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