Python 2.7: Using reduce to verify that elements are in a list

Question

While experimenting with the reduce function, I've observed behavior that I can't explain to myself. Say, there are 2 lists:

a = ["a", "b", "c", "z"]
b = ["b", "z", "a"]

I woud like to verify whether all elements of the list b are in the list a using reduce . So, I try:

reduce(lambda x,y: (x in a) and (y in a), b)

and get False instead of expected True .

So, why do I get False ?

PS: I know that there are other ways to verify if all list's elements are in another list, fi using sets and issuperset . I just wonder why the reduce function works this way.

Answer 1

Your code:

b = ["b", "z", "a"]
reduce(lambda x,y: (x in a) and (y in a), b)

is equivalent to:

(("b" in a) and ("z" in a)) in a and ("a" in a)

which calculates to:

(True in a) and ("a" in a)