title: pandas How to find all zeros in a column
body: <pre><code>import copy
  
  head6= copy.deepcopy(df)
  
  closed_day = head6[["DATEn","COUNTn"]]\
               .groupby(head6['DATEn']).sum()
  print closed_day.head(10)
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>     COUNTn
  DATEn               
  06-29-13    11326823
  06-30-13     5667746
  07-01-13     8694140
  07-02-13     7275701
  07-03-13     9948824
  07-04-13  1072542591
  07-05-13     7867611
  07-06-13     4733018
  07-07-13     4838404
  07-08-13    42962814
  </code></pre>
  
  <p>Now what if I want to find if <code>COUNTn</code> has any zeros and I want to return corresponding day? I've written something like this but I'm getting an error saying my df doesn't have any column called <code>COUNTn</code></p>
  
  <pre><code>ndf = closed_day[["DATEn","COUNTn"]][closed_day.COUNTn == 0]
  print ndf.head(1)
  </code></pre>