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JelleZijlstra merged 2 commits into from
Aug 13, 2026
Merged

Clarify ClassVar protocol conformance expectations #2340

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7ab5fee
Clarify ClassVar protocol conformance expectations
AlexWaygood Aug 12, 2026
cd8e1bd
Merge branch 'main' into alex/protocol-classvar-conformance
AlexWaygood Aug 13, 2026
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3 changes: 3 additions & 0 deletions conformance/results/mypy/protocols_definition.toml
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Expand Up @@ -51,6 +51,9 @@ protocols_definition.py:340: error: Incompatible types in assignment (expression
protocols_definition.py:340: note: Protocol member Template6.val1 expected settable variable, got read-only attribute
protocols_definition.py:341: error: Incompatible types in assignment (expression has type "Concrete6_Bad3", variable has type "Template6") [assignment]
protocols_definition.py:341: note: Protocol member Template6.val1 expected settable variable, got read-only attribute
protocols_definition.py:369: error: Incompatible types in assignment (expression has type "Concrete7_Bad1", variable has type "Template7") [assignment]
protocols_definition.py:369: note: Protocol member Template7.val1 expected class variable, got instance variable
protocols_definition.py:370: error: Incompatible types in assignment (expression has type "Concrete7_Bad2", variable has type "Template7") [assignment]
"""
conformance_automated = "Fail"
errors_diff = """
Expand Down
8 changes: 7 additions & 1 deletion conformance/results/pycroscope/protocols_definition.toml
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@@ -1,5 +1,10 @@
conformance_automated = "Pass"
conformance_automated = "Fail"
conformant = "Partial"
notes = """
Incorrectly accepts an instance-only attribute as satisfying a `ClassVar` protocol member.
"""
errors_diff = """
Line 369: Expected 1 errors
"""
output = """
./protocols_definition.py:30:10: Incompatible argument type for things: expected collections.abc.Iterable[./protocols_definition.py.SupportsClose] but got Literal[[1]] [incompatible_argument]
Expand All @@ -23,4 +28,5 @@ output = """
./protocols_definition.py:339:0: Incompatible assignment: expected ./protocols_definition.py.Template6, got ./protocols_definition.py.Concrete6_Bad1 [incompatible_assignment]
./protocols_definition.py:340:0: Incompatible assignment: expected ./protocols_definition.py.Template6, got ./protocols_definition.py.Concrete6_Bad2 [incompatible_assignment]
./protocols_definition.py:341:0: Incompatible assignment: expected ./protocols_definition.py.Template6, got ./protocols_definition.py.Concrete6_Bad3 [incompatible_assignment]
./protocols_definition.py:370:0: Incompatible assignment: expected ./protocols_definition.py.Template7, got ./protocols_definition.py.Concrete7_Bad2 [incompatible_assignment]
"""
2 changes: 2 additions & 0 deletions conformance/results/pyrefly/protocols_definition.toml
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Expand Up @@ -24,4 +24,6 @@ ERROR protocols_definition.py:289:22-38: `Concrete5_Bad5` is not assignable to `
ERROR protocols_definition.py:339:22-38: `Concrete6_Bad1` is not assignable to `Template6` [bad-assignment]
ERROR protocols_definition.py:340:22-38: `Concrete6_Bad2` is not assignable to `Template6` [bad-assignment]
ERROR protocols_definition.py:341:22-38: `Concrete6_Bad3` is not assignable to `Template6` [bad-assignment]
ERROR protocols_definition.py:369:22-38: `Concrete7_Bad1` is not assignable to `Template7` [bad-assignment]
ERROR protocols_definition.py:370:22-38: `Concrete7_Bad2` is not assignable to `Template7` [bad-assignment]
"""
6 changes: 6 additions & 0 deletions conformance/results/pyright/protocols_definition.toml
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Expand Up @@ -97,6 +97,12 @@ protocols_definition.py:341:22 - error: Type "Concrete6_Bad3" is not assignable
  "Concrete6_Bad3" is incompatible with protocol "Template6"
    "val1" is writable in protocol
    "val1" is not read-only in protocol (reportAssignmentType)
protocols_definition.py:369:22 - error: Type "Concrete7_Bad1" is not assignable to declared type "Template7"
  "Concrete7_Bad1" is incompatible with protocol "Template7"
    "val1" is defined as a ClassVar in protocol (reportAssignmentType)
protocols_definition.py:370:22 - error: Type "Concrete7_Bad2" is not assignable to declared type "Template7"
  "Concrete7_Bad2" is incompatible with protocol "Template7"
    "val1" is defined as a ClassVar in protocol (reportAssignmentType)
"""
conformance_automated = "Pass"
errors_diff = """
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11 changes: 8 additions & 3 deletions conformance/results/results.html
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2 changes: 2 additions & 0 deletions conformance/results/ty/protocols_definition.toml
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Expand Up @@ -23,4 +23,6 @@ protocols_definition.py:289:22: error[invalid-assignment] Object of type `Concre
protocols_definition.py:339:22: error[invalid-assignment] Object of type `Concrete6_Bad1` is not assignable to `Template6`
protocols_definition.py:340:22: error[invalid-assignment] Object of type `Concrete6_Bad2` is not assignable to `Template6`
protocols_definition.py:341:22: error[invalid-assignment] Object of type `Concrete6_Bad3` is not assignable to `Template6`
protocols_definition.py:369:22: error[invalid-assignment] Object of type `Concrete7_Bad1` is not assignable to `Template7`
protocols_definition.py:370:22: error[invalid-assignment] Object of type `Concrete7_Bad2` is not assignable to `Template7`
"""
3 changes: 3 additions & 0 deletions conformance/results/zuban/protocols_definition.toml
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Expand Up @@ -67,4 +67,7 @@ protocols_definition.py:340: error: Incompatible types in assignment (expression
protocols_definition.py:340: note: Protocol member Template6.val1 expected settable variable, got read-only attribute
protocols_definition.py:341: error: Incompatible types in assignment (expression has type "Concrete6_Bad3", variable has type "Template6") [assignment]
protocols_definition.py:341: note: Protocol member Template6.val1 expected settable variable, got read-only attribute
protocols_definition.py:369: error: Incompatible types in assignment (expression has type "Concrete7_Bad1", variable has type "Template7") [assignment]
protocols_definition.py:369: note: Protocol member Template7.val1 expected class variable, got instance variable
protocols_definition.py:370: error: Incompatible types in assignment (expression has type "Concrete7_Bad2", variable has type "Template7") [assignment]
"""
2 changes: 1 addition & 1 deletion conformance/tests/protocols_class_objects.py
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Expand Up @@ -105,5 +105,5 @@ class ConcreteC3(metaclass=CMeta):
pc2: ProtoC2 = ConcreteC1 # OK
pc3: ProtoC1 = ConcreteC2 # E
pc4: ProtoC2 = ConcreteC2 # E
pc5: ProtoC1 = ConcreteC3 # E
pc5: ProtoC1 = ConcreteC3 # E?: Explicit ClassVar matching is unspecified
pc6: ProtoC2 = ConcreteC3 # OK
31 changes: 30 additions & 1 deletion conformance/tests/protocols_definition.py
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Expand Up @@ -114,7 +114,7 @@ class Concrete2_Bad4:
v2_bad1: Template2 = Concrete2_Bad1() # E
v2_bad2: Template2 = Concrete2_Bad2() # E
v2_bad3: Template2 = Concrete2_Bad3() # E
v2_bad4: Template2 = Concrete2_Bad4() # E
v2_bad4: Template2 = Concrete2_Bad4() # E?: Explicit ClassVar matching is unspecified


class Template3(Protocol):
Expand Down Expand Up @@ -339,3 +339,32 @@ class Concrete6_Bad3:
v6_bad1: Template6 = Concrete6_Bad1() # E
v6_bad2: Template6 = Concrete6_Bad2() # E: named tuple is immutable
v6_bad3: Template6 = Concrete6_Bad3() # E: dataclass is frozen


# The specification leaves two possible interpretations of a ClassVar protocol
# member. One interpretation requires the implementing attribute to be explicitly
# declared with ClassVar. The other interpretation imposes only structural
# requirements: the attribute must be readable and writable on the class object
# and readable on instances of the class. Both interpretations therefore require
# the following assignments to be rejected.


class Template7(Protocol):
val1: ClassVar[int]


class Concrete7_Bad1:
def __init__(self) -> None:
self.val1: int = 42


class Concrete7Meta(type):
val1: int = 42


class Concrete7_Bad2(metaclass=Concrete7Meta):
pass


v7_bad1: Template7 = Concrete7_Bad1() # E: val1 is not readable on the class object
v7_bad2: Template7 = Concrete7_Bad2() # E: val1 is not readable on instances