I don't fully understand how the any() function in this script works

Question

I have a script that checks if there are one or more of the same items in a list. Here's the code:

items = ["Blue", "Black", "Red"]

def isUnique(item):
    seen = list()
    return not any(i in seen or seen.append(i) for i in item)


print(isUnique(items))

It prints "True" if all the items in the given list are unique and "False" if one or more items in the list are unique. Can someone please explain the any() part of the script for me as I don't fully understand how it works?

Answer 1

This code is kind of a hack, since it uses a generator expression with side-effects and exploits the fact that append returns None, which is falsy.

The equivalent code written in the imperative style is like so:

def isUnique(items):
    seen = list()
    for i in items:
        if i in seen or seen.append(i):
            return False
    return True

The or is still a bit strange there - it is being used for its short-circuiting behaviour, so that append is only called when i in seen is false - so we could rewrite it like this:

def isUnique(items):
    seen = list()
    for i in items:
        if i in seen:
            return False
        else:
            seen.append(i)
    return True

This is equivalent because append is only called when i in seen is false, and the call to append returns None which means the return False line shouldn't execute in that case.