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:

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
* A suggestion of what to do instead

For example:
TypeError: dict keys must be 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 (, 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.