title: Subtracting one list from another in Python
body: <p><strong>What I want to happen:</strong> When given two lists (list a and list b), remove the numbers in list a that are in list b.</p>
  
  <p><strong>What currently happens:</strong> My first function works only if list a has only one number to be removed.</p>
  
  <p><strong>What I've tried:</strong> Turning the lists into sets, then subtracting a - b</p>
  
  <pre><code>def array_diff(a, b):
    c = list(set(a) - set(b))
    return c
  </code></pre>
  
  <p><strong>Also tried:</strong> Turning the list into sets, looking for n in a and m in b, then if n = m to remove n.</p>
  
  <pre><code>def array_diff(a, b):
    list(set(a))
    list(set(b))
    for n in (a): 
      for m in (b):
        if n == m:
          n.remove()
  
          return a
  </code></pre>
  
  <p><strong>Possibly thought about:</strong> Using the "not in" function to determine if something is in b or not.</p>
  
  <p><strong>Sample Input/Output:</strong> </p>
  
  <p>INPUT: array_diff([1,2], [1]) OUTPUT: [2]</p>
  
  <p>INPUT: array_diff([1,2,2], [1]) OUTPUT: [2] (This should come out to be [2,2]</p>