`{}.pop([])` raises `KeyError` not `TypeError` - only on empty dict

[samuelcolvin]

Bug report

Bug description:

{}.pop([])

Raises a KeyError, not a TypeError as I would expect.

However

{1: 2}.pop([])

Raises TypeError: cannot use 'list' as a dict key (unhashable type: 'list') as you'd expected.

This isn't new, it's been around since at least 3.8. Is it really intended? If so, it should be documented.

CPython versions tested on:

3.14

Operating systems tested on:

macOS

[rhettinger]

This isn't a bug. dict.pop(m, k) is checking the arguments in the order provided, the dict first and then the key. It is perfectly reasonable to do zero processing on the key if the mapping is empty and it is known that the pop cannot succeed.

[samuelcolvin]

Okay that makes sense, but it is somewhat surprising behavior, I think it should at the very least be documented on the pop() method.

[serhiy-storchaka]

It makes sense, calculating the key hash is just a waste for empty dict.

But it is cheap to check tp_hash (without calling it) before raising an exception in C, so TypeError can be raised for some non-hashable types. Although this can make life harder for alternative implementations.

[johnslavik]

Shall we document this as an "implementation detail"?