problem1

def f(sz):
    assert sz, "Input cannot be empty/falsy"
    assert "__getitem__" in dir(sz), "Input must be subscribtable"
    result = []
    current = sz[0]
    count = 0;
    for c in sz:
        if c == current:
            count += 1
        else:
            result.append((current, count))
            current = c
            count = 1
    result.append((current, count))
    return result

if __name__ == "__main__":
    input = "aaaabbbcca"
    result = f(input)
    expected = [("a",4), ("b",3), ("c",2), ("a",1)]
    assert result == expected, "Should be {}".format(expected)
    print(result)