REGR: CategoricalIndex(None) (pandas-dev#41612)

Author: jbrockmendel

doc/source/whatsnew/v1.3.0.rst

@@ -655,6 +655,7 @@ Build
 Deprecations
 ~~~~~~~~~~~~
 - Deprecated allowing scalars to be passed to the :class:`Categorical` constructor (:issue:`38433`)
+- Deprecated constructing :class:`CategoricalIndex` without passing list-like data (:issue:`38944`)
 - Deprecated allowing subclass-specific keyword arguments in the :class:`Index` constructor, use the specific subclass directly instead (:issue:`14093`, :issue:`21311`, :issue:`22315`, :issue:`26974`)
 - Deprecated ``astype`` of datetimelike (``timedelta64[ns]``, ``datetime64[ns]``, ``Datetime64TZDtype``, ``PeriodDtype``) to integer dtypes, use ``values.view(...)`` instead (:issue:`38544`)
 - Deprecated :meth:`MultiIndex.is_lexsorted` and :meth:`MultiIndex.lexsort_depth`, use :meth:`MultiIndex.is_monotonic_increasing` instead (:issue:`32259`)

pandas/core/indexes/category.py

@@ -222,6 +222,17 @@ def __new__(
 
         name = maybe_extract_name(name, data, cls)
 
+        if data is None:
+            # GH#38944
+            warnings.warn(
+                "Constructing a CategoricalIndex without passing data is "
+                "deprecated and will raise in a future version. "
+                "Use CategoricalIndex([], ...) instead",
+                FutureWarning,
+                stacklevel=2,
+            )
+            data = []
+
         if is_scalar(data):
             raise cls._scalar_data_error(data)
 

pandas/tests/indexes/categorical/test_category.py

@@ -38,6 +38,11 @@ def test_can_hold_identifiers(self):
         key = idx[0]
         assert idx._can_hold_identifiers_and_holds_name(key) is True
 
+    def test_pickle_compat_construction(self):
+        # Once the deprecation is enforced, we can use the parent class's test
+        with tm.assert_produces_warning(FutureWarning, match="without passing data"):
+            self._index_cls()
+
     def test_insert(self, simple_index):
 
         ci = simple_index

pandas/tests/indexes/categorical/test_constructors.py

@@ -11,10 +11,17 @@
 
 
 class TestCategoricalIndexConstructors:
+    def test_construction_without_data_deprecated(self):
+        # Once the deprecation is enforced, we can add this case to
+        # test_construction_disallows_scalar
+        msg = "without passing data"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            CategoricalIndex(categories=list("abcd"), ordered=False)
+
     def test_construction_disallows_scalar(self):
         msg = "must be called with a collection of some kind"
         with pytest.raises(TypeError, match=msg):
-            CategoricalIndex(categories=list("abcd"), ordered=False)
+            CategoricalIndex(data=1, categories=list("abcd"), ordered=False)
 
     def test_construction(self):
 

pandas/tests/indexes/common.py

@@ -47,11 +47,17 @@ def create_index(self) -> Index:
 
     def test_pickle_compat_construction(self):
         # need an object to create with
-        msg = (
-            r"Index\(\.\.\.\) must be called with a collection of some "
-            r"kind, None was passed|"
-            r"__new__\(\) missing 1 required positional argument: 'data'|"
-            r"__new__\(\) takes at least 2 arguments \(1 given\)"
+        msg = "|".join(
+            [
+                r"Index\(\.\.\.\) must be called with a collection of some "
+                r"kind, None was passed",
+                r"DatetimeIndex\(\) must be called with a collection of some "
+                r"kind, None was passed",
+                r"TimedeltaIndex\(\) must be called with a collection of some "
+                r"kind, None was passed",
+                r"__new__\(\) missing 1 required positional argument: 'data'",
+                r"__new__\(\) takes at least 2 arguments \(1 given\)",
+            ]
         )
         with pytest.raises(TypeError, match=msg):
             self._index_cls()

pandas/tests/indexes/datetimes/test_datetimelike.py

@@ -32,9 +32,6 @@ def test_format(self, simple_index):
     def test_shift(self):
         pass  # handled in test_ops
 
-    def test_pickle_compat_construction(self):
-        pass
-
     def test_intersection(self):
         pass  # handled in test_setops
 

pandas/tests/indexes/period/test_period.py

@@ -35,8 +35,9 @@ def simple_index(self) -> Index:
     def index(self, request):
         return request.param
 
+    @pytest.mark.xfail(reason="Goes through a generate_range path")
     def test_pickle_compat_construction(self):
-        pass
+        super().test_pickle_compat_construction()
 
     @pytest.mark.parametrize("freq", ["D", "M", "A"])
     def test_pickle_round_trip(self, freq):

pandas/tests/indexes/timedeltas/test_timedelta.py

@@ -42,9 +42,6 @@ def test_numeric_compat(self):
     def test_shift(self):
         pass  # this is handled in test_arithmetic.py
 
-    def test_pickle_compat_construction(self):
-        pass
-
     def test_pickle_after_set_freq(self):
         tdi = timedelta_range("1 day", periods=4, freq="s")
         tdi = tdi._with_freq(None)