Rename Red Knot (#17820)

crates/ty_python_semantic/resources/mdtest/diagnostics/attribute_assignment.md

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