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popitem from UserDict in LIFO order #147957
Description
Feature or enhancement
Proposal:
collections.UserDict.popitem currently uses the collections.abc.MutableMapping default implementation, which pops items in the order of iter(self.data) and is thus FIFO:
from collections import UserDict
d = UserDict()
d['spam'] = 0
d['ham'] = 1
assert(d.popitem() == ('spam', 0))This does not match dict.popitem, which has been guaranteed to pop LIFO since Python 3.7:
d = dict()
d['spam'] = 0
d['ham'] = 1
assert(d.popitem() == ('ham', 1))Given that UserDict is meant to "[simulate] a dictionary", it seems preferable for UserDict instances to pop LIFO as well. (This has already been the subject of confusion, per a recent Stack Overflow question.) This change could be a CPython implementation detail, but it could (in the long run) be a spec guarantee as well.
The spec has not historically guaranteed an order for UserDict.popitem, nor for MutableMapping.popitem (nor specified that UserDict is a subclass of MutableMapping), but this change might break programs that relied on the FIFO behavior anyways. That said, the whole point of UserDict is to emulate dict for subclassing purposes, so I think the consistency still outweighs the risk.
Has this already been discussed elsewhere?
No response given
Links to previous discussion of this feature:
No response
Activity
pythongh-147957: pop items from UserDict in LIFO order
rhettinger commentedon Apr 1, 2026
Neither UserDict, nor MutableMapping makes an ordering guarantee:
Andrew5057 commentedon Apr 1, 2026
@rhettinger The "some" note is based on the
MutableMappingimplementation.UserDictjust doesn't have an implementation at all. However,dictmakes the ordering guarantee; those usingUserDictas an alternative to subclassingdictwill likely expect that behavior.Andrew5057 commentedon Apr 1, 2026
Is this a duplicate of gh-88913?
serhiy-storchaka commentedon Apr 1, 2026
This is an interesting question. UserDict inherits implementation from MutableMapping, but implements some methods by delegating to the underlying dict (because they are abstract in MutableMapping or for performance). What other methods are inherited from MutableMapping and not delegated to dict? What other methods are worth to delegate? The same questions for UserList and UserString.
rhettinger commentedon Apr 3, 2026
@Andrew5057 The "some" note was mentioned not as a restriction, but as freedom to proceed with your change.
Since the underlying dict is assignable, it can be anything. So, the docstring for the new
popitemshould be worded to say that it accesses entries in the same order as the underlying dict. In the case of a real dict (the normal case), it would be LIFO.pythongh-147957: pop items from UserDict in LIFO order
gh-147957: pop items from UserDict in LIFO order (gh-147958)
pythongh-147957: pop items from UserDict in LIFO order (pythongh-147958)