Detect if characters in string are in list1 or list2

I want to be able to search a string for characters in different lists.

For example:

list1=['a']
list2=['b']

If a user enters in 'a' for example. Python should print 'list1' and for 'b' list2.

Right now i'm trying to use this function to query the list, but I have to query each list in two separate if statements, like this.

def char_search(input_str):
    for char in input_str:
        if char in list1:
            print("lower")
        if char in list2:
            print("Upper")

I want to be able to combine the two if statements, and only print something if both conditions are True.