title: Removing letters from a list of both numbers and letters
body: <p>In a function I'm trying to write, the user enters a bunch of numbers e.g. "648392".
  I turn this string into a list like this: ['6', '4', '8', '3', '9', '2'].</p>
  
  <p>I wanted to be able to do sums with these numbers so I was turning the numbers in the list into integers rather than strings. This all worked fine, however I also wanted the user to be able to enter letters, and then I would just remove them from the list - and this is where I'm stuck. For example a user entering "6483A2".</p>
  
  <p>I can't check to see if an element is a digit with isDigit because the elements apparently have to be integers first, and I can't convert the elements in the list to integers because some of the elements are letters... I'm sure there's a simple solution but I am pretty terrible at python, so any help would be much appreciated!</p>