title: Python DataFrame: Replace values using dictionary, convert NaN if not in dictionary
body: <p>I understand how to replace column values with using a dictionary however I want to convert all of the values that are not in my dictionary to NaN or some other value. I am getting this: </p>
  
  <pre><code>Dictionary is:
  {'apple': 1, 'peach': 6, 'watermelon': 4, 'grapes': 5, 'orange': 2, 
  'banana': 3}
  
  DataFrame is: 
  fruit_tag
  apple
  orange
  banana
  watermelon
  red
  blue
  
  I use: 
  df["fruit_tag"].replace(dict, inplace=True)
  print(df)
  
  I get:
  fruit_tag
  1
  2
  3
  4
  red
  blue
  
  What I want to get:
  fruit_tag
  1
  2
  3
  4
  NaN
  NaN
  </code></pre>