title: df.head() sometimes doesn't work in Pandas, Python
body: <p>I'm a beginner in Python and the Pandas library, and I'm rather confused by some basic functionality of DataFrame. I've got a pandas DataFrame as below:  </p>
  
  <pre><code>&gt;&gt;&gt;df.head()  
                X  Y       unixtime
  0  652f5e69fcb3  1  1346689910622
  1        400292  1  1346614723542
  2  1c9d02e4f14e  1  1346862070161
  3        610449  1  1346806384518
  4        207664  1  1346723370096
  </code></pre>
  
  <p>However, after I performed some function:  </p>
  
  <pre><code>def unixTodate(unix):
    day = dt.datetime.utcfromtimestamp(unix/1000).strftime('%Y-%m-%d')
    return day
  
  df['day'] = df['unixtime'].apply(unixTodate)
  </code></pre>
  
  <p>I could no longer make use of the df.head() function:  </p>
  
  <pre><code>&gt;&gt;&gt;df.head()  
  
  &lt;class 'pandas.core.frame.DataFrame'&gt;
  Int64Index: 5 entries, 190648 to 626582
  Data columns:
  X              5  non-null values
  Y              5  non-null values
  unixtime       5  non-null values
  day            5  non-null values
  dtypes: int64(3), object(5)
  </code></pre>
  
  <p>I can't see why this is happening. Am I doing something wrong here? Any pointer is welcome! Thanks.</p>