"unhashable type" is a beginner-unfriendly error message

[cfbolz]

Feature or enhancement

Proposal:

Every time I'm teaching dicts and sets to beginners, someone tries to use lists or other mutable data structures as dict keys or set members. The error message in that case is unhelpful, because it describes the problem in very technical terms (when you're just starting you have no idea what a hash function is). it could be changed to also add what operation is impossible as a result:

>>> s = set()
>>> s.add({'pages': 12, 'grade': 'A'})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Cannot use 'dict' as a set element (unhashable type).
>>> d = {}
>>> l = [1, 2, 3]
>>> d[l] = 12
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Cannot use 'list' as a dict key (unhashable type).

That way, the problem is stated much more in terms of what the programmer is actually trying to do.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-132825: Enhance unhashable error messages for dict and set #132828
• gh-132825: Fix typo in dict_unhashable_type() name #132847

[vstinner]

I like your proposal. I wrote #132828 to implement your suggested error messages.

Examples:

>>> s=set(); s.add({})
TypeError: Cannot use 'dict' as a set element (unhashable type: 'dict')

>>> d = {}; l = [1, 2, 3]; d[l] = 12
TypeError: Cannot use 'list' as a dict key (unhashable type: 'list')

@StanFromIreland:

I don't see any way to improve the message.

@cfbolz proposes the change the error message. Currently, the error message is not helpful:

>>> s=set(); s.add({})
TypeError: unhashable type: 'dict'

>>> d = {}; l = [1, 2, 3]; d[l] = 12
TypeError: unhashable type: 'list'

[methane]

TypeError: Cannot use 'list' as a dict key (unhashable type: 'list')

Maybe, exception message should starts with lowercase?
It seems other exceptions start with "cannot...".

[cfbolz]

@vstinner thanks a lot Victor, this makes me super happy!

[UltimateLobster]

What about cases where the key is nested: {(1, 2, []): ""}?

The problem is not the tuple type, but the list type within. The error message would be false if it said that tuple cannot be used as a dict key. It would also be misleading if it said that list cannot be used a dict key, since the user did not use the list directly as a key.

[vstinner]

What about cases where the key is nested: {(1, 2, []): ""}?

You get this error message:

>>> {(1, 2, []): ""}
TypeError: cannot use 'tuple' as a dict key (unhashable type: 'list')

[merwok]

@hugovk This one could be added to What’s new section about improved error messages

[hugovk]

PR welcome :)

[merwok]

OK #133622