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[red-knot] Document current state of attribute assignment diagnostics (#16746)
## Summary A follow-up to #16705 which documents various kinds of diagnostics that can appear when assigning to an attribute. ## Test Plan New snapshot tests.
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‎crates/red_knot_python_semantic/resources/mdtest/diagnostics/attribute_assignment.md‎

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# Attribute assignment
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<!-- snapshot-diagnostics -->
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This test suite demonstrates various kinds of diagnostics that can be emitted in a
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`obj.attr = value` assignment.
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## Instance attributes with class-level defaults
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These can be set on instances and on class objects.
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```py
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class C:
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attr: int = 0
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instance = C()
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instance.attr = 1 # fine
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instance.attr = "wrong" # error: [invalid-assignment]
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C.attr = 1 # fine
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C.attr = "wrong" # error: [invalid-assignment]
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```
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## Pure instance attributes
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These can only be set on instances. When trying to set them on class objects, we generate a useful
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diagnostic that mentions that the attribute is only available on instances.
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```py
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class C:
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def __init__(self):
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self.attr: int = 0
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instance = C()
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instance.attr = 1 # fine
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instance.attr = "wrong" # error: [invalid-assignment]
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C.attr = 1 # error: [invalid-attribute-access]
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```
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## `ClassVar`s
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These can only be set on class objects. When trying to set them on instances, we generate a useful
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diagnostic that mentions that the attribute is only available on class objects.
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```py
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from typing import ClassVar
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class C:
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attr: ClassVar[int] = 0
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C.attr = 1 # fine
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C.attr = "wrong" # error: [invalid-assignment]
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instance = C()
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instance.attr = 1 # error: [invalid-attribute-access]
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```
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## Unknown attributes
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When trying to set an attribute that is not defined, we also emit errors:
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```py
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class C: ...
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C.non_existent = 1 # error: [unresolved-attribute]
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instance = C()
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instance.non_existent = 1 # error: [unresolved-attribute]
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```
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## Possibly-unbound attributes
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When trying to set an attribute that is not defined in all branches, we emit errors:
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```py
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def _(flag: bool) -> None:
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class C:
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if flag:
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attr: int = 0
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C.attr = 1 # error: [possibly-unbound-attribute]
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instance = C()
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instance.attr = 1 # error: [possibly-unbound-attribute]
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```
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## Data descriptors
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When assigning to a data descriptor attribute, we implicitly call the descriptor's `__set__` method.
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This can lead to various kinds of diagnostics.
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### Invalid argument type
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```py
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class Descriptor:
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def __set__(self, instance: object, value: int) -> None:
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pass
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class C:
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attr: Descriptor = Descriptor()
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instance = C()
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instance.attr = 1 # fine
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# TODO: ideally, we would mention why this is an invalid assignment (wrong argument type for `value` parameter)
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instance.attr = "wrong" # error: [invalid-assignment]
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```
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### Invalid `__set__` method signature
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```py
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class WrongDescriptor:
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def __set__(self, instance: object, value: int, extra: int) -> None:
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pass
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class C:
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attr: WrongDescriptor = WrongDescriptor()
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instance = C()
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# TODO: ideally, we would mention why this is an invalid assignment (wrong number of arguments for `__set__`)
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instance.attr = 1 # error: [invalid-assignment]
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```
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## Setting attributes on union types
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```py
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def _(flag: bool) -> None:
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if flag:
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class C1:
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attr: int = 0
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else:
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class C1:
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attr: str = ""
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# TODO: The error message here could be improved to explain why the assignment fails.
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C1.attr = 1 # error: [invalid-assignment]
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class C2:
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if flag:
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attr: int = 0
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else:
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attr: str = ""
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# TODO: This should be an error
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C2.attr = 1
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```

‎crates/red_knot_python_semantic/resources/mdtest/snapshots/attribute_assignment.md_-_Attribute_assignment_-_Data_descriptors_-_Invalid_`__set__`_method_signature.snap‎

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---
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source: crates/red_knot_test/src/lib.rs
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expression: snapshot
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---
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---
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mdtest name: attribute_assignment.md - Attribute assignment - Data descriptors - Invalid `__set__` method signature
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mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/attribute_assignment.md
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---
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# Python source files
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## mdtest_snippet.py
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```
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1 | class WrongDescriptor:
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2 | def __set__(self, instance: object, value: int, extra: int) -> None:
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3 | pass
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4 |
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5 | class C:
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6 | attr: WrongDescriptor = WrongDescriptor()
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7 |
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8 | instance = C()
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9 |
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10 | # TODO: ideally, we would mention why this is an invalid assignment (wrong number of arguments for `__set__`)
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11 | instance.attr = 1 # error: [invalid-assignment]
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```
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# Diagnostics
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```
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error: lint:invalid-assignment
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--> /src/mdtest_snippet.py:11:1
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|
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10 | # TODO: ideally, we would mention why this is an invalid assignment (wrong number of arguments for `__set__`)
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11 | instance.attr = 1 # error: [invalid-assignment]
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| ^^^^^^^^^^^^^ Invalid assignment to data descriptor attribute `attr` on type `C` with custom `__set__` method
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|
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```

‎crates/red_knot_python_semantic/resources/mdtest/snapshots/attribute_assignment.md_-_Attribute_assignment_-_Data_descriptors_-_Invalid_argument_type.snap‎

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---
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source: crates/red_knot_test/src/lib.rs
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expression: snapshot
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---
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---
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mdtest name: attribute_assignment.md - Attribute assignment - Data descriptors - Invalid argument type
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mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/attribute_assignment.md
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---
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# Python source files
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## mdtest_snippet.py
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```
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1 | class Descriptor:
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2 | def __set__(self, instance: object, value: int) -> None:
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3 | pass
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4 |
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5 | class C:
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6 | attr: Descriptor = Descriptor()
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7 |
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8 | instance = C()
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9 | instance.attr = 1 # fine
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10 |
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11 | # TODO: ideally, we would mention why this is an invalid assignment (wrong argument type for `value` parameter)
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12 | instance.attr = "wrong" # error: [invalid-assignment]
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```
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# Diagnostics
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```
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error: lint:invalid-assignment
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--> /src/mdtest_snippet.py:12:1
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|
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11 | # TODO: ideally, we would mention why this is an invalid assignment (wrong argument type for `value` parameter)
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12 | instance.attr = "wrong" # error: [invalid-assignment]
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| ^^^^^^^^^^^^^ Invalid assignment to data descriptor attribute `attr` on type `C` with custom `__set__` method
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|
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```

‎crates/red_knot_python_semantic/resources/mdtest/snapshots/attribute_assignment.md_-_Attribute_assignment_-_Instance_attributes_with_class-level_defaults.snap‎

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---
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source: crates/red_knot_test/src/lib.rs
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expression: snapshot
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---
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---
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mdtest name: attribute_assignment.md - Attribute assignment - Instance attributes with class-level defaults
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mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/attribute_assignment.md
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---
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# Python source files
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## mdtest_snippet.py
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```
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1 | class C:
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2 | attr: int = 0
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3 |
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4 | instance = C()
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5 | instance.attr = 1 # fine
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6 | instance.attr = "wrong" # error: [invalid-assignment]
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7 |
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8 | C.attr = 1 # fine
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9 | C.attr = "wrong" # error: [invalid-assignment]
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```
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# Diagnostics
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```
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error: lint:invalid-assignment
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--> /src/mdtest_snippet.py:6:1
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|
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4 | instance = C()
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5 | instance.attr = 1 # fine
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6 | instance.attr = "wrong" # error: [invalid-assignment]
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| ^^^^^^^^^^^^^ Object of type `Literal["wrong"]` is not assignable to attribute `attr` of type `int`
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7 |
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8 | C.attr = 1 # fine
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|
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```
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```
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error: lint:invalid-assignment
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--> /src/mdtest_snippet.py:9:1
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|
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8 | C.attr = 1 # fine
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9 | C.attr = "wrong" # error: [invalid-assignment]
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| ^^^^^^ Object of type `Literal["wrong"]` is not assignable to attribute `attr` of type `int`
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|
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```

‎crates/red_knot_python_semantic/resources/mdtest/snapshots/attribute_assignment.md_-_Attribute_assignment_-_Possibly-unbound_attributes.snap‎

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---
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source: crates/red_knot_test/src/lib.rs
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expression: snapshot
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---
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---
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mdtest name: attribute_assignment.md - Attribute assignment - Possibly-unbound attributes
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mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/attribute_assignment.md
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---
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# Python source files
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## mdtest_snippet.py
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```
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1 | def _(flag: bool) -> None:
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2 | class C:
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3 | if flag:
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4 | attr: int = 0
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5 |
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6 | C.attr = 1 # error: [possibly-unbound-attribute]
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7 |
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8 | instance = C()
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9 | instance.attr = 1 # error: [possibly-unbound-attribute]
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```
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# Diagnostics
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```
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warning: lint:possibly-unbound-attribute
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--> /src/mdtest_snippet.py:6:5
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|
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4 | attr: int = 0
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5 |
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6 | C.attr = 1 # error: [possibly-unbound-attribute]
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| ------ Attribute `attr` on type `Literal[C]` is possibly unbound
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7 |
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8 | instance = C()
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|
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```
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```
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warning: lint:possibly-unbound-attribute
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--> /src/mdtest_snippet.py:9:5
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|
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8 | instance = C()
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9 | instance.attr = 1 # error: [possibly-unbound-attribute]
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| ------------- Attribute `attr` on type `C` is possibly unbound
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|
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```

‎crates/red_knot_python_semantic/resources/mdtest/snapshots/attribute_assignment.md_-_Attribute_assignment_-_Pure_instance_attributes.snap‎

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---
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source: crates/red_knot_test/src/lib.rs
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expression: snapshot
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---
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---
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mdtest name: attribute_assignment.md - Attribute assignment - Pure instance attributes
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mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/attribute_assignment.md
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---
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# Python source files
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## mdtest_snippet.py
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```
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1 | class C:
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2 | def __init__(self):
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3 | self.attr: int = 0
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4 |
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5 | instance = C()
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6 | instance.attr = 1 # fine
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7 | instance.attr = "wrong" # error: [invalid-assignment]
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8 |
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9 | C.attr = 1 # error: [invalid-attribute-access]
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```
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# Diagnostics
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```
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error: lint:invalid-assignment
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--> /src/mdtest_snippet.py:7:1
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|
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5 | instance = C()
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6 | instance.attr = 1 # fine
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7 | instance.attr = "wrong" # error: [invalid-assignment]
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| ^^^^^^^^^^^^^ Object of type `Literal["wrong"]` is not assignable to attribute `attr` of type `int`
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8 |
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9 | C.attr = 1 # error: [invalid-attribute-access]
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|
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```
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```
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error: lint:invalid-attribute-access
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--> /src/mdtest_snippet.py:9:1
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|
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7 | instance.attr = "wrong" # error: [invalid-assignment]
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8 |
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9 | C.attr = 1 # error: [invalid-attribute-access]
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| ^^^^^^ Cannot assign to instance attribute `attr` from the class object `Literal[C]`
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|
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```

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