title: See if *any* characters are in string and remove them
body: <p>So I have a simple program in Python 3 that is just a basic calculator.
  here is a section of code:</p>
  
  <pre><code>calculation = input("What calculation do you want to do?\n")
  if "+" in calculation:
      numbers = calculation.split("+")
      answer = int(numbers[0]) + int(numbers[1])
  </code></pre>
  
  <p>I have some other operations setup below it.
  The issue is that if someone was to input anything as well as the operation (e.g. 10++2 or 10+abc2), the code throws back an error because it obviously can't add 10 to abc2.</p>
  
  <p>I thought that I could solve the issue by testing for each individual character but surely that is a long way around the issue. Is there a way to solve the issue using Python?</p>