Basic calculator

compute.py

import operator
def compute(expression):
    fn = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.div
    }

    # could use fn.keys but dictionaries are orderless
    operations = ['+', '-', '*', '/']
    for operation in operations:
        needle = expression.find(operation)
        if needle >= 0:
            return fn[expression[needle]](compute(expression[:needle]), compute(expression[needle + 1:]))
    try:
        return float(expression)
    except:
        raise SyntaxError