BUG: itemsize wrong for date32[day][pyarrow] dtype?

[MarcoGorelli]

Pandas version checks

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Reproducible Example

import pyarrow as pa
import pandas as pd

pd.ArrowDtype(pa.date32())  # date32[day][pyarrow]
pd.ArrowDtype(pa.date32()).itemsize  # 8

Issue Description

I think it should show 4? pa.date32() is 32 bits, so 4 bytes

Expected Behavior

pd.ArrowDtype(pa.date32()).itemsize # 4

Installed Versions

INSTALLED VERSIONS

commit : b033ca9
python : 3.10.12.final.0
python-bits : 64
OS : Linux
OS-release : 5.15.146.1-microsoft-standard-WSL2
Version : #1 SMP Thu Jan 11 04:09:03 UTC 2024
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.1.0.dev0+761.gb033ca94e7
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.8.2
setuptools : 69.1.1
pip : 24.0
Cython : 3.0.8
pytest : 8.0.2
hypothesis : 6.98.15
sphinx : 7.2.6
blosc : None
feather : None
xlsxwriter : 3.2.0
lxml.etree : 5.1.0
html5lib : 1.1
pymysql : 1.4.6
psycopg2 : 2.9.9
jinja2 : 3.1.3
IPython : 8.22.1
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
bottleneck : 1.3.8
fastparquet : 2024.2.0
fsspec : 2024.2.0
gcsfs : 2024.2.0
matplotlib : 3.8.3
numba : 0.59.0
numexpr : 2.9.0
odfpy : None
openpyxl : 3.1.2
pyarrow : 15.0.2
pyreadstat : 1.2.6
python-calamine : None
pyxlsb : 1.0.10
s3fs : 2024.2.0
scipy : 1.12.0
sqlalchemy : 2.0.27
tables : 3.9.2
tabulate : 0.9.0
xarray : 2024.2.0
xlrd : 2.0.1
zstandard : 0.22.0
tzdata : 2024.1
qtpy : 2.4.1
pyqt5 : None

[MarcoGorelli]

Not sure what to do here, as there isn't a numpy dtype corresponding to date32

pandas/pandas/core/dtypes/dtypes.py

Lines 2215 to 2218 in 77f9d7a

	@cache_readonly
	def itemsize(self) -> int:
	"""Return the number of bytes in this dtype"""
	return self.numpy_dtype.itemsize

[mroeschke]

I think it's OK to override ArrowDtype.itemsize to handle this case separately

[jorisvandenbossche]

PyArrow data types of fixed width have a bit_width attribute that could be used here. That does raise for nested types, though at the moment we just return 8 from the numpy object dtype, which also doesn't necessarily makes sense.

In [13]: pd.ArrowDtype(pa.list_(pa.int32())).itemsize
Out[13]: 8

[longovin]

take

[echerrin]

take

[Charlie-H7]

I'm having a hard time trying to find what and where to find the relationship between numpy_dtype and the itemsize method. Going to the definition of itemsize method does not show an implementation of the method, so I'm not sure what it is doing.

pandas/pandas/core/dtypes/dtypes.py#L2213-L2216

Any help is appreciated