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Starting in mypy 1.5.0 (at least when testing in versions available in playground), there are situations where an isinstance type guard that returns False will cause type checking to stop in the false branch. The examples below check correctly in 1.4.1 in the playground.
fromtypingimportOptionalclassFoo:
x: Optional[str]
assertisinstance(Foo.x, int)
Foo.x.y.z=1# expected error here
These examples are lame, but there are legitimate use cases for this to work. For example, we use metaclasses to transform these type annotations on fields into class attributes that have underlying data. So a more real example in my use case (that cant be reproduced in a playground), would be:
classFoo(Model):
x: Optional[int] =field(max_length=1)
assertisinstance(Foo.x, StringField) # under the hood, we transform `Foo.x` into a `StringField` because of the annotationassertFoo.x.max_lengt==1# expectd failures since max_lengt is a typo
Note that this behavior is mostly the same with if isinstance...: (and then the rest of the code within the block fails to check), though it seemed that some of those examples were also broken in 1.4.1, which is different than when using assert.
Expected Behavior
I would expect mypy to correctly type check even if it thinks an assert isinstance or if isinstance would result in the rest of the code being unreachable.
Note that I do not want to use warn-unreachable here because that would not actually solve the underlying problem, it would just give me extra warnings and still not actually let me write the isinstance checks that I want.
Actual Behavior
Mypy appears to give up type checking entirely - and silently - if it thinks that the rest of the code is unreachable.
Your Environment
My environment for all the examples is mypy playground
The text was updated successfully, but these errors were encountered:
Bug Report
Starting in mypy 1.5.0 (at least when testing in versions available in playground), there are situations where an
isinstance
type guard that returnsFalse
will cause type checking to stop in the false branch. The examples below check correctly in 1.4.1 in the playground.To Reproduce
Heres a pretty dumb example
Gist
Playground
Another example:
These examples are lame, but there are legitimate use cases for this to work. For example, we use metaclasses to transform these type annotations on fields into class attributes that have underlying data. So a more real example in my use case (that cant be reproduced in a playground), would be:
Note that this behavior is mostly the same with
if isinstance...:
(and then the rest of the code within the block fails to check), though it seemed that some of those examples were also broken in 1.4.1, which is different than when usingassert
.Expected Behavior
I would expect mypy to correctly type check even if it thinks an
assert isinstance
orif isinstance
would result in the rest of the code being unreachable.Note that I do not want to use
warn-unreachable
here because that would not actually solve the underlying problem, it would just give me extra warnings and still not actually let me write theisinstance
checks that I want.Actual Behavior
Mypy appears to give up type checking entirely - and silently - if it thinks that the rest of the code is unreachable.
Your Environment
My environment for all the examples is mypy playground
The text was updated successfully, but these errors were encountered: