[Python-Dev] dict contains raises TypeError on unhashable input (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sat Jul 20 12:21:41 CEST 2013

On Fri, 19 Jul 2013 16:47:36 -0700 Ethan Furman <ethan at stoneleaf.us> wrote:

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.value2membermap: 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.

Testing containment of an unhashable key in a dict is a programming error, so I'd prefer that we keep the TypeError here.

Same reasoning as for:

5 in "abc" Traceback (most recent call last): File "", line 1, in TypeError: 'in ' requires string as left operand, not int

Sure, 5 technically isn't in "abc" and therefore the "in" operator could return False, but it's better to highlight the programming error.

Regards

Antoine.

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