Filter list using Boolean index arrays

Question

How can I use boolean inddex arrays to filter a list without using numpy?

For example:

>>> l = ['a','b','c']
>>> b = [True,False,False]
>>> l[b]

The result should be:

['a']

I know numpy support it but want to know how to solve in Python.

>>> import numpy as np
>>> l = np.array(['a','b','c'])
>>> b = np.array([True,False,False])
>>> l[b]
array(['a'], 
      dtype='|S1')

Answer 1

Python does not support boolean indexing but the itertools.compress function does exactly what you want. It return an iterator with means you need to use the list constructor to return a list.

>>> from itertools import compress
>>> l = ['a', 'b', 'c']
>>> b = [True, False, False]
>>> list(compress(l, b))
['a']

Answer 2

[a for a, t in zip(l, b) if t]
# => ["a"]

A bit more efficient, use iterator version:

from itertools import izip
[a for a, t in izip(l, b) if t]
# => ["a"]

EDIT: user3100115's version is nicer.