BUG: Nonintuitive behaviour when indexing with a constant boolean function

[onnoeberhard]

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• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample, a copy-pastable example

import pandas as pd
series = pd.Series([1, 2, 3])
x = series[lambda s: s > 2]    # Indexing with a boolean function, works like a charm
y = series[lambda s: True]     # Indexing with a constant boolean function --> results in KeyError: True

Problem description

I would expect y to be equal to series in this example, because the callable function passed to __getitem__ returns True for all items in the Series. The same error arises when using a DataFrame. I know in this example, it seems like the function would be useless, because it would just return series, but in my use case the function that is passed to __getitem__ is actually a 'filter' function argument, and it would be very convenient to just turn off the filter by passing lambda df: True as the filter function.

A quick workaround I found useful is:

import numpy as np
y = series[lambda s: np.full(len(s), True)]

Output of pd.show_versions()

INSTALLED VERSIONS

commit : d9fff27
python : 3.8.3.final.0
python-bits : 64
OS : Linux
OS-release : 5.3.0-1030-aws
Version : #32~18.04.1-Ubuntu SMP Tue Jun 30 23:04:16 UTC 2020
machine : x86_64
processor :
byteorder : little
LC_ALL : en_US.UTF-8
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.1.0
numpy : 1.19.0
pytz : 2020.1
dateutil : 2.8.1
pip : 20.1.1
setuptools : 47.1.1
Cython : 0.29.21
pytest : 5.4.3
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : 1.2.9
lxml.etree : None
html5lib : None
pymysql : 0.9.3
psycopg2 : 2.8.5 (dt dec pq3 ext lo64)
jinja2 : 2.11.2
IPython : 7.16.1
pandas_datareader: None
bs4 : 4.9.1
bottleneck : None
fsspec : 0.7.4
fastparquet : None
gcsfs : None
matplotlib : 3.2.2
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.4
pandas_gbq : None
pyarrow : 0.17.1
pytables : None
pyxlsb : None
s3fs : 0.4.2
scipy : 1.4.1
sqlalchemy : 1.3.13
tables : 3.6.1
tabulate : 0.8.7
xarray : 0.15.1
xlrd : 1.2.0
xlwt : None
numba : 0.50.1

[dsaxton]

The lambda function takes the whole Series as input (it's not a "mapping" operation that returns a Series of True values) and needs to return a valid indexer. In this case the scalar True is not a valid index so an error is raised. It seems you're expecting it to behave like

series[lambda s: s.map(lambda x: True)]

but this isn't the same thing.

[simonjayhawkins]

closing as answered. Thanks @onnoeberhard for the report. Thanks @dsaxton for the reply.