fromtypingimportLiteraldeff1(a:str)->Literal[""]:returnaandexit()# E: Incompatible return value type (got "str", expected "Literal['']")deff2(a:int)->Literal[0]:returnaandexit()# E: Incompatible return value type (got "int", expected "Literal[0]")deff3(a:bytes)->Literal[b""]:returnaandexit()# E: Incompatible return value type (got "bytes", expected "Literal[b'']")

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brianschubertand others added2 commits

September 24, 2024 16:07

Narrow falsey str/bytes/int to literal type

e4e4fae

[pre-commit.ci] auto fixes from pre-commit.com hooks

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brianschubert commentedSep 24, 2024

This is consistent with how pyright applies narrowing:Pyright playground

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Update tests affected by falsey narrowing to literal types

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TeamSpen210 commentedSep 24, 2024

Looked through the primer results:

urllib3 andhomeassistant: False positive, caused by accessing an attribute twice, but the attribute got modified in-between by other code. Kind of a general issue with narrowing, and having@property with side-effects.
packaging: True positive, detects the redundant check inif a followed byif not a and b.
prefect: Fixes an existing true positive, code treats"" differently to other strings and mypy is now able to understand how strings get narrowed out.
kornia: Same asprefect but with0 instead.

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brianschubert commentedSep 24, 2024

Thanks for the analysis! I agree with your assessment.

Re: the false positives fromurllib3 andhomeassistant, this behavior is at least consistent with pyright, which also assumes that the type of a property doesn't change between evaluations:Pyright playground

hauntsaninja added the upnext label

Sep 24, 2024

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github-actionsbot commentedSep 24, 2024

Diff frommypy_primer, showing the effect of this PR on open source code:

urllib3 (https://github.com/urllib3/urllib3)+ test/test_response.py:179: error: Non-overlapping equality check (left operand type: "Literal[b'', ''] | None", right operand type: "Literal[b'foo']")  [comparison-overlap]packaging (https://github.com/pypa/packaging)+ src/packaging/version.py:496: error: Left operand of "and" is always true  [redundant-expr]core (https://github.com/home-assistant/core)+ homeassistant/components/vlc_telnet/media_player.py:181: error: Right operand of "and" is never evaluated  [unreachable]+ homeassistant/components/vlc_telnet/media_player.py:184: error: Statement is unreachable  [unreachable]prefect (https://github.com/PrefectHQ/prefect)- src/prefect/_internal/schemas/validators.py:820: error: Incompatible return value type (got "str", expected "Path")  [return-value]+ src/prefect/_internal/schemas/validators.py:820: error: Incompatible return value type (got "Literal['']", expected "Path")  [return-value]- src/prefect/deployments/runner.py:912: error: Item "str" of "str | DockerImage" has no attribute "reference"  [union-attr]- src/prefect/deployments/runner.py:917: error: Item "str" of "str | DockerImage" has no attribute "build"  [union-attr]- src/prefect/deployments/runner.py:921: error: Item "str" of "str | DockerImage" has no attribute "reference"  [union-attr]- src/prefect/deployments/runner.py:933: error: Item "str" of "str | DockerImage" has no attribute "push"  [union-attr]- src/prefect/deployments/runner.py:937: error: Item "str" of "str | DockerImage" has no attribute "reference"  [union-attr]- src/prefect/deployments/runner.py:941: error: Item "str" of "str | DockerImage" has no attribute "reference"  [union-attr]kornia (https://github.com/kornia/kornia)+ kornia/contrib/extract_patches.py:390: error: Unused "type: ignore" comment  [unused-ignore]+ kornia/contrib/extract_patches.py:395: error: Unused "type: ignore" comment  [unused-ignore]