title: pandas DataFrame DatetimeIndex slice error
body: <p>I am very curious why I can assign value to a slice, but cannot print it out directly. Following codes shows details:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  from datetime import datetime
  
  dt_start = datetime.strptime("20171010", "%Y%m%d")
  dt_end = datetime.strptime("20171020", "%Y%m%d")
  
  df = pd.DataFrame(np.nan, index=pd.date_range(start=dt_start, end=dt_end), columns=['sales', 'account'])
  
  df.loc[:1, 'sales'] = 100 # works well
  print(df.loc[:1, 'sales']) # error, why???
  </code></pre>
  
  <p>Error message:<br/>
  TypeError: cannot do slice indexing on class 'pandas.tseries.index.DatetimeIndex with these indexers [1] of class 'int'</p>
  
  <p>Why I can assign value but cannot print this slice?</p>
  
  <p>Thanks very much for checking.</p>