Fix calling df.agg with scalar dict values (#861)

Fixes #846

Author: hamdanal

pandas-stubs/core/frame.pyi

@@ -61,6 +61,7 @@ from pandas._typing import (
     S1,
     AggFuncTypeBase,
     AggFuncTypeDictFrame,
+    AggFuncTypeDictSeries,
     AggFuncTypeFrame,
     AnyArrayLike,
     ArrayLike,
@@ -1140,7 +1141,9 @@ class DataFrame(NDFrame, OpsMixin):
     ) -> DataFrame: ...
     def diff(self, periods: int = ..., axis: Axis = ...) -> DataFrame: ...
     @overload
-    def agg(self, func: AggFuncTypeBase, axis: Axis = ..., **kwargs) -> Series: ...
+    def agg(  # type: ignore[overload-overlap] # pyright: ignore[reportOverlappingOverload]
+        self, func: AggFuncTypeBase | AggFuncTypeDictSeries, axis: Axis = ..., **kwargs
+    ) -> Series: ...
     @overload
     def agg(
         self,
@@ -1149,8 +1152,8 @@ class DataFrame(NDFrame, OpsMixin):
         **kwargs,
     ) -> DataFrame: ...
     @overload
-    def aggregate(
-        self, func: AggFuncTypeBase, axis: Axis = ..., **kwargs
+    def aggregate(  # type: ignore[overload-overlap] # pyright: ignore[reportOverlappingOverload]
+        self, func: AggFuncTypeBase | AggFuncTypeDictSeries, axis: Axis = ..., **kwargs
     ) -> Series: ...
     @overload
     def aggregate(

tests/test_frame.py

@@ -1276,7 +1276,10 @@ def test_types_agg() -> None:
         assert_type(df.agg({"A": ["min", "max"], "B": "min"}), pd.DataFrame),
         pd.DataFrame,
     )
+    check(assert_type(df.agg({"A": ["mean"]}), pd.DataFrame), pd.DataFrame)
     check(assert_type(df.agg("mean", axis=1), pd.Series), pd.Series)
+    check(assert_type(df.agg({"A": "mean"}), pd.Series), pd.Series)
+    check(assert_type(df.agg({"A": "mean", "B": "sum"}), pd.Series), pd.Series)
 
 
 def test_types_aggregate() -> None:
@@ -1299,6 +1302,10 @@ def test_types_aggregate() -> None:
         assert_type(df.aggregate({"A": ["min", "max"], "B": "min"}), pd.DataFrame),
         pd.DataFrame,
     )
+    check(assert_type(df.aggregate({"A": ["mean"]}), pd.DataFrame), pd.DataFrame)
+    check(assert_type(df.aggregate("mean", axis=1), pd.Series), pd.Series)
+    check(assert_type(df.aggregate({"A": "mean"}), pd.Series), pd.Series)
+    check(assert_type(df.aggregate({"A": "mean", "B": "sum"}), pd.Series), pd.Series)
 
 
 def test_types_transform() -> None: