Combining 3 boolean masks in Python

Question

I have 3 lists:

a = [True, False, True]
b = [False, False, True]
c = [True, True, False]

When I type

a or b or c

I want to get back a list that's

[True, True, True]

but I'm getting back

[True, False, True]

Any ideas on why? And how can I combine these masks?

Answer 1

Your or operators are comparing the lists as entire objects, not their elements. Since a is not an empty list, it evaluates as true, and becomes the result of the or. b and c are not even evaluated.

To produce the logical OR of the three lists position-wise, you have to iterate over their contents and OR the values at each position. To convert a bunch of iterables into a list of their grouped elements, use zip(). To check if any element in an iterable is true (the OR of its entire contents), use any(). Do these two at once with a list comprehension:

mask = [any(tup) for tup in zip(a, b, c)]

Answer 2

How about this:

from numpy import asarray as ar

a = [True, False, True]
b = [False, False, True]
c = [True, True, False]

Try:

>>> ar(a) | ar(b) | ar(c)                  #note also the use `|` instead of `or`
array([ True, True, True], dtype=bool)

So no need for zip etc.