BUG: failed to roundtrip to pyarrow Table with ArrowDtype for DictionaryArray

[wenjuno]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pyarrow as pa
import pandas as pd

tbl = pa.table({'a': pa.array(['a', 'b']).dictionary_encode()})
df = tbl.to_pandas(types_mapper=pd.ArrowDtype)
tbl2 = pa.Table.from_pandas(df)
df2 = tbl2.to_pandas(types_mapper=pd.ArrowDtype)

Issue Description

It seems the arrow specific dtype from pandas is carried over to the pyarrow Table (tbl2 in the example) and when trying to convert to pandas again (last line in the example constructing df2) it somehow cannot understand the metadata. I get this exception from it:

Traceback (most recent call last):
  File "test_pandas_roundtrip.py", line 7, in <module>
    df2 = tbl2.to_pandas(types_mapper=pd.ArrowDtype)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "pyarrow/array.pxi", line 883, in pyarrow.lib._PandasConvertible.to_pandas
  File "pyarrow/table.pxi", line 4251, in pyarrow.lib.Table._to_pandas
  File "/tmp/arrow/lib/python3.11/site-packages/pyarrow/pandas_compat.py", line 769, in table_to_dataframe
    ext_columns_dtypes = _get_extension_dtypes(
                         ^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/arrow/lib/python3.11/site-packages/pyarrow/pandas_compat.py", line 828, in _get_extension_dtypes
    pandas_dtype = _pandas_api.pandas_dtype(dtype)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "pyarrow/pandas-shim.pxi", line 147, in pyarrow.lib._PandasAPIShim.pandas_dtype
  File "pyarrow/pandas-shim.pxi", line 150, in pyarrow.lib._PandasAPIShim.pandas_dtype
  File "/tmp/arrow/lib/python3.11/site-packages/pandas/core/dtypes/common.py", line 1645, in pandas_dtype
    npdtype = np.dtype(dtype)
              ^^^^^^^^^^^^^^^
  File "/tmp/arrow/lib/python3.11/site-packages/numpy/core/_internal.py", line 176, in _commastring
    raise ValueError(
ValueError: format number 1 of "dictionary<values=string, indices=int32, ordered=0>[pyarrow]" is not recognized

Expected Behavior

I expect to get the same DataFrame as the first DataFrame df.

Installed Versions

INSTALLED VERSIONS

commit : bdc79c1
python : 3.11.8.final.0
python-bits : 64
OS : Linux
OS-release : 6.8.4-arch1-1
Version : #1 SMP PREEMPT_DYNAMIC Fri, 05 Apr 2024 00:14:23 +0000
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.2.1
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.9.0
setuptools : 69.2.0
pip : 24.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 15.0.2
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2024.1
qtpy : None
pyqt5 : None

[bretttully]

This looks the same as #53011

[wenjuno]

Hi @bretttully , thanks for pointing that out, I didn't notice that ticket. Yes I think that the root cause of the problem is probably the same. I think my example is narrower in the scope where there is no file operation involved, the problem is in roundtripping the data type between pandas and pyarrow. I guess any nontrivial or pyarrow specific data type will trigger the same problem. Thanks.