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Bypass class attribute lookup for PEP 695 typevars #20292

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7feaead
Bypass class attribute lookup for PEP 695 typevars
A5rocks Nov 23, 2025
5feef39
Make the change less risky
A5rocks Nov 23, 2025
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16 changes: 8 additions & 8 deletions mypy/semanal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1842,7 +1842,7 @@ def push_type_args(
return None
tvs.append((p.name, tv))

if self.is_defined_type_param(p.name):
if self.get_defined_type_param(p.name):
self.fail(f'"{p.name}" already defined as a type parameter', context)
else:
assert self.add_symbol(
Expand All @@ -1851,15 +1851,15 @@ def push_type_args(

return tvs

def is_defined_type_param(self, name: str) -> bool:
def get_defined_type_param(self, name: str) -> TypeVarLikeExpr | None:
for names in self.locals:
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@sterliakov sterliakov Nov 26, 2025 •
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Ough, I spelled that out but didn't realize that this goes in the wrong direction (?). We are typically interested in the most recent locals frame, so shouldn't this be

Suggested change
for names in self.locals:
for names in reversed(self.locals):

? It didn't matter when we only requested a boolean "is defined" result, but now could potentially resolve to a wrong symbol. I don't have a testcase for that, but anyway the variable will normally be found in the most recent local frame, so checking it first makes more sense? Upd: no, it cannot resolve to the wrong symbol, we only accept type variables, and type param redefinitions are never added to symtable, but maybe it's still better to avoid the unnecessary work? reversed is fast

if names is None:
continue
if name in names:
Comment on lines 1856 to 1858
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@sterliakov sterliakov Nov 26, 2025

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And just to save a bit on branching,

Suggested change
if names is None:
continue
if name in names:
if names is not None and name in names:

node = names[name].node
if isinstance(node, TypeVarLikeExpr):
return True
return False
return node
return None

def analyze_type_param(
self, type_param: TypeParam, context: Context
Expand Down Expand Up @@ -2272,15 +2272,15 @@ class Foo(Bar, Generic[T]): ...
has_type_var_tuple = False
if defn.type_args is not None:
for p in defn.type_args:
node = self.lookup(p.name, context)
node = self.get_defined_type_param(p.name)
assert node is not None
assert isinstance(node.node, TypeVarLikeExpr)
if isinstance(node.node, TypeVarTupleExpr):
assert isinstance(node, TypeVarLikeExpr)
if isinstance(node, TypeVarTupleExpr):
if has_type_var_tuple:
self.fail("Can only use one type var tuple in a class def", context)
continue
has_type_var_tuple = True
declared_tvars.append((p.name, node.node))
declared_tvars.append((p.name, node))

for i, base_expr in enumerate(base_type_exprs):
if isinstance(base_expr, StarExpr):
Expand Down
7 changes: 7 additions & 0 deletions test-data/unit/check-python312.test
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Expand Up @@ -2237,3 +2237,10 @@ class D[*Ts](Generic[Unpack[Us]]): # E: Generic[...] base class is redundant \
# E: Can only use one type var tuple in a class def
pass
[builtins fixtures/tuple.pyi]

[case testPEP695TypeVarReusedName]
class C:
X = 5
class Inner[X]:
pass
[builtins fixtures/tuple.pyi]