freezable')`` </pre></div> </div> <p>Here are some code samples which show the intended semantics:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>class xset(set): def __freeze__(self): return frozenset(self) class xlist(list): def __freeze__(self): return tuple(self) class imdict(dict): def __hash__(self): return id(self) def _immutable(self, *args, **kws): raise TypeError('object is immutable') __setitem__ = _immutable __delitem__ = _immutable clear = _immutable update = _immutable setdefault = _immutable pop = _immutable popitem = _immutable class xdict(dict): def __freeze__(self): return imdict(self) >>> s = set([1, 2, 3]) >>> {s: 4} Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: set objects are unhashable >>> t = freeze(s) Traceback (most recent call last): File "<stdin>", line 1, in ? File "/usr/tmp/python-lWCjBK.py", line 9, in freeze TypeError: object is not freezable >>> t = xset(s) >>> u = freeze(t) >>> {u: 4} {frozenset([1, 2, 3]): 4} >>> x = 'hello' >>> freeze(x) is x True >>> d = xdict(a=7, b=8, c=9) >>> hash(d) Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: dict objects are unhashable >>> hash(freeze(d)) -1210776116 >>> {d: 4} Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: dict objects are unhashable >>> {freeze(d): 4} {{'a': 7, 'c': 9, 'b': 8}: 4} </pre></div> </div> </section> <section id="reference-implementation"> <h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference implementation</a></h2> <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 freeze() built-in, along with implementations of the __freeze__() method for lists and sets. Dictionaries are not easily freezable in current Python, so an implementation of dict.__freeze__() is not provided yet.</p> </section> <section id="open-issues"> <h2><a class="toc-backref" href="#open-issues" role="doc-backlink">Open issues</a></h2> <ul class="simple"> <li>Should we define a similar protocol for thawing frozen objects?</li> <li>Should dicts and sets automatically freeze their mutable keys?</li> <li>Should we support “temporary freezing” (perhaps with a method called __congeal__()) a la __as_temporarily_immutable__() in sets.Set?</li> <li>For backward compatibility with sets.Set, should we support __as_immutable__()? Or should __freeze__() just be renamed to __as_immutable__()?</li> </ul> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> <p>This document has been placed in the public domain.</p> </section> </section> <hr class="docutils" /> <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> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0351.rst">2025-02-01 08:59:27 GMT</a></p> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rejection-notice">Rejection Notice</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#sample-implementations">Sample implementations</a></li> <li><a class="reference internal" href="#reference-implementation">Reference implementation</a></li> <li><a class="reference internal" href="#open-issues">Open issues</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <br> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0351.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>

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PEP 351 – The freeze protocol | peps.python.org