op_solver.py

from operator import mul, add, sub, truediv
from itertools import product, permutations

ops = [
  [ mul, "{a} x {b}" ],
  [ add, "{a} + {b}" ],
  [ sub, "{a} - {b}" ],
  [ lambda a, b: sub(b, a), "{b} - {a}" ],
  [ truediv, "{a} / {b}" ],
  [ lambda a, b: truediv(b, a), "{b} / {a}" ],
]

# Implement search for all possible combinations with an operand stack (starting with input values in some order)
# and an operator stack. Apply the operators from the stack to the last two values, pushing on the result, until
# there is just one value remaing 

def calc_result(values, ops):
  values = list(values)
  ops = list(ops)
  top_str = str(values[-1])
  while len(values) > 1:
    a = values.pop()
    b = values.pop()
    op = ops.pop()
    try:
      r = op[0](a, b)
      top_str = "({})".format(op[1].format(a=top_str, b=str(b)))
      values.append(r)
    except ZeroDivisionError:
      return None, ""
  return values[0], top_str

# Example use: print "10 = {}".format(find_op([1, 1, 5, 8], 10))
def find_op(values, answer):
  for ordered_values in permutations(values):
    for operations in product(ops, repeat=len(values)-1):
      result, formula = calc_result(ordered_values, operations)
      if result == answer:
        return formula
      if result and abs(result - answer) < .001:
        return "~ {}".format(formula)
  return False