parsing - Postfix expression parser in Python -

i trying complete program running few things need with. need trying to: make sure user input correct , error test. like.....

if token == "" or token == " " 

i not sure how make sure user input correct.

assignment:

write program (postfix.py) extracts postfix expression user , evaluates described in class. program should extract expression user via standard input. expression should formatted such there @ least 1 blank space between each operator , operand. example,

enter postfix expression: 5 6 * 13 2 * + assume operators , operands valid. operands should integer values while valid operators include + - / * %

after extracting expression, use split() method on string split individual tokens stored in python list. note multi-digit operand (i.e. 13) considered single token.

the postfix expression should printed standard output, 1 token @ time, being evaluated. in addition, if expression valid, result should printed on next line following expression shown below

5 6 * 13 2 * + = 56 

if expression invalid, stop evaluating expression once determined there error , print 3 dollar signs. print error message on next line indicate error. example, if there many operators given operands encountered, program should produce results shown below

enter postfix expression: 17 6 / + 8 * 17 6 / + $$$ error: insufficient number of operands. 

when there few operators given operands, program should produce results shown below

enter postfix expression: 17 6 / 25 5 + 17 6 / 25 5 + $$$ error: insufficient number of operators. 

my code:

  input("enter postfix expression: ")   operorlist("*, /, -, +, %")    def postfixeval(postfixexpr):      operandstack = stack()      tokenlist = postfixexpr.split()       token in tokenlist:         if token in "0123456789":           operandstack.push(int(token))      else:         operand2 = operandstack.pop()         operand1 = operandstack.pop()         result = domath(token,operand1,operand2)         operandstack.push(result)      return operandstack.pop()   def domath(op, op1, op2):      if op == "*":       return op1 * op2      elif op == "/":       return op1 / op2      elif op == "+":       return op1 + op2      elif op == "%":       return op1 % op2      else:       return op1 - op2 

the assignmment tells

assume operators , operands valid. operands should integer values while valid operators include + - / * %

thus 1 can assume nothing else input, , errors happen many or not enough operands on stack, need test those.

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the line

if token in "0123456789": 

only checks if whole token substring of 0123456789, a.k.a single digit, 01, 4567 or so.

what want instead check whether token digits:

if token.isdigit():     ... 

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as too few operands, either 1 of pops throw exception indexerror; you'd catch , report error:

try:     operand2 = operandstack.pop()     operand1 = operandstack.pop()         except indexerror:     # handle appropriately 

similarly many operands mean operandstack has more 1 item @ end, check len(operandstack); if greater 1 report "too many operands".