>>> ops = {u"раздели":"/", u"умножь":"*", u"сложи": "+"}
>>> import re
>>> seeking = re.compile(r"(.*)\s+(\d+)\s+.*(\d+)")
>>> def my_eval(in_oper):
... res = seeking.match(in_oper)
... if res:
... my_op = ops.get(res.group(1).lower(), None)
... if my_op:
... return eval('{}{}{}'.format(res.group(2), my_op, res.group(3)))
...
>>> print my_eval(u"Раздели 8 на 4")
2
>>> print my_eval(u"Умножь 3 на 4")
12
>>> print my_eval(u"Сложи 3 и 4")
7