(Codewars): Find the Unknown Digit
In this code, I find it misleading that n is first assigned zeros,
and then its content is replaced with strings.


n = [0]*3
n[0], op, n[1], x, n[2] = parse_op.search(s).groups()



I think it would be more clear this way:

num1, op, num2, _, num3 = parse_op.search(s).groups()
nums = [num1, num2, num3]


Notice that I replaced x with _. It's a common convention when a variable is unused.

Some doctests with example inputs and expected outputs would be great too,
making the code easier to digest, and play with.