Python - using append in a def function, but return isn't used to give back anything

Question

int_list = [1,2,3,4]
mix_list = ["AB",7235,"C"]

def using_append(list1, list2):
    list1.append(list2)
    print(list1) # [1,2,3,4,["AB,7235,"C"]]

using_append(int_list, mix_list)
print(int_list)  # [1,2,3,4,["AB,7235,"C"]]  #why did int_list update? 

As seen in the program, within the function using_append, there was no return used. That said, when i do a print(int_list), shouldn't I expect to get [1,2,3,4] rather than [1,2,3,4,["AB,7235,"C"]]? How does this work?