Fixed bug with setting a Series and the consistency of the column type. Issue: #pandas-dev#43996

Author: Damian Barabonkov

pandas/core/indexing.py

@@ -1817,8 +1817,7 @@ def _setitem_single_column(self, loc: int, value, plane_indexer):
         if com.is_null_slice(pi) or com.is_full_slice(pi, len(self.obj)):
             ser = value
         elif is_array_like(value) and is_exact_shape_match(ser, value):
-            ser = value
-
+            ser.update(value)
         else:
             # set the item, possibly having a dtype change
             ser = ser.copy()

pandas/core/internals/blocks.py

@@ -924,11 +924,11 @@ def setitem(self, indexer, value):
             # otherwise should have _can_hold_element
 
             return self.astype(dtype).setitem(indexer, value)
-
-        if self.dtype.kind in ["m", "M"]:
-            arr = self.array_values().T
-            arr[indexer] = value
-            return self
+        else:
+            if self.dtype.kind in ["m", "M"]:
+                arr = self.array_values().T
+                arr[indexer] = value
+                return self
 
         # value must be storable at this moment
         if is_extension_array_dtype(getattr(value, "dtype", None)):