Move categorical casting check further up

Author: keechongtan

pandas/core/internals/blocks.py

@@ -828,6 +828,14 @@ def setitem(self, indexer, value):
             if lib.is_scalar(value):
                 value = convert_scalar(values, value)
 
+            if (
+                hasattr(value, "dtype")
+                and self.is_categorical_astype(value.dtype)
+                and not is_categorical_dtype(values)
+            ):
+                values[indexer] = value
+                return self.make_block(Categorical(self.values, dtype=value.dtype))
+
         else:
             # current dtype cannot store value, coerce to common dtype
             find_dtype = False
@@ -883,13 +891,6 @@ def setitem(self, indexer, value):
         ):
             values[indexer] = value
             try:
-                # GH25495 may need to convert to categorical block
-                if self.is_categorical_astype(
-                    arr_value.dtype
-                ) and not is_categorical_dtype(values):
-                    return self.make_block(
-                        Categorical(self.values, dtype=arr_value.dtype)
-                    )
 
                 values = values.astype(arr_value.dtype)
             except ValueError: