[red-knot] Attribute access and the descriptor protocol (#16416)

## Summary

* Attributes/method are now properly looked up on metaclasses, when
called on class objects
* We properly distinguish between data descriptors and non-data
descriptors (but we do not yet support them in store-context, i.e.
`obj.data_descr = …`)
* The descriptor protocol is now implemented in a single unified place
for instances, classes and dunder-calls. Unions and possibly-unbound
symbols are supported in all possible stages of the process by creating
union types as results.
* In general, the handling of "possibly-unbound" symbols has been
improved in a lot of places: meta-class attributes, attributes,
descriptors with possibly-unbound `__get__` methods, instance
attributes, …
* We keep track of type qualifiers in a lot more places. I anticipate
that this will be useful if we import e.g. `Final` symbols from other
modules (see relevant change to typing spec:
python/typing#1937).
* Detection and special-casing of the `typing.Protocol` special form in
order to avoid lots of changes in the test suite due to new `@Todo`
types when looking up attributes on builtin types which have `Protocol`
in their MRO. We previously
looked up attributes in a wrong way, which is why this didn't come up
before.

closes #16367
closes #15966

## Context

The way attribute lookup in `Type::member` worked before was simply
wrong (mostly my own fault). The whole instance-attribute lookup should
probably never have been integrated into `Type::member`. And the
`Type::static_member` function that I introduced in my last descriptor
PR was the wrong abstraction. It's kind of fascinating how far this
approach took us, but I am pretty confident that the new approach
proposed here is what we need to model this correctly.

There are three key pieces that are required to implement attribute
lookups:

- **`Type::class_member`**/**`Type::find_in_mro`**: The
`Type::find_in_mro` method that can look up attributes on class bodies
(and corresponding bases). This is a partial function on types, as it
can not be called on instance types like`Type::Instance(…)` or
`Type::IntLiteral(…)`. For this reason, we usually call it through
`Type::class_member`, which is essentially just
`type.to_meta_type().find_in_mro(…)` plus union/intersection handling.
- **`Type::instance_member`**: This new function is basically the
type-level equivalent to `obj.__dict__[name]` when called on
`Type::Instance(…)`. We use this to discover instance attributes such as
those that we see as declarations on class bodies or as (annotated)
assignments to `self.attr` in methods of a class.
- The implementation of the descriptor protocol. It works slightly
different for instances and for class objects, but it can be described
by the general framework:
- Call `type.class_member("attribute")` to look up "attribute" in the
MRO of the meta type of `type`. Call the resulting `Symbol` `meta_attr`
(even if it's unbound).
- Use `meta_attr.class_member("__get__")` to look up `__get__` on the
*meta type* of `meta_attr`. Call it with `__get__(meta_attr, self,
self.to_meta_type())`. If this fails (either the lookup or the call),
just proceed with `meta_attr`. Otherwise, replace `meta_attr` in the
following with the return type of `__get__`. In this step, we also probe
if a `__set__` or `__delete__` method exists and store it in
`meta_attr_kind` (can be either "data descriptor" or "normal attribute
or non-data descriptor").
  - Compute a `fallback` type.
    - For instances, we use `self.instance_member("attribute")`
- For class objects, we use `class_attr =
self.find_in_mro("attribute")`, and then try to invoke the descriptor
protocol on `class_attr`, i.e. we look up `__get__` on the meta type of
`class_attr` and call it with `__get__(class_attr, None, self)`. This
additional invocation of the descriptor protocol on the fallback type is
one major asymmetry in the otherwise universal descriptor protocol
implementation.
- Finally, we look at `meta_attr`, `meta_attr_kind` and `fallback`, and
handle various cases of (possible) unboundness of these symbols.
- If `meta_attr` is bound and a data descriptor, just return `meta_attr`
- If `meta_attr` is not a data descriptor, and `fallback` is bound, just
return `fallback`
- If `meta_attr` is not a data descriptor, and `fallback` is unbound,
return `meta_attr`
- Return unions of these three possibilities for partially-bound
symbols.

This allows us to handle class objects and instances within the same
framework. There is a minor additional detail where for instances, we do
not allow the fallback type (the instance attribute) to completely
shadow the non-data descriptor. We do this because we (currently) don't
want to pretend that we can statically infer that an instance attribute
is always set.

Dunder method calls can also be embedded into this framework. The only
thing that changes is that *there is no fallback type*. If a dunder
method is called on an instance, we do not fall back to instance
variables. If a dunder method is called on a class object, we only look
it up on the meta class, never on the class itself.

## Test Plan

New Markdown tests.

Author: sharkdp

crates/red_knot_python_semantic/resources/mdtest/annotations/literal_string.md

@@ -73,12 +73,12 @@ qux = (foo, bar)
 reveal_type(qux)  # revealed: tuple[Literal["foo"], Literal["bar"]]
 
 # TODO: Infer "LiteralString"
-reveal_type(foo.join(qux))  # revealed: @Todo(overloaded method)
+reveal_type(foo.join(qux))  # revealed: @Todo(return type of decorated function)
 
 template: LiteralString = "{}, {}"
 reveal_type(template)  # revealed: Literal["{}, {}"]
 # TODO: Infer `LiteralString`
-reveal_type(template.format(foo, bar))  # revealed: @Todo(overloaded method)
+reveal_type(template.format(foo, bar))  # revealed: @Todo(return type of decorated function)
 ```
 
 ### Assignability

crates/red_knot_python_semantic/resources/mdtest/annotations/stdlib_typing_aliases.md

@@ -70,8 +70,7 @@ import typing
 
 class ListSubclass(typing.List): ...
 
-# TODO: should have `Generic`, should not have `Unknown`
-# revealed: tuple[Literal[ListSubclass], Literal[list], Unknown, Literal[object]]
+# revealed: tuple[Literal[ListSubclass], Literal[list], Literal[MutableSequence], Literal[Sequence], Literal[Reversible], Literal[Collection], Literal[Iterable], Literal[Container], @Todo(protocol), Literal[object]]
 reveal_type(ListSubclass.__mro__)
 
 class DictSubclass(typing.Dict): ...

crates/red_knot_python_semantic/resources/mdtest/assignment/augmented.md

@@ -75,8 +75,7 @@ def _(flag: bool):
 
     f = Foo()
 
-    # TODO: We should emit an `unsupported-operator` error here, possibly with the information
-    # that `Foo.__iadd__` may be unbound as additional context.
+    # error: [unsupported-operator] "Operator `+=` is unsupported between objects of type `Foo` and `Literal["Hello, world!"]`"
     f += "Hello, world!"
 
     reveal_type(f)  # revealed: int | Unknown

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -155,7 +155,9 @@ reveal_type(c_instance.declared_in_body_and_init)  # revealed: str | None
 
 reveal_type(c_instance.declared_in_body_defined_in_init)  # revealed: str | None
 
-reveal_type(c_instance.bound_in_body_declared_in_init)  # revealed: str | None
+# TODO: This should be `str | None`. Fixing this requires an overhaul of the `Symbol` API,
+# which is planned in https://github.com/astral-sh/ruff/issues/14297
+reveal_type(c_instance.bound_in_body_declared_in_init)  # revealed: Unknown | str | None
 
 reveal_type(c_instance.bound_in_body_and_init)  # revealed: Unknown | None | Literal["a"]
 ```
@@ -704,8 +706,91 @@ reveal_type(Derived().declared_in_body)  # revealed: int | None
 reveal_type(Derived().defined_in_init)  # revealed: str | None
 ```
 
+## Accessing attributes on class objects
+
+When accessing attributes on class objects, they are always looked up on the type of the class
+object first, i.e. on the metaclass:
+
+```py
+from typing import Literal
+
+class Meta1:
+    attr: Literal["meta class value"] = "meta class value"
+
+class C1(metaclass=Meta1): ...
+
+reveal_type(C1.attr)  # revealed: Literal["meta class value"]
+```
+
+However, the meta class attribute only takes precedence over a class-level attribute if it is a data
+descriptor. If it is a non-data descriptor or a normal attribute, the class-level attribute is used
+instead (see the [descriptor protocol tests] for data/non-data descriptor attributes):
+
+```py
+class Meta2:
+    attr: str = "meta class value"
+
+class C2(metaclass=Meta2):
+    attr: Literal["class value"] = "class value"
+
+reveal_type(C2.attr)  # revealed: Literal["class value"]
+```
+
+If the class-level attribute is only partially defined, we union the meta class attribute with the
+class-level attribute:
+
+```py
+def _(flag: bool):
+    class Meta3:
+        attr1 = "meta class value"
+        attr2: Literal["meta class value"] = "meta class value"
+
+    class C3(metaclass=Meta3):
+        if flag:
+            attr1 = "class value"
+            # TODO: Neither mypy nor pyright show an error here, but we could consider emitting a conflicting-declaration diagnostic here.
+            attr2: Literal["class value"] = "class value"
+
+    reveal_type(C3.attr1)  # revealed: Unknown | Literal["meta class value", "class value"]
+    reveal_type(C3.attr2)  # revealed: Literal["meta class value", "class value"]
+```
+
+If the *meta class* attribute is only partially defined, we emit a `possibly-unbound-attribute`
+diagnostic:
+
+```py
+def _(flag: bool):
+    class Meta4:
+        if flag:
+            attr1: str = "meta class value"
+
+    class C4(metaclass=Meta4): ...
+    # error: [possibly-unbound-attribute]
+    reveal_type(C4.attr1)  # revealed: str
+```
+
+Finally, if both the meta class attribute and the class-level attribute are only partially defined,
+we union them and emit a `possibly-unbound-attribute` diagnostic:
+
+```py
+def _(flag1: bool, flag2: bool):
+    class Meta5:
+        if flag1:
+            attr1 = "meta class value"
+
+    class C5(metaclass=Meta5):
+        if flag2:
+            attr1 = "class value"
+
+    # error: [possibly-unbound-attribute]
+    reveal_type(C5.attr1)  # revealed: Unknown | Literal["meta class value", "class value"]
+```
+
 ## Union of attributes
 
+If the (meta)class is a union type or if the attribute on the (meta) class has a union type, we
+infer those union types accordingly:
+
 ```py
 def _(flag: bool):
     if flag:
@@ -716,14 +801,35 @@ def _(flag: bool):
         class C1:
             x = 2
 
+    reveal_type(C1.x)  # revealed: Unknown | Literal[1, 2]
+
     class C2:
         if flag:
             x = 3
         else:
             x = 4
 
-    reveal_type(C1.x)  # revealed: Unknown | Literal[1, 2]
     reveal_type(C2.x)  # revealed: Unknown | Literal[3, 4]
+
+    if flag:
+        class Meta3(type):
+            x = 5
+
+    else:
+        class Meta3(type):
+            x = 6
+
+    class C3(metaclass=Meta3): ...
+    reveal_type(C3.x)  # revealed: Unknown | Literal[5, 6]
+
+    class Meta4(type):
+        if flag:
+            x = 7
+        else:
+            x = 8
+
+    class C4(metaclass=Meta4): ...
+    reveal_type(C4.x)  # revealed: Unknown | Literal[7, 8]
 ```
 
 ## Inherited class attributes
@@ -883,7 +989,7 @@ def _(flag: bool):
                 self.x = 1
 
     # error: [possibly-unbound-attribute]
-    reveal_type(Foo().x)  # revealed: int
+    reveal_type(Foo().x)  # revealed: int | Unknown
 ```
 
 #### Possibly unbound
@@ -1105,8 +1211,8 @@ Most attribute accesses on bool-literal types are delegated to `builtins.bool`,
 bools are instances of that class:
 
 ```py
-reveal_type(True.__and__)  # revealed: @Todo(overloaded method)
-reveal_type(False.__or__)  # revealed: @Todo(overloaded method)
+reveal_type(True.__and__)  # revealed: <bound method `__and__` of `Literal[True]`>
+reveal_type(False.__or__)  # revealed: <bound method `__or__` of `Literal[False]`>
 ```
 
 Some attributes are special-cased, however:
@@ -1262,6 +1368,7 @@ reveal_type(C.a_none)  # revealed: None
 Some of the tests in the *Class and instance variables* section draw inspiration from
 [pyright's documentation] on this topic.
 
+[descriptor protocol tests]: descriptor_protocol.md
 [pyright's documentation]: https://microsoft.github.io/pyright/#/type-concepts-advanced?id=class-and-instance-variables
 [typing spec on `classvar`]: https://typing.readthedocs.io/en/latest/spec/class-compat.html#classvar
 [`typing.classvar`]: https://docs.python.org/3/library/typing.html#typing.ClassVar

crates/red_knot_python_semantic/resources/mdtest/binary/integers.md

@@ -10,8 +10,7 @@ reveal_type(-3 // 3)  # revealed: Literal[-1]
 reveal_type(-3 / 3)  # revealed: float
 reveal_type(5 % 3)  # revealed: Literal[2]
 
-# TODO: Should emit `unsupported-operator` but we don't understand the bases of `str`, so we think
-#       it inherits `Unknown`, so we think `str.__radd__` is `Unknown` instead of nonexistent.
+# error: [unsupported-operator] "Operator `+` is unsupported between objects of type `Literal[2]` and `Literal["f"]`"
 reveal_type(2 + "f")  # revealed: Unknown
 
 def lhs(x: int):

crates/red_knot_python_semantic/resources/mdtest/call/dunder.md

@@ -46,6 +46,17 @@ class DunderOnMetaClass(metaclass=Meta):
 reveal_type(DunderOnMetaClass[0])  # revealed: str
 ```
 
+If the dunder method is only present on the class itself, it will not be called:
+
+```py
+class ClassWithNormalDunder:
+    def __getitem__(self, key: int) -> str:
+        return str(key)
+
+# error: [non-subscriptable]
+ClassWithNormalDunder[0]
+```
+
 ## Operating on instances
 
 When invoking a dunder method on an instance of a class, it is looked up on the class:
@@ -79,13 +90,32 @@ reveal_type(this_fails[0])  # revealed: Unknown
 However, the attached dunder method *can* be called if accessed directly:
 
 ```py
-# TODO: `this_fails.__getitem__` is incorrectly treated as a bound method. This
-# should be fixed with https://github.com/astral-sh/ruff/issues/16367
-# error: [too-many-positional-arguments]
-# error: [invalid-argument-type]
 reveal_type(this_fails.__getitem__(this_fails, 0))  # revealed: Unknown | str
 ```
 
+The instance-level method is also not called when the class-level method is present:
+
+```py
+def external_getitem1(instance, key) -> str:
+    return "a"
+
+def external_getitem2(key) -> int:
+    return 1
+
+def _(flag: bool):
+    class ThisFails:
+        if flag:
+            __getitem__ = external_getitem1
+
+        def __init__(self):
+            self.__getitem__ = external_getitem2
+
+    this_fails = ThisFails()
+
+    # error: [call-possibly-unbound-method]
+    reveal_type(this_fails[0])  # revealed: Unknown | str
+```
+
 ## When the dunder is not a method
 
 A dunder can also be a non-method callable:
@@ -126,3 +156,64 @@ class_with_descriptor_dunder = ClassWithDescriptorDunder()
 
 reveal_type(class_with_descriptor_dunder[0])  # revealed: str
 ```
+
+## Dunders can not be overwritten on instances
+
+If we attempt to overwrite a dunder method on an instance, it does not affect the behavior of
+implicit dunder calls:
+
+```py
+class C:
+    def __getitem__(self, key: int) -> str:
+        return str(key)
+
+    def f(self):
+        # TODO: This should emit an `invalid-assignment` diagnostic once we understand the type of `self`
+        self.__getitem__ = None
+
+# This is still fine, and simply calls the `__getitem__` method on the class
+reveal_type(C()[0])  # revealed: str
+```
+
+## Calling a union of dunder methods
+
+```py
+def _(flag: bool):
+    class C:
+        if flag:
+            def __getitem__(self, key: int) -> str:
+                return str(key)
+        else:
+            def __getitem__(self, key: int) -> bytes:
+                return key
+
+    c = C()
+    reveal_type(c[0])  # revealed: str | bytes
+
+    if flag:
+        class D:
+            def __getitem__(self, key: int) -> str:
+                return str(key)
+
+    else:
+        class D:
+            def __getitem__(self, key: int) -> bytes:
+                return key
+
+    d = D()
+    reveal_type(d[0])  # revealed: str | bytes
+```
+
+## Calling a possibly-unbound dunder method
+
+```py
+def _(flag: bool):
+    class C:
+        if flag:
+            def __getitem__(self, key: int) -> str:
+                return str(key)
+
+    c = C()
+    # error: [call-possibly-unbound-method]
+    reveal_type(c[0])  # revealed: str
+```

crates/red_knot_python_semantic/resources/mdtest/call/getattr_static.md

@@ -12,7 +12,7 @@ import inspect
 
 class Descriptor:
     def __get__(self, instance, owner) -> str:
-        return 1
+        return "a"
 
 class C:
     normal: int = 1
@@ -59,7 +59,7 @@ import sys
 reveal_type(inspect.getattr_static(sys, "platform"))  # revealed: LiteralString
 reveal_type(inspect.getattr_static(inspect, "getattr_static"))  # revealed: Literal[getattr_static]
 
-reveal_type(inspect.getattr_static(1, "real"))  # revealed: Literal[1]
+reveal_type(inspect.getattr_static(1, "real"))  # revealed: Literal[real]
 ```
 
 (Implicit) instance attributes can also be accessed through `inspect.getattr_static`:
@@ -72,6 +72,23 @@ class D:
 reveal_type(inspect.getattr_static(D(), "instance_attr"))  # revealed: int
 ```
 
+And attributes on metaclasses can be accessed when probing the class:
+
+```py
+class Meta(type):
+    attr: int = 1
+
+class E(metaclass=Meta): ...
+
+reveal_type(inspect.getattr_static(E, "attr"))  # revealed: int
+```
+
+Metaclass attributes can not be added when probing an instance of the class:
+
+```py
+reveal_type(inspect.getattr_static(E(), "attr", "non_existent"))  # revealed: Literal["non_existent"]
+```
+
 ## Error cases
 
 We can only infer precise types if the attribute is a literal string. In all other cases, we fall