stubgen: Use Generator type var defaults (#17670)

Fixes #17669

Author: srittau

mypy/stubgen.py

@@ -588,8 +588,8 @@ def _get_func_return(self, o: FuncDef, ctx: FunctionContext) -> str | None:
         if has_yield_expression(o) or has_yield_from_expression(o):
             generator_name = self.add_name("collections.abc.Generator")
             yield_name = "None"
-            send_name = "None"
-            return_name = "None"
+            send_name: str | None = None
+            return_name: str | None = None
             if has_yield_from_expression(o):
                 yield_name = send_name = self.add_name("_typeshed.Incomplete")
             else:
@@ -600,7 +600,14 @@ def _get_func_return(self, o: FuncDef, ctx: FunctionContext) -> str | None:
                         send_name = self.add_name("_typeshed.Incomplete")
             if has_return_statement(o):
                 return_name = self.add_name("_typeshed.Incomplete")
-            return f"{generator_name}[{yield_name}, {send_name}, {return_name}]"
+            if return_name is not None:
+                if send_name is None:
+                    send_name = "None"
+                return f"{generator_name}[{yield_name}, {send_name}, {return_name}]"
+            elif send_name is not None:
+                return f"{generator_name}[{yield_name}, {send_name}]"
+            else:
+                return f"{generator_name}[{yield_name}]"
         if not has_return_statement(o) and o.abstract_status == NOT_ABSTRACT:
             return "None"
         return None

test-data/unit/stubgen.test

@@ -1640,11 +1640,11 @@ def all():
 from _typeshed import Incomplete
 from collections.abc import Generator
 
-def f() -> Generator[Incomplete, None, None]: ...
-def g() -> Generator[None, Incomplete, None]: ...
-def h1() -> Generator[None, None, None]: ...
+def f() -> Generator[Incomplete]: ...
+def g() -> Generator[None, Incomplete]: ...
+def h1() -> Generator[None]: ...
 def h2() -> Generator[None, None, Incomplete]: ...
-def h3() -> Generator[None, None, None]: ...
+def h3() -> Generator[None]: ...
 def all() -> Generator[Incomplete, Incomplete, Incomplete]: ...
 
 [case testFunctionYieldsNone]
@@ -1656,8 +1656,8 @@ def g():
 [out]
 from collections.abc import Generator
 
-def f() -> Generator[None, None, None]: ...
-def g() -> Generator[None, None, None]: ...
+def f() -> Generator[None]: ...
+def g() -> Generator[None]: ...
 
 [case testGeneratorAlreadyDefined]
 class Generator:
@@ -1671,7 +1671,7 @@ from collections.abc import Generator as _Generator
 
 class Generator: ...
 
-def f() -> _Generator[Incomplete, None, None]: ...
+def f() -> _Generator[Incomplete]: ...
 
 [case testGeneratorYieldFrom]
 def g1():
@@ -1692,10 +1692,10 @@ def g5():
 from _typeshed import Incomplete
 from collections.abc import Generator
 
-def g1() -> Generator[Incomplete, Incomplete, None]: ...
-def g2() -> Generator[Incomplete, Incomplete, None]: ...
-def g3() -> Generator[Incomplete, Incomplete, None]: ...
-def g4() -> Generator[Incomplete, Incomplete, None]: ...
+def g1() -> Generator[Incomplete, Incomplete]: ...
+def g2() -> Generator[Incomplete, Incomplete]: ...
+def g3() -> Generator[Incomplete, Incomplete]: ...
+def g4() -> Generator[Incomplete, Incomplete]: ...
 def g5() -> Generator[Incomplete, Incomplete, Incomplete]: ...
 
 [case testGeneratorYieldAndYieldFrom]
@@ -1728,13 +1728,13 @@ def g7():
 from _typeshed import Incomplete
 from collections.abc import Generator
 
-def g1() -> Generator[Incomplete, Incomplete, None]: ...
-def g2() -> Generator[Incomplete, Incomplete, None]: ...
-def g3() -> Generator[Incomplete, Incomplete, None]: ...
-def g4() -> Generator[Incomplete, Incomplete, None]: ...
-def g5() -> Generator[Incomplete, Incomplete, None]: ...
+def g1() -> Generator[Incomplete, Incomplete]: ...
+def g2() -> Generator[Incomplete, Incomplete]: ...
+def g3() -> Generator[Incomplete, Incomplete]: ...
+def g4() -> Generator[Incomplete, Incomplete]: ...
+def g5() -> Generator[Incomplete, Incomplete]: ...
 def g6() -> Generator[Incomplete, Incomplete, Incomplete]: ...
-def g7() -> Generator[Incomplete, Incomplete, None]: ...
+def g7() -> Generator[Incomplete, Incomplete]: ...
 
 [case testCallable]
 from typing import Callable