[Python-Dev] dict __contains__ raises TypeError on unhashable input

[Benjamin Peterson]

2013/7/19 Ethan Furman <ethan at stoneleaf.us>:
> While working on issue #18508 I stumbled across this:
>
> Traceback (most recent call last):
> ...
>   File "/usr/local/lib/python3.4/enum.py", line 417, in __new__
>     if value in cls._value2member_map:
> TypeError: unhashable type: 'list'
>
> I'll wrap it in a try-except block, but I must admit I was surprised the
> answer wasn't False.  After all, if the input is unhashable then obviously
> it's not in the dict; furthermore, if I were to compare the number 5 with a
> set() I would get False, not a TypeMismatch error, and dict keys are
> basically done by equality, the hash is just (?) a speed-up.

I'm not exactly sure what the last part of that sentence means.

Anyway, it's well established that operations on a key in a dict are
going to involve looking up the key, and thus hashing it. You wouldn't
expect, {}.get(unhashable, None) not to raise, right?



--
Regards,
Benjamin