Python Filter Not Working As Expected?

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

Solution 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:

0NoneFalse''
[]
()
# and all other empty containers

Solution 2:

It's not a bug. filter takes elements where your function returns True-y values.

In the first case, you unpack the tuple as 1 and (1,2,3) and you look at the second one ((1,2,3)) -- It's true (non-empty), so filter returns the whole thing.

In the second case, you look at the tuple (1,(1,2,3)). That's not empty either, so it returns the whole thing again.