Narrow falsey str/bytes/int to literal type

[brianschubert]

Closes #16891

Before

from typing import Literal

def f1(a: str) -> Literal[""]:
    return a and exit()  # E: Incompatible return value type (got "str", expected "Literal['']")

def f2(a: int) -> Literal[0]:
    return a and exit()  # E: Incompatible return value type (got "int", expected "Literal[0]")

def f3(a: bytes) -> Literal[b""]:
    return a and exit()  # E: Incompatible return value type (got "bytes", expected "Literal[b'']")

After

Success: no issues found in 1 source file

[brianschubert]

This is consistent with how pyright applies narrowing: Pyright playground

[TeamSpen210]

Looked through the primer results:

• urllib3 and homeassistant: 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 in if a followed by if 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 as prefect but with 0 instead.

[brianschubert]

Thanks for the analysis! I agree with your assessment.

Re: the false positives from urllib3 and homeassistant, this behavior is at least consistent with pyright, which also assumes that the type of a property doesn't change between evaluations: Pyright playground

[github-actions]

Diff from mypy_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]

[hauntsaninja]

Nice, thank you!