title: pandas dataframe drop rows by multiindex
body: <p>I'd like to drop rows from a pandas dataframe using the MultiIndex value.</p>
  
  <p>I've tried quite a few things but I put below what I think was closer. (Actually I will explain the full problem since there might be an alternative solutions using a completely different approach). From a correlation matrix, I'd like to get the pair of columns that correlate more. I use <code>unstack</code> and put the results in a dataframe:</p>
  
  <pre><code>In [263]: corr_df = pd.DataFrame(total.corr().unstack())
  </code></pre>
  
  <p>Then get the higher correlations (actually I should get the negatives as well).</p>
  
  <pre><code>In [264]: high = corr_df[(corr_df[0] &gt; 0.5) &amp; (corr_df[0] &lt; 1.0)]
  
  In [236]: print high
                                                    0
  residual sugar       density               0.552517
  free sulfur dioxide  total sulfur dioxide  0.720934
  total sulfur dioxide free sulfur dioxide   0.720934
                       wine                  0.700357
  density              residual sugar        0.552517
  wine                 total sulfur dioxide  0.700357
  </code></pre>
  
  <p>Closed enough, but there are duplicates, that's actually the point of the correlation matrix. In order to clean them up, my idea is to iterate the high values to remove duplicates:</p>
  
  <pre><code>In [267]:
  for row in high.iterrows():
      print row[0][0], ",", row[0][1]
      print high.loc[row[0][1]].loc[row[0][0]].index
      high.drop(high.loc[row[0][1]].loc[row[0][0]].index)
  residual sugar , density
  Int64Index([0], dtype='int64')
  ---------------------------------------------------------------------------
  KeyError                                  Traceback (most recent call last)
  &lt;ipython-input-267-1258da2a4772&gt; in &lt;module&gt;()
        2     print row[0][0], ",", row[0][1]
        3     print high.loc[row[0][1]].loc[row[0][0]].index
  ----&gt; 4     high.drop(high.loc[row[0][1]].loc[row[0][0]].index)
  
  ...
  [huge stack of errors]
  ...
  KeyError: 0
  </code></pre>
  
  <p>The method <code>drop</code> is working perfectly when the index is normal (see <a href="http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.drop.html" rel="nofollow">drop</a>), but, how do I build the <code>label</code> when I got a <code>MultiIndex</code>?</p>