BUG: invalid result of reindex on columns after unstack with Period data #60980

[Pranav970]

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

series1 = pd.DataFrame(
   [(0, "s2", pd.Period(2022)), (0, "s1", pd.Period(2021))],
   columns=["A", "B", "C"]
).set_index(["A", "B"])["C"]

series2 = series1.astype(str)

print(series1.unstack("B").reindex(["s2"], axis=1))
print(series2.unstack("B").reindex(["s2"], axis=1))

Issue Description

The example code prints

B s2
A
0 2021

B s2
A  
0 2022

Expected Behavior

Expect the result for both pd.Period and str data to be 2022:

B s2
A  
0 2022

B s2
A  
0 2022

(actually observed with older Pandas 2.0.3)

Installed Versions

INSTALLED VERSIONS
commit : 0691c5c
python : 3.11.10
python-bits : 64
OS : Linux
OS-release : 6.2.16
Version : #1 SMP PREEMPT_DYNAMIC Tue Jan 1 00:00:00 UTC 1980
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 2.2.3
numpy : 2.2.3
pytz : 2025.1
dateutil : 2.9.0.post0
pip : 24.0
Cython : None
sphinx : None
IPython : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : None
lxml.etree : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : None
pyreadstat : None
pytest : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2025.1
qtpy : None
pyqt5 : None

[rhshadrach]

Thanks for the report! Confirmed on main. There has to be some internal state that gets set wrong. Constructing the unstacked result directly:

series1 = pd.DataFrame(
   [(0, "s2", pd.Period(2022)), (0, "s1", pd.Period(2021))],
   columns=["A", "B", "C"]
).set_index(["A", "B"])["C"]
unstacked = series1.unstack("B")
df = pd.DataFrame({"s1": pd.Period(2021), "s2": pd.Period(2022)}, index=pd.Index([0], name="A"))
df.columns.name = "B"
tm.assert_frame_equal(unstacked, df)

does not exhibit the buggy behavior either when using reindex. Further investigations and PRs to fix are welcome!

[rhshadrach]

@Pranav970 - why was this issue opened? It is identical to #60980.

[rhshadrach]

Closing this as the other issue was opened first.