Please explain (python): example_list[a < b]

I was reading Is there a numpy builtin to reject outliers from a list and came across a python list syntax which I am unfamiliar with.

Question: What does the the use of < or > do inside a list's []s?

e.g. example_list[a < b]

I played around in Terminal some but that didn't help me understand anything:

>>> ex = [1,2,3,4]
>>> ex[0<5]
2
>>> ex[0>5]
1
>>> ex[0>3]
1
>>> ex[0>0]
1
>>> ex[0<0]
1
>>> ex[1<0]
1
>>> ex[1<5]
2

Solution

< will return either True or False, and they are equal to 1 and 0 in Python. Hence you'll get either first or second item.

>>> True == 1
True
>>> False == 0
True
>>> 'ab'[True]
'b'
>>> 'ab'[False]
'a'

This thing was helpful in older versions of Python when the conditional expressions were not introduced:

>>> a = 'aa'
>>> b = 'bb'
>>> [10, 20][a>b]
10
>>> 20 if a > b else 10
10