[ty] Validate signatures of dataclass __post_init__ methods (#22730)

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/dataclasses/post_init.md

@@ -0,0 +1,175 @@
+# Tests for the `__post_init__` method
+
+At runtime, if a dataclass has a `__post_init__` method then all `InitVar`-annotated fields will be
+passed as positional arguments to that method as the final statement in the dataclass's generated
+`__init__` method. The `__post_init__` signature must therefore be compatible with the fields of the
+dataclass:
+
+```py
+from dataclasses import dataclass, InitVar
+
+@dataclass
+class Empty1:
+    def __post_init__(self): ...  # fine
+
+@dataclass
+class Empty2:
+    def __post_init__(self) -> None: ...  # fine
+
+@dataclass
+class Empty3:
+    # The returned value is discarded,
+    # so arbitrary return annotations are allowed
+    def __post_init__(self) -> int:
+        return 42
+
+@dataclass
+class Empty4:
+    def __post_init__(self, *args): ...  # fine
+
+@dataclass
+class Empty5:
+    def __post_init__(self, **kwargs): ...  # fine
+
+@dataclass
+class Empty6:
+    def __post_init__(self, *args, **kargs): ...  # fine
+
+@dataclass
+class Empty7:
+    # no arguments will be passed to this method at runtime,
+    # because there are no `InitVar` fields on the class,
+    # so this is an error:
+    #
+    # error: [invalid-dataclass]
+    def __post_init__(self, required_argument: int): ...
+
+@dataclass
+class Empty8:
+    # error: [invalid-dataclass]
+    def __post_init__(self, *, required_argument): ...
+
+@dataclass
+class Empty9:
+    # error: [invalid-dataclass]
+    def __post_init__(self, required_argument, /): ...
+
+@dataclass
+class SingleField:
+    x: int
+
+    # `x` will not be passed to `__post_init__`,
+    # because it is not an `InitVar`, so this is an
+    #
+    # error: [invalid-dataclass]
+    def __post_init__(self, x: int) -> None: ...
+
+@dataclass
+class SingleFieldGood:
+    x: int
+
+    # this is fine!
+    def __post_init__(self) -> None: ...
+
+@dataclass
+class HasInitVarNoParameter:
+    x: InitVar[int]
+
+    # error: [invalid-dataclass]
+    def __post_init__(self) -> None: ...
+
+@dataclass
+class HasInitVarDifferentParameterName:
+    x: InitVar[int]
+
+    # because arguments are always passed in positionally
+    # to `__post_init__` methods, we allow a parameter to
+    # have an arbitrary name as long as it is inferred has
+    # having a compatible type. So this is fine:
+    def __post_init__(self, xx) -> None: ...
+
+@dataclass
+class HasInitVarBadParameterType:
+    x: InitVar[int]
+
+    # error: [invalid-dataclass]
+    def __post_init__(self, x: str) -> None: ...
+
+@dataclass
+class HasInitVarBadParameterKind:
+    x: InitVar[int]
+
+    # error: [invalid-dataclass]
+    def __post_init__(self, *, x: int) -> None: ...
+
+@dataclass
+class HasInitVarGood:
+    x: InitVar[int]
+
+    def __post_init__(self, x: int) -> None: ...
+
+@dataclass
+class HasInitVarGoodPositionalOnly:
+    x: InitVar[int]
+
+    # arguments are always passed to `__post_init__` positionally at runtime,
+    # so this is fine
+    def __post_init__(self, x: int, /) -> None: ...
+
+@dataclass
+class LotsOfInitVarsBad:
+    a: int
+    b: InitVar[str]
+    c: InitVar[bytes]
+    d: int
+    e: int
+    f: InitVar[range]
+    g: int
+
+    # Only `InitVar` fields are passed in at runtime, so this is an
+    # error: [invalid-dataclass]
+    def __post_init__(self, a: int, b: str, c: bytes, d: int, e: int, f: range, g: int): ...
+
+@dataclass
+class LotsOfInitVarsOutOfOrder:
+    a: int
+    b: InitVar[str]
+    c: InitVar[bytes]
+    d: int
+    e: int
+    f: InitVar[range]
+    g: int
+
+    # the parameters are in the wrong order, so this is an
+    # error: [invalid-dataclass]
+    def __post_init__(self, c: bytes, b: str, f: range): ...
+
+@dataclass
+class LotsOfInitVarsGood:
+    a: int
+    b: InitVar[str]
+    c: InitVar[bytes]
+    d: int
+    e: int
+    f: InitVar[range]
+    g: int
+
+    def __post_init__(self, b: str, c: bytes, f: range): ...
+
+@dataclass
+class InitVarSubclassGood(LotsOfInitVarsGood):
+    h: InitVar[list[int]]
+    i: str
+    j: InitVar[bool]
+
+    def __post_init__(self, b: str, c: bytes, f: range, h: list[int], j: bool): ...
+
+@dataclass
+class InitVarSubclassBad(LotsOfInitVarsGood):
+    h: InitVar[list[int]]
+    i: str
+    j: InitVar[bool]
+
+    # error: [invalid-dataclass]
+    def __post_init__(self, h: list[int], j: bool, b: str, c: bytes, f: range): ...
+```

crates/ty_python_semantic/resources/mdtest/liskov.md

@@ -411,7 +411,7 @@ python-version = "3.13"
 ```
 
 ```pyi
-from dataclasses import dataclass
+from dataclasses import dataclass, InitVar
 from typing_extensions import Self
 
 class Grandparent: ...
@@ -425,14 +425,14 @@ class Child(Parent):
 
 @dataclass(init=False)
 class DataSuper:
-    x: int
+    x: InitVar[int]
 
     def __post_init__(self, x: int) -> None:
         self.x = x
 
 @dataclass(init=False)
 class DataSub(DataSuper):
-    y: str
+    y: InitVar[str]
 
     def __post_init__(self, x: int, y: str) -> None:
         self.y = y