input: Write a function to evaluate a postfix expression.
  assert expression_evaluator("7 4 * 2 + 1 -") == 29
  assert expression_evaluator("3 4 + 2 * 7 /") == 2.0
  assert expression_evaluator("5 1 2 + 4 * + 3 -") == 14
output: def expression_evaluator(expr):
      stack = []
      tokens = expr.split()
      
      for token in tokens:
          if token.isdigit() or (token.startswith('-') and token[1:].isdigit()):
              stack.append(int(token))
          else:
              b = stack.pop()
              a = stack.pop()
              if token == '+':
                  stack.append(a + b)
              elif token == '-':
                  stack.append(a - b)
              elif token == '*':
                  stack.append(a * b)
              elif token == '/':
                  stack.append(a / b)
      
      return stack[0]