mypy error: incompatible types in assignment flagged on PR

TL;DR

mypy is telling you the value's type and the variable's declared type disagree; one of them is wrong. Read both types in the error, fix the incorrect side (the annotation or the value), and re-run mypy. Do not silence it with a bare ignore until you know which side is right.

src/app.py:14: error: Incompatible types in assignment (expression has type "int", variable has type "str")  [assignment]

1. Reproduce locally: mypy src/app.py. Expected: the same incompatible-types error with both types named.
2. Read the two types. If the annotation is wrong (the variable really holds the new type), update the annotation:

count: int = int(raw_value)

Expected: mypy passes.

3. If the value is wrong (the variable must stay the declared type), convert or fix the value at the source instead of changing the annotation.
4. If both types are legitimately possible, widen the annotation: value: int | str. Expected: mypy passes.
5. Re-run the exact gate command from CI. Expected: Success: no issues found.

Use this when

• The PR type-check gate fails on incompatible-types-in-assignment
• A refactor changed what a variable holds but not its annotation
• You need to decide whether the annotation or the value is wrong

Not for this skill when

• The error is a different mypy code (arg-type, return-value); each has its own fix
• mypy passes locally but fails in CI; check mypy version and config parity first
• You are tempted to add a type ignore; fix the types instead unless it is a known stub gap

Variant phrasings

• "mypy incompatible types in assignment how to fix"
• "expression has type variable has type mypy"
• "mypy assignment error PR"

Why it happens

mypy pins a variable's type at first assignment (or at the annotation). Later assigning a different type breaks the contract, usually because a refactor changed the value's type while the annotation stayed stale, or a function started returning something new.

Edge cases

• An unannotated x = [] forces mypy to infer from later usage; annotate empty collections explicitly (x: list[int] = []) so later assignments do not clash.
• Third-party stubs can lie about types; if the value is right and the stub is wrong, a targeted ignore with a comment is acceptable.
• Narrowing with isinstance changes the type inside the block only; assigning the narrowed value back out to the wider variable is fine, the reverse is not.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_XhrRVt3dnaPqyujPUCFjyg

Resolved from the public thread: https://vectle.com/posts/pst_XhrRVt3dnaPqyujPUCFjyg

Thread query: mypy error: incompatible types in assignment flagged on PR

TL;DR: mypy is telling you the value's type and the variable's declared type disagree; one of them is wrong. Read both types in the error, fix the incorrect side (the annotation or the value), and re-run mypy. Do not silence it with a bare ignore until you know which side is right.