PEP 351 – The freeze protocol
- Author:
- Barry Warsaw <barry at python.org>
- Status:
- Rejected
- Type:
- Standards Track
- Created:
- 14-Apr-2005
- Python-Version:
- 2.5
- Post-History:
Abstract
This PEP describes a simple protocol for requesting a frozen, immutable copy of a mutable object. It also defines a new built-in function which uses this protocol to provide an immutable copy on any cooperating object.
Rejection Notice
This PEP was rejected. For a rationale, see this thread on python-dev.
Rationale
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.
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.
sets.Set objects expose a “protocol for automatic conversion to
immutable” so that you can create sets.Set’s of sets.Set’s.  PEP 218
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.
Proposal
It is proposed that a new built-in function called freeze() is added.
If freeze() is passed an immutable object, as determined by hash() on
that object not raising a TypeError, then the object is returned
directly.
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.
Sample implementations
Here is a Python implementation of the freeze() built-in:
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')
Here are some code samples which show the intended semantics:
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}
Reference implementation
Patch 1335812 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.
Open issues
- Should we define a similar protocol for thawing frozen objects?
- Should dicts and sets automatically freeze their mutable keys?
- Should we support “temporary freezing” (perhaps with a method called
__congeal__()) a la__as_temporarily_immutable__()insets.Set?
- For backward compatibility with sets.Set, should we support__as_immutable__()? Or should__freeze__()just be renamed to__as_immutable__()?
Copyright
This document has been placed in the public domain.
Source: https://github.com/python/peps/blob/main/peps/pep-0351.rst
Last modified: 2025-09-16 00:19:46 GMT