REF: de-nest Series.__setitem__ (pandas-dev#32078)

Author: jbrockmendel

pandas/core/series.py

@@ -970,14 +970,15 @@ def __setitem__(self, key, value):
         key = com.apply_if_callable(key, self)
         cacher_needs_updating = self._check_is_chained_assignment_possible()
 
+        if key is Ellipsis:
+            key = slice(None)
+
         try:
             self._set_with_engine(key, value)
         except (KeyError, ValueError):
             values = self._values
             if is_integer(key) and not self.index.inferred_type == "integer":
                 values[key] = value
-            elif key is Ellipsis:
-                self[:] = value
             else:
                 self.loc[key] = value
 
@@ -995,9 +996,10 @@ def __setitem__(self, key, value):
                     self._where(~key, value, inplace=True)
                     return
                 except InvalidIndexError:
-                    pass
+                    self._set_values(key.astype(np.bool_), value)
 
-            self._set_with(key, value)
+            else:
+                self._set_with(key, value)
 
         if cacher_needs_updating:
             self._maybe_update_cacher()
@@ -1038,13 +1040,13 @@ def _set_with(self, key, value):
             else:
                 key_type = lib.infer_dtype(key, skipna=False)
 
+            # Note: key_type == "boolean" should not occur because that
+            #  should be caught by the is_bool_indexer check in __setitem__
             if key_type == "integer":
                 if self.index.inferred_type == "integer":
                     self._set_labels(key, value)
                 else:
                     return self._set_values(key, value)
-            elif key_type == "boolean":
-                self._set_values(key.astype(np.bool_), value)
             else:
                 self._set_labels(key, value)