PEP: | 351 |
---|---|
Title: | The freeze protocol |
Version: | 2.5 |
Last-Modified: | 2007-06-28 19:53:41 +0000 (Thu, 28 Jun 2007) |
Author: | Barry Warsaw <barry at python.org> |
Status: | Rejected |
Type: | Standards Track |
Content-Type: | text/x-rst |
Created: | 14-Apr-2005 |
Post-History: |
Contents
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 [1].
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.Sets of sets.Sets. 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 [2] 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__() in sets.Set?
- For backward compatibility with sets.Set, should we support __as_immutable__()? Or should __freeze__() just be renamed to __as_immutable__()?
References
[1] | http://mail.python.org/pipermail/python-dev/2006-February/060793.html |
[2] | http://sourceforge.net/tracker/index.php?func=detail&aid=1335812&group_id=5470&atid=305470 |
Copyright
This document has been placed in the public domain.