stubgen: Add support for PEP 570 positional-only parameters (#16904)

This only adds support for Python modules (x-ref #14138)

mypy/stubgen.py

@@ -110,6 +110,7 @@
     Var,
 )
 from mypy.options import Options as MypyOptions
+from mypy.sharedparse import MAGIC_METHODS_POS_ARGS_ONLY
 from mypy.stubdoc import ArgSig, FunctionSig
 from mypy.stubgenc import InspectionStubGenerator, generate_stub_for_c_module
 from mypy.stubutil import (
@@ -480,6 +481,9 @@ def get_default_function_sig(self, func_def: FuncDef, ctx: FunctionContext) -> F
     def _get_func_args(self, o: FuncDef, ctx: FunctionContext) -> list[ArgSig]:
         args: list[ArgSig] = []
 
+        # Ignore pos-only status of magic methods whose args names are elided by mypy at parse
+        actually_pos_only_args = o.name not in MAGIC_METHODS_POS_ARGS_ONLY
+        pos_only_marker_position = 0  # Where to insert "/", if any
         for i, arg_ in enumerate(o.arguments):
             var = arg_.variable
             kind = arg_.kind
@@ -500,6 +504,9 @@ def _get_func_args(self, o: FuncDef, ctx: FunctionContext) -> list[ArgSig]:
                 if not isinstance(get_proper_type(annotated_type), AnyType):
                     typename = self.print_annotation(annotated_type)
 
+            if actually_pos_only_args and arg_.pos_only:
+                pos_only_marker_position += 1
+
             if kind.is_named() and not any(arg.name.startswith("*") for arg in args):
                 args.append(ArgSig("*"))
 
@@ -518,6 +525,8 @@ def _get_func_args(self, o: FuncDef, ctx: FunctionContext) -> list[ArgSig]:
             args.append(
                 ArgSig(name, typename, default=bool(arg_.initializer), default_value=default)
             )
+        if pos_only_marker_position:
+            args.insert(pos_only_marker_position, ArgSig("/"))
 
         if ctx.class_info is not None and all(
             arg.type is None and arg.default is False for arg in args

test-data/unit/stubgen.test

@@ -4231,3 +4231,25 @@ o = int | None
 
 def f1(a: int | tuple[int, int | None] | None) -> int: ...
 def f2(a: int | x.Union[int, int] | float | None) -> int: ...
+
+[case testPEP570PosOnlyParams]
+def f(x=0, /): ...
+def f1(x: int, /): ...
+def f2(x: int, y: float = 1, /): ...
+def f3(x: int, /, y: float): ...
+def f4(x: int, /, y: float = 1): ...
+def f5(x: int, /, *, y: float): ...
+def f6(x: int = 0, /, *, y: float): ...
+def f7(x: int, /, *, y: float = 1): ...
+def f8(x: int = 0, /, *, y: float = 1): ...
+
+[out]
+def f(x: int = 0, /) -> None: ...
+def f1(x: int, /): ...
+def f2(x: int, y: float = 1, /): ...
+def f3(x: int, /, y: float): ...
+def f4(x: int, /, y: float = 1): ...
+def f5(x: int, /, *, y: float): ...
+def f6(x: int = 0, /, *, y: float): ...
+def f7(x: int, /, *, y: float = 1): ...
+def f8(x: int = 0, /, *, y: float = 1): ...