def swip_swap(source, c1, c2):
    """
    This function takes a string source and characters c1 and c2
    and returns the string source with all occurrences of c1 and c2 swapped.
    """
    result = ""
    for i in source:
        if i is c1:
            result += c2
        elif i is c2:
            result += c1
        else:
            result += i
    return result