UserDict.popitem is not LIFO

Question

Consider the following snippet:

from collections import UserDict

class D(dict): ...

d = D(foo="bar", baz=42)
print(d.popitem())  # ('baz', 42)

class UD(UserDict): ...

ud = UD(foo="bar", baz=42)
print(ud.data)  # {'foo': 'bar', 'baz': 42}
print(ud.popitem())  # ('foo', 'bar')

The order seems to be preserved in the UserDict implementation as it should since it internally uses a data dict which is ordered since 3.7.

Why is the popped item the first one and not the last one? Am I misunderstanding something? Is this a bug or a feature?

Tested on 3.13.9 and 3.14.0 in fresh environments.

This actually seems like a bug to me. I'd report it on CPython github.. — wim
– wim, Commented Apr 1 at 3:34
For the record, there seems to be an easy workaround if you want LIFO: you can use ud.data.popitem() instead of ud.popitem(). — wim
– wim, Commented Apr 1 at 3:40

Anerdw · Accepted Answer · 2026-05-05 16:01:41Z

4

UserDict wraps a dictionary, but it doesn't subclass dict or even collections.OrderedDict. Instead, it subclasses collections.abc.MutableMapping. As such, UserDict.popitem doesn't have the LIFO guarantees that dict.popitem does. In 3.14 and earlier, it instead uses MutableMapping.popitem, which is implemented using dict.__iter__ (and is thus FIFO, except for value mutations):

def popitem(self):
    '''D.popitem() -> (k, v), remove and return some (key, value) pair
       as a 2-tuple; but raise KeyError if D is empty.
    '''
    try:
        key = next(iter(self))
    except StopIteration:
        raise KeyError from None
    value = self[key]
    del self[key]
    return key, value

In 3.15, UserDict.popitem will have a new implementation that will guarantee the same removal order as the underyling dictionary.

edited May 5 at 16:01

answered Apr 1 at 2:14

Anerdw

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