Question

When I try to filter [1,2,0,3,8] with if x < 3: return x I end up with [1,2]. Why is the 0 not included in this list?

def TestFilter(x):
    if x < 3: 
        return x

a = [1,2,0,3,8]
b = filter(TestFilter, a)
print b

Answer 1

Every time your function returns True filter() will add the current element from the original list to the new list. Python considers 0 to be False and any other number to be True. Therefore you will want to have the function return True instead of the number.

def TestFilter(x):
    if x < 3: 
       return True

EDIT: Here’s a lambda example:

a = [1, 2, 3, 0, 4, 8]
print filter(lambda x: x < 3, a)