BUG: Change IntervalDtype.kind from None to "O"

doc/source/whatsnew/v1.0.0.rst

@@ -843,6 +843,7 @@ Interval
 
 - Bug in :meth:`IntervalIndex.get_indexer` where a :class:`Categorical` or :class:`CategoricalIndex` ``target`` would incorrectly raise a ``TypeError`` (:issue:`30063`)
 - Bug in ``pandas.core.dtypes.cast.infer_dtype_from_scalar`` where passing ``pandas_dtype=True`` did not infer :class:`IntervalDtype` (:issue:`30337`)
+- Bug in :class:`IntervalDtype` where the ``kind`` attribute was incorrectly set as ``None`` instead of ``"O"`` (:issue:`30568`)
 
 Indexing
 ^^^^^^^^

pandas/core/dtypes/common.py

@@ -633,7 +633,14 @@ def is_string_dtype(arr_or_dtype) -> bool:
 
     # TODO: gh-15585: consider making the checks stricter.
     def condition(dtype) -> bool:
-        return dtype.kind in ("O", "S", "U") and not is_period_dtype(dtype)
+        return dtype.kind in ("O", "S", "U") and not is_excluded_dtype(dtype)
+
+    def is_excluded_dtype(dtype) -> bool:
+        """
+        These have kind = "O" but aren't string dtypes so need to be explicitly excluded
+        """
+        is_excluded_checks = (is_period_dtype, is_interval_dtype)
+        return any(is_excluded(dtype) for is_excluded in is_excluded_checks)
 
     return _is_dtype(arr_or_dtype, condition)
 

pandas/core/dtypes/dtypes.py

@@ -974,7 +974,7 @@ class IntervalDtype(PandasExtensionDtype):
     """
 
     name = "interval"
-    kind: Optional[str_type] = None
+    kind: str_type = "O"
     str = "|O08"
     base = np.dtype("O")
     num = 103

pandas/tests/dtypes/test_dtypes.py

@@ -685,6 +685,10 @@ def test_caching(self):
         tm.round_trip_pickle(dtype)
         assert len(IntervalDtype._cache) == 0
 
+    def test_not_string(self):
+        # GH30568: though IntervalDtype has object kind, it cannot be string
+        assert not is_string_dtype(IntervalDtype())
+
 
 class TestCategoricalDtypeParametrized:
     @pytest.mark.parametrize(

pandas/tests/extension/base/dtype.py

@@ -16,8 +16,7 @@ def test_name(self, dtype):
 
     def test_kind(self, dtype):
         valid = set("biufcmMOSUV")
-        if dtype.kind is not None:
-            assert dtype.kind in valid
+        assert dtype.kind in valid
 
     def test_construct_from_string_own_name(self, dtype):
         result = dtype.construct_from_string(dtype.name)