title: Error when selecting rows in pandas dataframe based on column value
body: <p>I have a dataframe <code>df</code> which looks like this:</p>
  <pre><code>col1  col2 col3
   A     45    4
   A     3     5
   B     2     5
  </code></pre>
  <p>I want to make a separate dataframe, <code>df2</code>, which only has the rows where <code>col</code> in <code>df</code> equals <code>A</code>. Hence it should look like:</p>
  <pre><code>col1  col2 col3
   A     45    4
   A     3     5
  </code></pre>
  <p>I just use <code>df2=df1.loc[df1['col1']=='A']</code>. However this returns the error: <code>ValueError: Cannot index with multidimensional key</code>. Any idea what is going wrong here?</p>