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Prevent assignment to None for non-Optional class variables #20054

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Prevent assignment to None for non-Optional class variables #20054

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8a9935b
Prevent assignment to None for non-Optional class variables
hauntsaninja Oct 12, 2025
02a5c1b
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Oct 12, 2025
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21 changes: 3 additions & 18 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3175,7 +3175,7 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
# as X | Y.
if not (s.is_alias_def and self.is_stub):
with self.enter_final_context(s.is_final_def):
self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None)

if s.is_alias_def:
self.check_type_alias_rvalue(s)
Expand Down Expand Up @@ -3223,11 +3223,7 @@ def check_type_alias_rvalue(self, s: AssignmentStmt) -> None:
self.store_type(s.lvalues[-1], alias_type)

def check_assignment(
self,
lvalue: Lvalue,
rvalue: Expression,
infer_lvalue_type: bool = True,
new_syntax: bool = False,
self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type: bool = True
) -> None:
"""Type check a single assignment: lvalue = rvalue."""
if isinstance(lvalue, (TupleExpr, ListExpr)):
Expand Down Expand Up @@ -3295,15 +3291,6 @@ def check_assignment(
# Try to infer a partial type. No need to check the return value, as
# an error will be reported elsewhere.
self.infer_partial_type(lvalue_type.var, lvalue, rvalue_type)
elif (
is_literal_none(rvalue)
and isinstance(lvalue, NameExpr)
and isinstance(lvalue.node, Var)
and lvalue.node.is_initialized_in_class
and not new_syntax
):
# Allow None's to be assigned to class variables with non-Optional types.
rvalue_type = lvalue_type
elif (
isinstance(lvalue, MemberExpr) and lvalue.kind is None
): # Ignore member access to modules
Expand Down Expand Up @@ -5056,9 +5043,7 @@ def visit_operator_assignment_stmt(self, s: OperatorAssignmentStmt) -> None:
# There is no __ifoo__, treat as x = x <foo> y
expr = OpExpr(s.op, s.lvalue, s.rvalue)
expr.set_line(s)
self.check_assignment(
lvalue=s.lvalue, rvalue=expr, infer_lvalue_type=True, new_syntax=False
)
self.check_assignment(lvalue=s.lvalue, rvalue=expr, infer_lvalue_type=True)
self.check_final(s)

def visit_assert_stmt(self, s: AssertStmt) -> None:
Expand Down
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