Understanding Python list with function as output and conditional

For some function that can return None or some other value and list of values:

def fn(x):
    if x == 4:
        return None
    else return x

lst = [1, 2, 3, 4, 5, 6, 7]

      

        
        
        
      

    

I want to have a list of outputs fn()

that do not return None

I have some code that works like this:

output = []
for i in lst:
    result = fn(i)
    if result:
        output.append(result)

      

        
        
        
      

    

I can express it as a list comprehension like this:

output = [fn(i) for i in lst if fn(i)]

      

        
        
        
      

    

but it runs fn(i)

on anything that doesn't return None

twice, which is undesirable if it's fn

an expensive feature.

Is there a way to have a good understanding of pythonic without executing the function twice?

Possible Solution:

output = [fn(i) for i in lst]
output = [o for o in f if o]