title: return key by value in dictionary
body: <p>I am trying to return the key in a dictionary given a value </p>
  
  <p>in this case if 'b' is in the dictionary, I want it to return the key at which 'b' is (i.e 2)</p>
  
  <pre><code>def find_key(input_dict, value):
      if value in input_dict.values():
          return UNKNOWN            #This is a placeholder
      else:
          return "None"
  
  print(find_key({1:'a', 2:'b', 3:'c', 4:'d'}, 'b'))
  </code></pre>
  
  <p>The answer I want to get is the key 2, but I am unsure what to put in order to get the answer, any help would be much appreciated</p>