BUG: assert_index_equal does not raise error for check_categorical=False when comparing 2 CategoricalIndex objects

doc/source/whatsnew/v0.23.1.txt

@@ -46,6 +46,11 @@ Bug Fixes
 -
 -
 
+Categorical
+^^^^^^^^^^^
+
+- Bug in :func:`pandas.util.testing.assert_index_equal` which raised ``AssertionError`` incorrectly, when comparing two :class:`CategoricalIndex` objects with param ``check_categorical=False`` (:issue:`19776`)
+
 Conversion
 ^^^^^^^^^^
 

pandas/tests/util/test_testing.py

@@ -503,6 +503,25 @@ def test_index_equal_metadata_message(self):
         with tm.assert_raises_regex(AssertionError, expected):
             assert_index_equal(idx1, idx2)
 
+    def test_categorical_index_equality(self):
+        expected = """Index are different
+
+Attribute "dtype" are different
+\\[left\\]:  CategoricalDtype\\(categories=\\[u?'a', u?'b'\\], ordered=False\\)
+\\[right\\]: CategoricalDtype\\(categories=\\[u?'a', u?'b', u?'c'\\], \
+ordered=False\\)"""
+
+        with tm.assert_raises_regex(AssertionError, expected):
+            assert_index_equal(pd.Index(pd.Categorical(['a', 'b'])),
+                               pd.Index(pd.Categorical(['a', 'b'],
+                                        categories=['a', 'b', 'c'])))
+
+    def test_categorical_index_equality_relax_categories_check(self):
+        assert_index_equal(pd.Index(pd.Categorical(['a', 'b'])),
+                           pd.Index(pd.Categorical(['a', 'b'],
+                                    categories=['a', 'b', 'c'])),
+                           check_categorical=False)
+
 
 class TestAssertSeriesEqual(object):
 
@@ -600,6 +619,25 @@ def test_series_equal_message(self):
             assert_series_equal(pd.Series([1, 2, 3]), pd.Series([1, 2, 4]),
                                 check_less_precise=True)
 
+    def test_categorical_series_equality(self):
+        expected = """Attributes are different
+
+Attribute "dtype" are different
+\\[left\\]:  CategoricalDtype\\(categories=\\[u?'a', u?'b'\\], ordered=False\\)
+\\[right\\]: CategoricalDtype\\(categories=\\[u?'a', u?'b', u?'c'\\], \
+ordered=False\\)"""
+
+        with tm.assert_raises_regex(AssertionError, expected):
+            assert_series_equal(pd.Series(pd.Categorical(['a', 'b'])),
+                                pd.Series(pd.Categorical(['a', 'b'],
+                                          categories=['a', 'b', 'c'])))
+
+    def test_categorical_series_equality_relax_categories_check(self):
+        assert_series_equal(pd.Series(pd.Categorical(['a', 'b'])),
+                            pd.Series(pd.Categorical(['a', 'b'],
+                                      categories=['a', 'b', 'c'])),
+                            check_categorical=False)
+
 
 class TestAssertFrameEqual(object):
 

pandas/util/testing.py

@@ -778,8 +778,12 @@ def assert_index_equal(left, right, exact='equiv', check_names=True,
 
     def _check_types(l, r, obj='Index'):
         if exact:
-            assert_class_equal(left, right, exact=exact, obj=obj)
-            assert_attr_equal('dtype', l, r, obj=obj)
+            assert_class_equal(l, r, exact=exact, obj=obj)
+
+            # Skip exact dtype checking when `check_categorical` is False
+            if check_categorical:
+                assert_attr_equal('dtype', l, r, obj=obj)
+
             # allow string-like to have different inferred_types
             if l.inferred_type in ('string', 'unicode'):
                 assert r.inferred_type in ('string', 'unicode')
@@ -829,7 +833,8 @@ def _get_ilevel_values(index, level):
             # get_level_values may change dtype
             _check_types(left.levels[level], right.levels[level], obj=obj)
 
-    if check_exact:
+    # skip exact index checking when `check_categorical` is False
+    if check_exact and check_categorical:
         if not left.equals(right):
             diff = np.sum((left.values != right.values)
                           .astype(int)) * 100.0 / len(left)
@@ -950,23 +955,23 @@ def is_sorted(seq):
 
 
 def assert_categorical_equal(left, right, check_dtype=True,
-                             obj='Categorical', check_category_order=True):
+                             check_category_order=True, obj='Categorical'):
     """Test that Categoricals are equivalent.
 
     Parameters
     ----------
-    left, right : Categorical
-        Categoricals to compare
+    left : Categorical
+    right : Categorical
     check_dtype : bool, default True
         Check that integer dtype of the codes are the same
-    obj : str, default 'Categorical'
-        Specify object name being compared, internally used to show appropriate
-        assertion message
     check_category_order : bool, default True
         Whether the order of the categories should be compared, which
         implies identical integer codes.  If False, only the resulting
         values are compared.  The ordered attribute is
         checked regardless.
+    obj : str, default 'Categorical'
+        Specify object name being compared, internally used to show appropriate
+        assertion message
     """
     _check_isinstance(left, right, Categorical)
 
@@ -1020,7 +1025,7 @@ def raise_assert_detail(obj, message, left, right, diff=None):
 
 def assert_numpy_array_equal(left, right, strict_nan=False,
                              check_dtype=True, err_msg=None,
-                             obj='numpy array', check_same=None):
+                             check_same=None, obj='numpy array'):
     """ Checks that 'np.ndarray' is equivalent
 
     Parameters
@@ -1033,11 +1038,11 @@ def assert_numpy_array_equal(left, right, strict_nan=False,
         check dtype if both a and b are np.ndarray
     err_msg : str, default None
         If provided, used as assertion message
+    check_same : None|'copy'|'same', default None
+        Ensure left and right refer/do not refer to the same memory area
     obj : str, default 'numpy array'
         Specify object name being compared, internally used to show appropriate
         assertion message
-    check_same : None|'copy'|'same', default None
-        Ensure left and right refer/do not refer to the same memory area
     """
 
     # instance validation