This is a follow up of bpo-34641.

>>> f(lambda x: x = 1)
  File "<stdin>", line 1
SyntaxError: lambda cannot contain assignment
>>> f(x.y = 1)
  File "<stdin>", line 1
SyntaxError: keyword can't be an expression

The error message "keyword can't be an expression" still looks confusing to me. This is because the term "keyword" is ambiguous. Usually it means reserved identifier like "if" or "def". Some keywords, like "None" and "True" can be expressions. Perhaps "keyword name can't be an expression" would be better.

But I think that in these cases it is most likely that "=" was used instead of "==". And it would be better to generalize the error message for lambdas and point on a possible typo.

>>> f(x.y = 1)
  File "<stdin>", line 1
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?

The proposed PR changes this error message. It makes also an error message for forbidden assignment more concrete: "cannot assign to __debug__" instead of "assignment to keyword" (the latter is actually incorrect, because __debug__ is not true keyword in tokenizer). This restores Python 2 error messages. Improved also other error messages for forbidden assigning: dict and set displays, f-string expressions are not literals.

Benjamin, could you please take a look at this?

New changeset 97f1efb by Serhiy Storchaka in branch 'master':
bpo-35169: Improve error messages for forbidden assignments. (GH-10342)
97f1efb