fix: DataFrame.assign None (#1238)

Author: cmp0xff

pandas-stubs/core/frame.pyi

@@ -794,7 +794,7 @@ class DataFrame(NDFrame, OpsMixin, _GetItemHack):
         value: Scalar | ListLikeU | None,
         allow_duplicates: _bool = ...,
     ) -> None: ...
-    def assign(self, **kwargs: IntoColumn) -> Self: ...
+    def assign(self, **kwargs: IntoColumn | None) -> Self: ...
     def align(
         self,
         other: NDFrameT,

tests/test_frame.py

@@ -339,6 +339,7 @@ def my_named_func_2(df: pd.DataFrame) -> pd.Series[Any]:
     check(assert_type(df.assign(c=my_unnamed_func), pd.DataFrame), pd.DataFrame)
     check(assert_type(df.assign(c=my_named_func_1), pd.DataFrame), pd.DataFrame)
     check(assert_type(df.assign(c=my_named_func_2), pd.DataFrame), pd.DataFrame)
+    check(assert_type(df.assign(c=None), pd.DataFrame), pd.DataFrame)
 
 
 def test_types_sample() -> None: