BUG: `pd.api.types.is_string_dtype()` returns inconsistent results for Categorical series vs dtype

[nsarang]

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Reproducible Example

import pandas as pd

series = pd.Categorical(['A', 'B', 'C'])
print(f"is_string_dtype(series): {pd.api.types.is_string_dtype(series)}") # True  
print(f"is_string_dtype(series.dtype): {pd.api.types.is_string_dtype(series.dtype)}") # False

Issue Description

pd.api.types.is_string_dtype() returns inconsistent results when passed a Categorical series directly versus the dtype of that series.

Expected Behavior

IMO, both calls should return False since a Categorical is not a string dtype.

Installed Versions

INSTALLED VERSIONS

commit : c888af6
python : 3.11.0
python-bits : 64
OS : Darwin
OS-release : 24.3.0
Version : Darwin Kernel Version 24.3.0: Thu Jan 2 20:24:23 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T8122
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 2.3.1
numpy : 2.3.1
pytz : 2025.2
dateutil : 2.9.0.post0
pip : 25.0.1
Cython : None
sphinx : None
IPython : 9.4.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.13.4
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : 2025.3.0
html5lib : None
hypothesis : 6.138.0
gcsfs : 2025.3.0
jinja2 : 3.1.6
lxml.etree : None
matplotlib : 3.10.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 19.0.1
pyreadstat : None
pytest : 8.3.5
python-calamine : None
pyxlsb : None
s3fs : 2025.3.0
scipy : 1.15.2
sqlalchemy : 2.0.39
tables : None
tabulate : 0.9.0
xarray : 2025.6.1
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None

[rhshadrach]

Thanks for the report! object is also not a string dtype, yet

series = pd.Series(["x", "y"], dtype="object")
print(pd.api.types.is_string_dtype(series))
# True

I had thought the intention here was "Does the array contain strings and only strings", so I would expect these to return True.

cc @jbrockmendel @mroeschke @jorisvandenbossche

[jbrockmendel]

i guess that's reasonable, though ideally we'd avoid value-dependent behavior

[rhshadrach]

Agreed that after some time we start moving away from treating object as the defacto string dtype if it has all strings, and can change both object and Categorical behavior here. But I think it'd be best to do them at the same time.