How do I run the program

Quote:

Every time I run it, it says return outside function. What do I do?

chars = "ACGT"

def neighbors(pattern, d):
    assert(d <= len(pattern))

    if d == 0:
        return [pattern]
    
    r2 = neighbors(pattern[1:], d-1)
    r = [c + r3 for r3 in r2 for c in chars if c != pattern[0]]
    
    if (d < len(pattern)):
        r2 = neighbors(pattern[1:], d)
        r += [pattern[0] + r3 for r3 in r2]
    
    return r