problem3.py

import sys
from itertools import product

with open(sys.argv[1], "rt") as fh:
    digits = {}
    specials = set()
    for i, line in enumerate(fh):
        line = line.rstrip()
        begin = None
        l = ""
        for j, c in enumerate(line):
            if c.isdigit():
                l += c
                if begin is None:
                    begin = j
            else:
                if begin is not None:
                    digits[int(l)] = (i, begin, j)
                    begin = None
                    l = ""
                if c != ".":
                    specials.add((i, j))

res = 0
for k, (row, start, end) in digits.items():
    for r, c in product([row-1, row, row+1], range(start-1, end+1)):
        if (r, c) in specials:
            res += k
            break
print(res)