I'm composing a Python list from an input list run through a transforming function. I would like to include only those items in the output list for which the result isn't None. This works:
def transform(n):
# expensive irl, so don't execute twice
return None if n == 2 else n**2
a = [1, 2, 3]
lst = []
for n in a:
t = transform(n)
if t is not None:
lst.append(t)
print(lst)
[1, 9]
I have a hunch that this can be simplified with a comprehension. However, the straighforward solution
def transform(n):
return None if n == 2 else n**2
a = [1, 2, 3]
lst = [transform(n) for n in a if transform(n) is not None]
print(lst)
is no good since transform() is applied twice to each entry. Any way around this?
Use the := operator for python >=3.8.
lst = [t for n in a if (t:= transform(n)) is not None]