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Fix narrowing with chained comparison by hauntsaninja · Pull Request #21150 · python/mypy · GitHub

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Fix narrowing with chained comparison #21150

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hauntsaninja merged 2 commits into from
Apr 3, 2026
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Fix narrowing with chained comparison #21150

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2b1f15c
Fix narrowing with chained comparison
hauntsaninja Apr 2, 2026
36e50c9
more tests
hauntsaninja Apr 3, 2026
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4 changes: 2 additions & 2 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6674,8 +6674,8 @@ def comparison_type_narrowing_helper(self, node: ComparisonExpr) -> tuple[TypeMa
# If we have found non-trivial restrictions from the regular comparisons,
# then return soon. Otherwise try to infer restrictions involving `len(x)`.
# TODO: support regular and len() narrowing in the same chain.
if any(m != ({}, {}) for m in partial_type_maps):
return reduce_conditional_maps(partial_type_maps)
if any(len(m[0]) or len(m[1]) for m in partial_type_maps):

ilevkivskyi Apr 3, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

TBH I don't understand how is this different from what it was before. Is this just some drive-by performance micro-optimization?

hauntsaninja Apr 3, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Yeah, just a micro optimisation
return reduce_conditional_maps(partial_type_maps, use_meet=True)

ilevkivskyi Apr 3, 2026

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I am in favour of this, but just as a warning, this may have some subtle side-effect w.r.t. Any, so you may want to add some tests involving Any as well.

hauntsaninja Apr 3, 2026

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Added some tests, but couldn't find one with a behaviour change. If there is one, I think it should result in fewer Any's, and we don't see anything on primer, so that's good
else:
# Use meet for `and` maps to get correct results for chained checks
# like `if 1 < len(x) < 4: ...`
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33 changes: 33 additions & 0 deletions test-data/unit/check-narrowing.test
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Expand Up @@ -3264,6 +3264,39 @@ def bad_but_should_pass(has_key: bool, key: bool, s: tuple[bool, ...]) -> None:
reveal_type(key) # N: Revealed type is "builtins.bool"
[builtins fixtures/primitives.pyi]

[case testNarrowChainedComparisonMeet]
# flags: --strict-equality --warn-unreachable
from __future__ import annotations
from typing import Any

def f1(a: str | None, b: str | None) -> None:
if None is not a == b:
reveal_type(a) # N: Revealed type is "builtins.str"
reveal_type(b) # N: Revealed type is "builtins.str | None"

hauntsaninja Apr 3, 2026

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This should also be str. I will fix this in a subsequent PR (this one is not a regression, it will need net new code)

if (None is not a) and (a == b):
reveal_type(a) # N: Revealed type is "builtins.str"
reveal_type(b) # N: Revealed type is "builtins.str"

def f2(a: Any | None, b: str | None) -> None:
if None is not a == b:
reveal_type(a) # N: Revealed type is "Any"
reveal_type(b) # N: Revealed type is "builtins.str | None"

if (None is not a) and (a == b):
reveal_type(a) # N: Revealed type is "Any"
reveal_type(b) # N: Revealed type is "builtins.str | None"

def f3(a: str | None, b: Any | None) -> None:
if None is not a == b:
reveal_type(a) # N: Revealed type is "builtins.str"
reveal_type(b) # N: Revealed type is "Any | builtins.str | None"

if (None is not a) and (a == b):
reveal_type(a) # N: Revealed type is "builtins.str"
reveal_type(b) # N: Revealed type is "Any | builtins.str"
[builtins fixtures/primitives.pyi]

[case testNarrowTypeObject]
# flags: --strict-equality --warn-unreachable
from typing import Any
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