Code review changes.

Author: Oleh Kozynets

pandas/core/arrays/categorical.py

@@ -1062,7 +1062,7 @@ def remove_unused_categories(self, inplace=no_default):
         Returns
         -------
         cat : Categorical or None
-            Categorical with unused categories dropped.
+            Categorical with unused categories dropped or None if ``inplace=True``.
 
         See Also
         --------
@@ -1079,8 +1079,11 @@ def remove_unused_categories(self, inplace=no_default):
                 FutureWarning,
                 stacklevel=2,
             )
+        else:
+            inplace = False
 
-        cat = self.copy()
+        inplace = validate_bool_kwarg(inplace, "inplace")
+        cat = self if inplace else self.copy()
         idx, inv = np.unique(cat._codes, return_inverse=True)
 
         if idx.size != 0 and idx[0] == -1:  # na sentinel
@@ -1093,7 +1096,8 @@ def remove_unused_categories(self, inplace=no_default):
         cat._dtype = new_dtype
         cat._codes = coerce_indexer_dtype(inv, new_dtype.categories)
 
-        return cat
+        if not inplace:
+            return cat
 
     # ------------------------------------------------------------------
 

pandas/tests/arrays/categorical/test_analytics.py

@@ -354,8 +354,10 @@ def test_validate_inplace_raises(self, value):
         with pytest.raises(ValueError, match=msg):
             cat.remove_categories(removals=["D", "E", "F"], inplace=value)
 
-        with tm.assert_produces_warning(FutureWarning):
-            cat.remove_unused_categories(inplace=value)
+        with pytest.raises(ValueError, match=msg):
+            with tm.assert_produces_warning(FutureWarning):
+                # issue #37643 inplace kwarg deprecated
+                cat.remove_unused_categories(inplace=value)
 
         with pytest.raises(ValueError, match=msg):
             cat.sort_values(inplace=value)

pandas/tests/arrays/categorical/test_api.py

@@ -372,7 +372,11 @@ def test_remove_unused_categories(self):
         tm.assert_index_equal(c.categories, exp_categories_all)
 
         with tm.assert_produces_warning(FutureWarning):
-            c.remove_unused_categories(inplace=True)
+            # issue #37643 inplace kwarg deprecated
+            res = c.remove_unused_categories(inplace=True)
+
+        tm.assert_index_equal(c.categories, exp_categories_dropped)
+        assert res is None
 
         # with NaN values (GH11599)
         c = Categorical(["a", "b", "c", np.nan], categories=["a", "b", "c", "d", "e"])