PERF: improve efficiency of BaseMaskedArray.__setitem__

This somewhat deals with #44172, though that won't be fully resolved until 2D `ExtensionArray`s are supported (per the comments there).

pandas/core/apply.py

@@ -85,7 +85,7 @@ def frame_apply(
     args=None,
     kwargs=None,
 ) -> FrameApply:
-    """construct and return a row or column based frame apply object"""
+    """Construct and return a row- or column-based frame apply object."""
     axis = obj._get_axis_number(axis)
     klass: type[FrameApply]
     if axis == 0:
@@ -693,7 +693,7 @@ def dtypes(self) -> Series:
         return self.obj.dtypes
 
     def apply(self) -> DataFrame | Series:
-        """compute the results"""
+        """Compute the results."""
         # dispatch to agg
         if is_list_like(self.f):
             return self.apply_multiple()
@@ -1011,7 +1011,7 @@ def result_columns(self) -> Index:
     def wrap_results_for_axis(
         self, results: ResType, res_index: Index
     ) -> DataFrame | Series:
-        """return the results for the columns"""
+        """Return the results for the columns."""
         result: DataFrame | Series
 
         # we have requested to expand

pandas/core/arrays/masked.py

@@ -77,7 +77,7 @@
 
 class BaseMaskedDtype(ExtensionDtype):
     """
-    Base class for dtypes for BasedMaskedArray subclasses.
+    Base class for dtypes for BaseMaskedArray subclasses.
     """
 
     name: str
@@ -209,18 +209,17 @@ def _coerce_to_array(self, values) -> tuple[np.ndarray, np.ndarray]:
         raise AbstractMethodError(self)
 
     def __setitem__(self, key, value) -> None:
-        _is_scalar = is_scalar(value)
-        if _is_scalar:
-            value = [value]
-        value, mask = self._coerce_to_array(value)
-
-        if _is_scalar:
-            value = value[0]
-            mask = mask[0]
-
         key = check_array_indexer(self, key)
-        self._data[key] = value
-        self._mask[key] = mask
+        if is_scalar(value):
+            if isna(value):
+                self._mask[key] = True
+            else:
+                self._data[key] = value
+                self._mask[key] = False
+        else:
+            value, mask = self._coerce_to_array(value)
+            self._data[key] = value
+            self._mask[key] = mask
 
     def __iter__(self):
         if self.ndim == 1: