BUG: conversion from Int64 to string introduces decimals in Pandas 2.2.0

[mplatzer]

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 pandas as pd
pd.Series([12, pd.NA], dtype="Int64[pyarrow]").astype("string[pyarrow]")

yields

0    12.0
1    <NA>
dtype: string

Issue Description

Converting from Int64[pyarrow] to string[pyarrow] introduces float notation, if NAs are present. I.e. 12 will be converted to "12.0". This bug got introduced in Pandas 2.2.0, is still present on Pandas 2.2.x branch, but is not present on Pandas main branch.

Expected Behavior

import pandas as pd
x = pd.Series([12, pd.NA], dtype="Int64[pyarrow]").astype("string[pyarrow]")
assert x[0] == "12"

Installed Versions

INSTALLED VERSIONS

commit : fd3f571
python : 3.11.7.final.0
python-bits : 64
OS : Darwin
OS-release : 23.2.0
Version : Darwin Kernel Version 23.2.0: Wed Nov 15 21:59:33 PST 2023; root:xnu-10002.61.3~2/RELEASE_ARM64_T8112
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 2.2.0
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.8.2
setuptools : 69.0.3
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 : 15.0.0
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

[rhshadrach]

Thanks for the report. @mplatzer - can you give this issue a meaningful title? Currently it is BUG:.

[mplatzer]

@rhshadrach sorry, my bad, I've overlooked that. A meaningful title has now been added.

[rhshadrach]

Confirmed on main. Further investigations and PRs to fix are welcome.

[mplatzer]

I suspect the issue is related to this change (taken from the 2.2.0 release notes):

integer dtypes with missing values are cast to NumPy float dtypes and NaN is used as missing value indicator
but wouldn't know any further details

[rhshadrach]

This bug... is not present on Pandas main branch.

I'm still seeing it on main. Can you recheck, make sure we're seeing the same thing?

[rhshadrach]

Result of a git bisect:

Author: Patrick Hoefler
Date:   Thu Dec 21 23:15:49 2023 +0100

    Convert ArrowExtensionArray to proper NumPy dtype (#56290)

cc @phofl

[mplatzer]

yes, you are right, it's still happening for main

>>> import pandas as pd
>>> pd.__version__
'3.0.0.dev0+355.g92a52e231'
>>> pd.Series([12, pd.NA], dtype="Int64[pyarrow]").astype("string[pyarrow]")
0    12.0
1    <NA>