Question

Why do the following two filter expressions return the same result?

A = [(1,(1,2,3))]
A1 = filter(lambda (a,b): b, A)
A2 = filter(lambda ab: ab, A)
A1 == A2
>>>> True

Answer 1

filter filters out arguments that when passed into the function, returns a False-ish value. Both (1, 2, 3) and (1, (1, 2, 3)) return True in a boolean context, and therefore remain in the returned list.

You want map instead.

A1 = map(lambda (a,b): b, A)
A2 = map(lambda ab: ab, A)

FYI, the follwing values are False-ish values, while everything else is True-ish:

0
None
False
''
[]
()
# and all other empty containers