title: Storing Series into a pandas DataFrame cell using apply
body: <p>I have a function that returns a Series.  I apply the function to my DataFrame using apply(), and I would like to store the resulting Series into a new column of the dataframe. e.g.,</p>
  
  <pre><code>   | 1 |                  2
  ---+---+------------------------------------
   0 | 1 | &lt;class 'pandas.core.series.Series'&gt;
   1 | 2 | &lt;class 'pandas.core.series.Series'&gt;
   2 | 3 | &lt;class 'pandas.core.series.Series'&gt;
  </code></pre>
  
  <p>However, it seems that pandas wants to expand the series and store it into multiple columns, so I get a ValueError: Wrong number of items passed 3, placement implies 1.</p>
  
  <pre><code>import pandas as pd
  
  df = pd.DataFrame({1: [1, 2, 3]})
  
  
  def f(x):
      return pd.Series(['a', 'b', 'c'])
  
  df[2] = df.apply(f, axis=1)
  </code></pre>
  
  <p>How should deal with this?</p>