Description
Pandas version checks
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I have checked that this issue has not already been reported.
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I have confirmed this bug exists on the latest version of pandas.
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I have confirmed this bug exists on the main branch of pandas.
Reproducible Example
import pandas as pd
# Create two DataFrames with MultiIndex
index1 = pd.MultiIndex.from_tuples([('A', 'one'), ('A', 'two')])
index2 = pd.MultiIndex.from_tuples([('B', 'one'), ('B', 'two')])
df1 = pd.DataFrame([[1, 2]], columns=index1)
df2 = pd.DataFrame([[3, 4]], columns=index2)
# Adding DataFrames with different MultiIndex
result = df1.add(df2, fill_value=0)
print(result)Issue Description
I have two data frames with unaligned multi-indices. When adding the two data frames, with fill_value=0, I'd expect the missing values to be replaced with zero before performing the addition operation, as described in the documentation of DataFrame.add. However, the above example produces this output:
A B
one two one two
0 NaN NaN NaN NaN
The problem affects the current main branch (tested with commit e557039). No released version is affected (yet). The behavior was introduced by #60538. Before this PR, the code was producing the expected output.
Expected Behavior
I'd expect this output:
A B
one two one two
0 1.0 2.0 3.0 4.0
Installed Versions
pandas : 3.0.0.dev0+1839.g72fd708761
numpy : 1.26.4
dateutil : 2.9.0.post0
pip : 24.2
Cython : 3.0.11
sphinx : 8.1.3
IPython : 8.30.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
blosc : None
bottleneck : 1.4.2
fastparquet : 2024.11.0
fsspec : 2024.10.0
html5lib : 1.1
hypothesis : 6.122.4
gcsfs : 2024.10.0
jinja2 : 3.1.4
lxml.etree : 5.3.0
matplotlib : 3.10.0
numba : 0.60.0
numexpr : 2.10.2
odfpy : None
openpyxl : 3.1.5
psycopg2 : 2.9.10
pymysql : 1.4.6
pyarrow : 18.1.0
pyreadstat : 1.2.8
pytest : 8.3.4
python-calamine : None
pytz : 2024.2
pyxlsb : 1.0.10
s3fs : 2024.10.0
scipy : 1.14.1
sqlalchemy : 2.0.36
tables : 3.10.1
tabulate : 0.9.0
xarray : 2024.9.0
xlrd : 2.0.1
xlsxwriter : 3.2.0
zstandard : 0.23.0
tzdata : 2024.2
qtpy : None
pyqt5 : None