Comparing two lists in python if elements exist

Question

I have two lists where I want to check if elements from a exists in b

a=[1,2,3,4]
b=[4,5,6,7,8,1]

This is what I tried ( didn't work though!)

a=[1,2,3,4]
b=[4,5,6,7,3,1]

def detect(list_a, list_b):
    for item in list_a:
        if item in list_b:
            return True
    return False  # not found

detect(a,b)

I want to check if elements from a exists in b and should set a flag accordingly. Any thoughts?

Answer 1

Your code returns as soon as first element exists in both lists. To check all elements you could try this for example:

def detect(list_a, list_b):
    return set(list_a).issubset(list_b)

Other possibility, without creating a set:

def detect(list_a, list_b):
    return all(x in list_b for x in list_a)

And in case you were curious what exactly in your code is wrong, this is the fix in its current form (but it's not very pythonic):

def detect(list_a, list_b):
    for item in list_a:
        if item not in list_b:
            return False       # at least one doesn't exist in list_b

    return True   # no element found that doesn't exist in list_b
                  # therefore all exist in list_b

And finally, your function name isn't very readable. detect is too ambiguous. Consider other more verbose names, like isSubset etc.