infix to postfix.py

OPERATORS = set(['+', '-', '*', '/', '(', ')', '^'])  # set of operators
PRIORITY = {'+':1, '-':1, '*':2, '/':2, '^':3} # dictionary having priorities 
def infix_to_postfix(expression): #input expression
    stack = [] # initially stack empty
    output = '' # initially output empty
    for ch in expression:
        if ch not in OPERATORS:  # if an operand then put it directly in postfix expression
            output+= ch
        elif ch == '(' :  # else operators should be put in stack

            stack.append('(')
        elif ch==')':
            while stack and stack[-1]!= '(':
                output+=stack.pop()
            stack.pop()
        else:
            # lesser priority can't be on top on higher or equal priority    
            # so pop and put in output   
            while stack and stack[-1]!='(' and PRIORITY[ch]<=PRIORITY[stack[-1]]:  ## Comparing the operator with the top of stack 
                output+=stack.pop()  ## pop the lesser priority operator and show it in output
            stack.append(ch)  ## PUSH the higher priority operator onto stack
    while stack:
        output+=stack.pop()  ## Pop each element from the stack one by one and show them in output at the end 
    return output  ## Return the postfix expression
expression = input('Enter infix expression')   ## enter infix expression
print('infix expression: ',expression)   ## printing the intfix expression  
print('postfix expression: ',infix_to_postfix(expression))  ## printing the post fix expression