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Check property decorators stricter #19313

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b6411cf
Check property setter/deleter decorators stricter
sterliakov Jun 18, 2025
29a47f1
Fix selfcheck
sterliakov Jun 18, 2025
e0092bf
Update one more test
sterliakov Jun 18, 2025
25ea889
Document and test `.getter` rejection
sterliakov Jun 21, 2025
2f22993
Merge remote-tracking branch 'upstream/master' into bugfix/gh-19312-s…
sterliakov Jun 21, 2025
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31 changes: 24 additions & 7 deletions mypy/semanal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1527,33 +1527,33 @@ def analyze_property_with_multi_part_definition(
assert isinstance(first_item, Decorator)
deleted_items = []
bare_setter_type = None
func_name = first_item.func.name
for i, item in enumerate(items[1:]):
if isinstance(item, Decorator):
item.func.accept(self)
if item.decorators:
first_node = item.decorators[0]
if isinstance(first_node, MemberExpr):
if self._is_valid_property_decorator(first_node, func_name):
# Get abstractness from the original definition.
item.func.abstract_status = first_item.func.abstract_status
if first_node.name == "setter":
# The first item represents the entire property.
first_item.var.is_settable_property = True
# Get abstractness from the original definition.
item.func.abstract_status = first_item.func.abstract_status
setter_func_type = function_type(
item.func, self.named_type("builtins.function")
)
assert isinstance(setter_func_type, CallableType)
bare_setter_type = setter_func_type
defn.setter_index = i + 1
if first_node.name == "deleter":
item.func.abstract_status = first_item.func.abstract_status
for other_node in item.decorators[1:]:
other_node.accept(self)
else:
self.fail(
f"Only supported top decorator is @{first_item.func.name}.setter", item
f"Only supported top decorators are @{func_name}.setter and @{func_name}.deleter",
item,
)
else:
self.fail(f'Unexpected definition for property "{first_item.func.name}"', item)
self.fail(f'Unexpected definition for property "{func_name}"', item)
deleted_items.append(i + 1)
for i in reversed(deleted_items):
del items[i]
Expand All @@ -1567,6 +1567,23 @@ def analyze_property_with_multi_part_definition(
)
return bare_setter_type

def _is_valid_property_decorator(
self, deco: Expression, property_name: str
) -> TypeGuard[MemberExpr]:
if not isinstance(deco, MemberExpr):
return False
if not isinstance(deco.expr, NameExpr) or deco.expr.name != property_name:
return False
if deco.name not in {"setter", "deleter"}:
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@brianschubert brianschubert Jun 18, 2025 •
edited
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Suggested change
if deco.name not in {"setter", "deleter"}:
if deco.name not in {"getter", "setter", "deleter"}:

@x.getter is valid at runtime (albeit not common). We probably don't want to emit an "Only supported top decorators are ..." error for it.

I guess we don't have any tests for it 🤷

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Hm. This definitely needs a test, but I think rejecting @prop.getter is a reasonable thing to do: it should override the existing getter, supporting it properly won't be trivial, so explicitly saying "we don't support redefined getters" is not that bad (given that such usage should be really rare, 0 primer hits)

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@sterliakov sterliakov Jun 21, 2025 •
edited
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By following this github search, I identified two common patterns with @prop.getter (excluding partial subclass overrides as they don't trigger this code and aren't supported anyway):

  1. Mistake with getter instead of setter (followed by signature def (self, value))

  2. Obscure way of defining properties:

     class A:
     @property
     def foo(self): pass
     
     @foo.getter
     def foo(self):
         ...  # Some logic here
     
     @foo.setter
     def foo(self, new_foo):
         ...

    Honestly this makes my eyes bleed. I don't know how popular this is as regex code search is limited to 5 pages, and the file count estimate is never close to reality, they don't actually regex-match whole github. I propose to accept the risk and wait - if we get a couple of tickets about this in the next release, we can whitelist getter as well.

As a secondary reference, pyright handles this in the most confusing manner possible, we don't want to replicate that behaviour. It silently accepts any @prop.getter even with incompatible return type and ignores it completely - see my ticket microsoft/pyright#10633

# This intentionally excludes getter. While `@prop.getter` is valid at
# runtime, that would mean replacing the already processed getter type.
# Such usage is almost definitely a mistake (except for overrides in
# subclasses but we don't support them anyway) and might be a typo
# (only one letter away from `setter`), it's likely almost never used,
# so supporting it properly won't pay off.
return False
return True

def add_function_to_symbol_table(self, func: FuncDef | OverloadedFuncDef) -> None:
if self.is_class_scope():
assert self.type is not None
Expand Down
49 changes: 45 additions & 4 deletions test-data/unit/check-classes.test
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Original file line number Diff line number Diff line change
Expand Up @@ -772,7 +772,7 @@ class A:
class B(A):
@property
def f(self) -> Callable[[object], None]: pass
@func.setter
@f.setter
def f(self, x: object) -> None: pass
[builtins fixtures/property.pyi]

Expand All @@ -786,7 +786,7 @@ class A:
class B(A):
@property
def f(self) -> Callable[[object], None]: pass
@func.setter
@f.setter
def f(self, x: object) -> None: pass
[builtins fixtures/property.pyi]

Expand Down Expand Up @@ -1622,7 +1622,48 @@ class A:
self.x = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
return ''
[builtins fixtures/property.pyi]
[out]

[case testPropertyNameIsChecked]
class A:
@property
def f(self) -> str: ...
@not_f.setter # E: Only supported top decorators are @f.setter and @f.deleter
def f(self, val: str) -> None: ...

a = A()
reveal_type(a.f) # N: Revealed type is "builtins.str"
a.f = '' # E: Property "f" defined in "A" is read-only

class B:
@property
def f(self) -> str: ...
@not_f.deleter # E: Only supported top decorators are @f.setter and @f.deleter
def f(self) -> None: ...

class C:
@property
def f(self) -> str: ...
@not_f.setter # E: Only supported top decorators are @f.setter and @f.deleter
def f(self, val: str) -> None: ...
@not_f.deleter # E: Only supported top decorators are @f.setter and @f.deleter
def f(self) -> None: ...
[builtins fixtures/property.pyi]

[case testPropertyAttributeIsChecked]
class A:
@property
def f(self) -> str: ...
@f.unknown # E: Only supported top decorators are @f.setter and @f.deleter
def f(self, val: str) -> None: ...
[builtins fixtures/property.pyi]

[case testPropertyGetterDecoratorIsRejected]
class A:
@property
def f(self) -> str: ...
@f.getter # E: Only supported top decorators are @f.setter and @f.deleter
def f(self, val: str) -> None: ...
[builtins fixtures/property.pyi]

[case testDynamicallyTypedProperty]
import typing
Expand Down Expand Up @@ -7729,7 +7770,7 @@ class A:
def y(self) -> int: ...
@y.setter
def y(self, value: int) -> None: ...
@dec # E: Only supported top decorator is @y.setter
@dec # E: Only supported top decorators are @y.setter and @y.deleter
def y(self) -> None: ...

reveal_type(A().y) # N: Revealed type is "builtins.int"
Expand Down
4 changes: 1 addition & 3 deletions test-data/unit/check-functions.test
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Expand Up @@ -2752,7 +2752,7 @@ class B:
@property
@dec
def f(self) -> int: pass
@dec # E: Only supported top decorator is @f.setter
@dec # E: Only supported top decorators are @f.setter and @f.deleter
@f.setter
def f(self, v: int) -> None: pass

Expand All @@ -2764,7 +2764,6 @@ class C:
@dec
def f(self, v: int) -> None: pass
[builtins fixtures/property.pyi]
[out]

[case testInvalidArgCountForProperty]
from typing import Callable, TypeVar
Expand All @@ -2783,7 +2782,6 @@ class A:
@property
def h(self, *args, **kwargs) -> int: pass # OK
[builtins fixtures/property.pyi]
[out]

[case testSubtypingUnionGenericBounds]
from typing import Callable, TypeVar, Union, Sequence
Expand Down
2 changes: 1 addition & 1 deletion test-data/unit/semanal-errors.test
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Expand Up @@ -1236,7 +1236,7 @@ class A:
@overload # E: Decorators on top of @property are not supported
@property
def f(self) -> int: pass
@property # E: Only supported top decorator is @f.setter
@property # E: Only supported top decorators are @f.setter and @f.deleter
@overload
def f(self) -> int: pass
[builtins fixtures/property.pyi]
Expand Down