Someone please explain this one. I’m going crazy.

edit: BTW - same behaviour on python3 (3.4.4).

For those that can’t stand video for reading code, the code in question is:

class A(object):
    def __getitem__(self, item):
        print item
        return item

a = A()
if "A" in a:
    print "YEP"

A few things to remember:

__getitem__(...)
    x.__getitem__(y) <==> x[y]

With that in mind, remember that "a" in x is roughly equivalent to:

i = 0
try:
    while True:
        if x[i] == "a":
            return True
        else:
            i += 1
except IndexError:
    return False

The original A implementation never bothers to validate the position of getitem