Python list comprehension ignore None results?-CodePudding

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.

CodePudding user response：

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.