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Description
Pandas version checks
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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
empty_ix = pd.MultiIndex.from_product([
pd.Index([], name='a', dtype=object),
pd.Index([], name='i', dtype='f4')
])
empty_ix.factorize() # Fails
# Names are lost
pd.MultiIndex.from_product([
pd.Index(['a', 'b'], name='a'),
pd.Index([0, 1, 0], name='i', dtype='f4')
]).factorize()[1].namesIssue Description
The first example should succeed, but actually it fails:
File ~/opt/conda/envs/mamba/envs/py3_2/lib/python3.11/site-packages/pandas/core/base.py:1203, in IndexOpsMixin.factorize(self, sort, use_na_sentinel)
1199 uniques = uniques.astype(np.float32)
1201 if isinstance(self, ABCIndex):
1202 # preserve e.g. MultiIndex
-> 1203 uniques = self._constructor(uniques)
1204 else:
1205 from pandas import Index
File ~/opt/conda/envs/mamba/envs/py3_2/lib/python3.11/site-packages/pandas/core/indexes/multi.py:222, in names_compat.<locals>.new_meth(self_or_cls, *args, **kwargs)
219 if "name" in kwargs:
220 kwargs["names"] = kwargs.pop("name")
--> 222 return meth(self_or_cls, *args, **kwargs)
File ~/opt/conda/envs/mamba/envs/py3_2/lib/python3.11/site-packages/pandas/core/indexes/multi.py:609, in MultiIndex.from_tuples(cls, tuples, sortorder, names)
607 if len(tuples) == 0:
608 if names is None:
--> 609 raise TypeError("Cannot infer number of levels from empty list")
610 # error: Argument 1 to "len" has incompatible type "Hashable";
611 # expected "Sized"
612 arrays = [[]] * len(names) # type: ignore[arg-type]
TypeError: Cannot infer number of levels from empty list
Probably because of the same underlying issue, MultiIndex.factorize always loses the names of the original index. It should preserve the original names instead.
Expected Behavior
First factorize() should return (np.array([], dtype=np.intp), empty_ix)
Second example should return ['a', 'i'] instead of [None, None]
Installed Versions
INSTALLED VERSIONS
------------------
commit : fd3f571
python : 3.11.6.final.0
python-bits : 64
OS : Linux
OS-release : 4.18.0-348.20.1.el8_5.x86_64
Version : #1 SMP Thu Mar 10 20:59:28 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8
pandas : 2.2.0
numpy : 1.26.3
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 69.0.3
pip : 23.3.2
Cython : None
pytest : 7.4.4
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : 3.1.9
lxml.etree : 5.1.0
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 3.1.3
IPython : 8.20.0
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
bottleneck : 1.3.7
dataframe-api-compat : None
fastparquet : None
fsspec : 2023.12.2
gcsfs : None
matplotlib : 3.8.2
numba : 0.58.1
numexpr : None
odfpy : None
openpyxl : 3.1.2
pandas_gbq : None
pyarrow : 14.0.2
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.12.0
sqlalchemy : None
tables : None
tabulate : None
xarray : 2024.1.0
xlrd : 2.0.1
zstandard : None
tzdata : 2023.4
qtpy : 2.4.1
pyqt5 : None