Improve error messages when positional arguments are missing #20591
+294
−12
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This comment has been minimized.
This comment has been minimized.
JukkaL
reviewed
Jan 16, 2026
test-data/unit/check-functions.test
Outdated
| f("wrong", 123) | ||
| [builtins fixtures/primitives.pyi] | ||
| [out] | ||
| main:3: error: Incompatible arguments for "f"; check for missing arguments |
Collaborator
There was a problem hiding this comment.
I think in this case we can fall back to the pre-existing way of generating messages. It could be useful to show which types are expected vs actual, etc.
test-data/unit/check-functions.test
Outdated
| f(1, b'x', 1) | ||
| [builtins fixtures/primitives.pyi] | ||
| [out] | ||
| main:3: error: Expected "str" for parameter "y"; did you forget argument "y"? |
Collaborator
There was a problem hiding this comment.
This seems inconsistent with other existing messages about argument type mismatches. Can you check these for non-overloads and see if these could be more consistent?
Contributor
|
According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Summary
This PR improves error messages when positional arguments are missing from a function call.
Problem
When a user forgets a positional argument, mypy would previously emit multiple confusing type errors because the subsequent arguments would be "shifted" and mismatched with their expected types.
Solution
Detect patterns where a forgotten positional argument causes subsequent arguments to be misaligned with their expected parameters. Instead of showing multiple type errors, emit a single consolidated message that suggests which argument might be missing.