False negative when lvalue has union type
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affects-mypyc
bug
topic-union-types
- Dominant language
- Python
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Description
Mypy generates no error, even though the code is unsafe:
class C[T]:
a: T
def f(x: C[int] | C[str]) -> None:
x.a = 1 # No error
c = C[str]()
f(c)
print(c.a, type(c.a)) # 1, int
I'd expect x.a = 1 to generate an error, since the assignment is unsafe.
I suspect that the lvalue type is inferred as int | str, and mypy thinks it's fine. instead, assignment should be valid for each union item separately.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the provided generic-union example with mypy and inspect the type-checking path for assignments to union-typed lvalues. Done means the assignment is rejected when it is unsafe for any union member, while valid assignments continue to pass.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers, devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100