So today I came across a situation where I need to use ternary conditional operator in list comprehensions. My friend worked it out for me and I feel like it’s better to note it down.
List Comprehensions
List comprehensions provide a concise way to create lists in python. Instead of a for loop, we can write simply one line of code like this:
l = [-1, 2, 3, 5, 10]
a = [e+1 for e in l]
print(a) # [0, 3, 4, 6, 11]
Iterator filter
On top of it, we can even add an if statement to the list comprehension.
b = [e for e in l if e>2]
print(b) # [3, 5, 10]
if here is an iterator filter. This is cool. But the problem is: what if you want to keep elements that are larger than 2, and, for example, multiply the other elements by 2 and make a new list?
Ternary conditional operator
Now what we need is a ternary conditional operator.
c = [e if e>2 else e*2 for e in l]
print(c) # c = [-2, 4, 3, 5, 10]
The ternary conditional operator writes in general:
<expression1> if <condition> else <expression2>
And you can go further with nested if-elif-else-like statements:
c = [e if e>2 else e*2 if e>0 else e+10 for e in l]
print(c) # [9, 4, 3, 5, 10]
Why position matters
As you may notice, when we use only the if statement, it’s positioned after the for-in iterator. It’s because the iterator filter if can be only used on iterators or iteration generators, which is a list here. But a ternary conditional operator can’t be used on iterators since an iterator can’t be evaluated.
Performance
My friend also told me that it’s quicker to iterate over a set instead of a list. Iterating over a set needs constant time O(1) per step only. Good catch!