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Avoid false possibly-undefined errors due to omitted unrequired else statements. #20149

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Merged
hauntsaninja merged 7 commits into from
Jan 18, 2026
Merged

Avoid false possibly-undefined errors due to omitted unrequired else statements. #20149

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a1ed9c3
Avoid false `possibly-undefined` errors due to omitted unrequired els…
tyralla Oct 30, 2025
7e9b592
else_irrelevant_for_possibly_undefined -> unreachable_else
tyralla Oct 31, 2025
b9c59be
else_irrelevant_for_possibly_undefined -> unreachable_else
tyralla Oct 31, 2025
97275b2
Merge branch 'master' into fix/possibly_undefined_superfluous_else_block
tyralla Nov 15, 2025
123d5a4
Explain (the difference between) `else_body.is_unreachable` and `unre…
tyralla Nov 15, 2025
2901f69
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Nov 15, 2025
c2d4351
fix format
tyralla Nov 15, 2025
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else_irrelevant_for_possibly_undefined -> unreachable_else
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@tyralla
tyralla committed Oct 31, 2025
commit 7e9b592d0a15c0da26a8c073eb90f45e2c0ebeba
2 changes: 1 addition & 1 deletion mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -5036,7 +5036,7 @@ def visit_if_stmt(self, s: IfStmt) -> None:

if_map, else_map = self.find_isinstance_check(e)

s.else_irrelevant_for_possibly_undefined = else_map is None
s.unreachable_else = else_map is None

# XXX Issue a warning if condition is always False?
with self.binder.frame_context(can_skip=True, fall_through=2):
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6 changes: 3 additions & 3 deletions mypy/nodes.py
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Expand Up @@ -1792,21 +1792,21 @@ def accept(self, visitor: StatementVisitor[T]) -> T:


class IfStmt(Statement):
__slots__ = ("expr", "body", "else_body", "else_irrelevant_for_possibly_undefined")
__slots__ = ("expr", "body", "else_body", "unreachable_else")

__match_args__ = ("expr", "body", "else_body", "else_body_irrelevant_for_possibly_undefined")

expr: list[Expression]
body: list[Block]
else_body: Block | None
else_irrelevant_for_possibly_undefined: bool
unreachable_else: bool

@sterliakov sterliakov Nov 1, 2025

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Could you add a brief comment explaining the attribute here? It would be nice to immediately see a notice with this attr's meaning and "volatile" nature

@tyralla tyralla Nov 15, 2025 •
edited
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Again, sorry for the delay. I added some explanations that hopefully clarify things.


def __init__(self, expr: list[Expression], body: list[Block], else_body: Block | None) -> None:
super().__init__()
self.expr = expr
self.body = body
self.else_body = else_body
self.else_irrelevant_for_possibly_undefined = False
self.unreachable_else = False

def accept(self, visitor: StatementVisitor[T]) -> T:
return visitor.visit_if_stmt(self)
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2 changes: 1 addition & 1 deletion mypy/partially_defined.py
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Expand Up @@ -395,7 +395,7 @@ def visit_if_stmt(self, o: IfStmt) -> None:
continue
b.accept(self)
self.tracker.next_branch()
if o.else_irrelevant_for_possibly_undefined:
if o.unreachable_else:
self.tracker.skip_branch()
elif o.else_body:
if o.else_body.is_unreachable:
Expand Down
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