boolean selection with loc in python

Question

s = pd.Series(np.nan, index=[49,48,47,46,45, 1, 2, 3, 4, 5])

output of s is

49   NaN
48   NaN
47   NaN
46   NaN
45   NaN
1    NaN
2    NaN
3    NaN
4    NaN
5    NaN

s.loc[[False,True]]

it gives output as-:

48   NaN

.loc Access a group of rows and columns by label(s), I have given list of false and true and it is also not equal to length of axis being sliced. My doubt is if we gave list of boolean array to loc it slice the dataframe/series with position instead of label?

s.loc[[False,True]] does not (or at least, should not) return what you stated — rafaelc
– rafaelc, Commented Dec 9, 2019 at 17:43

erncyp · Accepted Answer · 2019-12-09 18:15:32Z

1

I certainly get an error when I am running this:

import pandas as pd
import numpy as np
s = pd.Series(np.nan, index=[49,48,47,46,45, 1, 2, 3, 4, 5])
s.loc[[False,True]]

the error (as expected) is:

IndexError: Item wrong length 2 instead of 10.

Maybe your problem is specific to a certain version of pandas? Maybe an old one? I used pandas version 0.25.3

edited Dec 9, 2019 at 18:15

answered Dec 9, 2019 at 17:57

erncyp

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1 Comment

Iguananaut Over a year ago

That used to work but it was a bug which was fixed: github.com/pandas-dev/pandas/pull/26911 What OP is doing is essentially equivalent to s.loc[[False, True] + [False] * (len(s) - 2)] which in fact returns only the second element in the series, and I'm not sure what else they were expecting here.

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