Python list comprehension ignore None results?

Question

I have the following toy example function and list comprehension:

def foo(lst):
    if lst:
        return lst[0] + lst[1]

[foo(l) for l in [[], [1,2], [1,4]]]

The result is:

[None, 3, 4]

How can I avoid the Nones, I would like to avoid calling if foo(l) is not None inside the list comp. Please advise.

Answer 1

If you want to avoid calling the function more than once, you can make a generator that yields based on the result of the function. It's a little more code, but avoids making a list with a bunch of None values which have to be filtered later, and also avoids calling the function twice in the list comprehension:

def expensive_function(lst):
    # sometimes returns None...hard to know when without calling
    if lst:
        return lst[0] + lst[1]

def gen_results(l):
    for a in l:
        res = expensive_function(a)
        if res:
            yield res
    
inp = [[], [1,2], [1,4]]

list(gen_results(inp))
# [3, 5]

Also, since generators are lazy, you don't need to make a list if you don't need a list.