Fixes is_string_dtype (#34294)

Author: ieaves

doc/source/whatsnew/v1.1.0.rst

@@ -341,6 +341,7 @@ Backwards incompatible API changes
   will now result in a float column instead of an object dtyped column (:issue:`33607`)
 - :meth:`Series.to_timestamp` now raises a ``TypeError`` if the axis is not a :class:`PeriodIndex`. Previously an ``AttributeError`` was raised (:issue:`33327`)
 - :meth:`Series.to_period` now raises a ``TypeError`` if the axis is not a :class:`DatetimeIndex`. Previously an ``AttributeError`` was raised (:issue:`33327`)
+- :func: `pandas.api.dtypes.is_string_dtype` no longer incorrectly identifies categorical series as string.
 
 ``MultiIndex.get_indexer`` interprets `method` argument differently
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

pandas/core/dtypes/common.py

@@ -599,7 +599,7 @@ 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)
+        is_excluded_checks = (is_period_dtype, is_interval_dtype, is_categorical_dtype)
         return any(is_excluded(dtype) for is_excluded in is_excluded_checks)
 
     return _is_dtype(arr_or_dtype, condition)

pandas/tests/dtypes/test_dtypes.py

@@ -191,6 +191,10 @@ def test_dtype_specific_categorical_dtype(self):
         result = str(Categorical(DatetimeIndex([])).categories.dtype)
         assert result == expected
 
+    def test_not_string(self):
+        # though CategoricalDtype has object kind, it cannot be string
+        assert not is_string_dtype(CategoricalDtype())
+
 
 class TestDatetimeTZDtype(Base):
     @pytest.fixture