Title: Improve error messages for assignment
Type: enhancement Stage: resolved
Components: Interpreter Core Versions: Python 3.8
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: Nosy List: benjamin.peterson, brett.cannon, serhiy.storchaka, yselivanov
Priority: normal Keywords: patch

Created on 2018-11-05 19:17 by serhiy.storchaka, last changed 2018-11-20 18:07 by serhiy.storchaka. This issue is now closed.

Pull Requests
URL Status Linked Edit
PR 10342 merged serhiy.storchaka, 2018-11-05 19:21
Messages (3)
msg329313 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2018-11-05 19:17
This is a follow up of issue34641.

>>> 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.
msg330056 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2018-11-18 17:30
Benjamin, could you please take a look at this?
msg330138 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2018-11-20 17:27
New changeset 97f1efb6062188645a470daaa91e3669d739c75f by Serhiy Storchaka in branch 'master':
bpo-35169: Improve error messages for forbidden assignments. (GH-10342)
Date User Action Args
2018-11-20 18:07:05serhiy.storchakasetstatus: open -> closed
resolution: fixed
stage: patch review -> resolved
2018-11-20 17:27:19serhiy.storchakasetmessages: + msg330138
2018-11-18 17:30:57serhiy.storchakasetmessages: + msg330056
2018-11-06 18:42:14gvanrossumsetnosy: - gvanrossum
2018-11-05 19:21:18serhiy.storchakasetkeywords: + patch
stage: patch review
pull_requests: + pull_request9645
2018-11-05 19:17:32serhiy.storchakacreate