title: The truth value of a Series is ambiguous in dataframe
body: <p>I have the same code,I'm trying to create new field in pandas dataframe with simple conditions:</p>
  
  <pre><code>if df_reader['email1_b']=='NaN':
      df_reader['email1_fin']=df_reader['email1_a']
  else:
      df_reader['email1_fin']=df_reader['email1_b']
  </code></pre>
  
  <h2>But I see this strange mistake:</h2>
  
  <pre><code>ValueError                                Traceback (most recent call last)
  &lt;ipython-input-92-46d604271768&gt; in &lt;module&gt;()
  ----&gt; 1 if df_reader['email1_b']=='NaN':
        2     df_reader['email1_fin']=df_reader['email1_a']
        3 else:
        4     df_reader['email1_fin']=df_reader['email1_b']
  
  /home/user/GL-env_py-gcc4.8.5/lib/python2.7/site-packages/pandas/core/generic.pyc in __nonzero__(self)
      953         raise ValueError("The truth value of a {0} is ambiguous. "
      954                          "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
  --&gt; 955                          .format(self.__class__.__name__))
      956 
      957     __bool__ = __nonzero__
  
  ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
  </code></pre>
  
  <p>Can anybody explain me, what I need to with this?</p>