The error message for using a mutable sequence as a set item or map key
seems to frequently confuse Python beginners. The current wording is:
>>> {[1, 2, 3]: [4, 5, 6]}
TypeError: unhashable type: 'list'
The first thing a Google search finds for "unhashable type" is ~4k Stack
Overflow results like:
https://stackoverflow.com/questions/13264511/typeerror-unhashable-type-dict
The message does not include:
* The type on which the operation is performed
* Which input to the operation is the problem
* The word "hashable" verbatim, which appears in the glossary
* A link to https://docs.python.org/3/glossary.html#term-hashable
* A suggestion of what to do instead
For example:
TypeError: dict keys must be hashable (
https://docs.python.org/glossary.html#term-hashable), 'list' is not.
Consider using a primitive type (e.g. int, str) or immutable sequence (e.g.
tuple, frozenset).
(That could be too much stuff, my point is I think there's room for
improvement over "unhashable type: 'list'".)
I filed a bug about this (https://bugs.python.org/issue41114), but it was
closed by a core contributor, who suggested discussing that on this list
instead. While it's true that you can Google the error message and find the
relevant information, I think the error message text would be worth
improving.
Peace,
-Sam
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