Python operators

Question

I am learning Python for the past few days and I have written this piece of code to evaluate a postfix expression.

postfix_expression = "34*34*+"

stack = []

for char in postfix_expression :
    try :
     char = int(char);
     stack.append(char);
    except ValueError:
     if char == '+' :
      stack.append(stack.pop() + stack.pop())
     elif char == '-' :
      stack.append(stack.pop() - stack.pop())
     elif char == '*' :
      stack.append(stack.pop() * stack.pop())
     elif char == '/' :
      stack.append(stack.pop() / stack.pop())

print stack.pop()

Is there a way I can avoid that huge if else block? As in, is there module that takes a mathematical operator in the string form and invokes the corresponding mathematical operator or some python idiom that makes this simple?

Answer 1

The operator module has functions that implement the standard arithmetic operators. With that, you can set up a mapping like:

OperatorFunctions = {
    '+': operator.add,
    '-': operator.sub,
    '*': operator.mul,
    '/': operator.div,
    # etc
}

Then your main loop can look something like this:

for char in postfix_expression:
    if char in OperatorFunctions:
        stack.append(OperatorFunctions[char](stack.pop(), stack.pop()))
    else:
        stack.append(char)

You will want to take care to ensure that the operands to subtraction and division are popped off the stack in the correct order.

Answer 2

Just use eval along with string generation:

postfix_expression = "34*34*+"
stack = []
for char in postfix_expression:
    if char in '+-*/':
        expression = '%d%s%d' % (stack.pop(), char, stack.pop())
        stack.append(eval(expression))
    else:
        stack.append(int(char))
print stack.pop()

EDIT: made an even nicer version without the exception handling.

Answer 3

[untested]
from operator import add, sub, mul, div
# read the docs; this is a tiny part of the operator module

despatcher = {
    '+': add,
    '-': sub,
    # etc
    }

opfunc = despatcher[op_char]
operand2 = stack.pop()
# your - and / are bassackwards
stack[-1] = opfunc(stack[-1], operand2)