title: Conditional If Statement: If value contains string then set another column equal to string
body: <p>I write one python 3 script</p>
  
  <hr>
  
  <p>I have a column 'original_title', where I have different film titles i.a. all films of Star Wars (+ the name of the episode) and Star Trek (+ the name of the episode). I want to create one column which will show me only 'star trek' (without the name of episode), 'star wars' and 'na'.</p>
  
  <p>This is my code for the new column: </p>
  
  <pre><code>df['Trek_Wars'] = pd.np.where(df.original_title.str.contains("Star Wars"), "star_wars", 
                pd.np.where(df.original_title.str.contains("Star Trek"), "star_trek"))
  </code></pre>
  
  <p>However, it doesn't work</p>
  
  <pre><code>---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-33-5472b36a2193&gt; in &lt;module&gt;()
        1 df['Trek_Wars'] = pd.np.where(df.original_title.str.contains("Star Wars"), "star_wars",
  ----&gt; 2                    pd.np.where(df.original_title.str.contains("Star Trek"), "star_trek"))
  
  ValueError: either both or neither of x and y should be given
  </code></pre>
  
  <p>What should I do?</p>