Convert an OSGB grid ref with a quadrant to one without (TQ27SE to TQ2570)

quad-to-num.py

def quad2num(ref):
    ref = ref.upper()
    grid = ref[:2]
    assert grid.isalpha()
    nums = ref[2:-2]
    if nums:
        assert nums.isdigit()
    quad = ref[-2:]
    assert quad.isalpha()
    num_nums = len(nums)
    assert num_nums % 2 == 0
    nums_x = nums[:int(num_nums/2)]
    nums_y = nums[int(num_nums/2):]
    quad_lookup = {"N": "5", "E": "5", "S": "0", "W": "0"}
    quad_x = quad_lookup[quad[1]]
    quad_y = quad_lookup[quad[0]]
    return grid + nums_x + quad_x + nums_y + quad_y


if __name__ == '__main__':
    ref = input().strip()
    print(quad2num(ref), end="")