title: pandas dataframe merge merging between different levels can give an unintended result
body: <p>I am doing a left join between two dataframes right and left using <code>merge</code>:</p>
  <pre><code>left.merge(right, on='A', how='left')
  </code></pre>
  <p>I get the warning</p>
  <pre><code>UserWarning: merging between different levels can give an unintended result (2 levels on the left, 1 on the right)
  </code></pre>
  <p>But if I do
  <code>right.index.nlevels</code> and <code>left.index.nlevels</code> I get 1 for both.</p>
  <p>should i ignore that warning?</p>
  <p>Note: if I get the index of both I get:</p>
  <pre><code>&gt;&gt;&gt; left = mshareFrame
  &gt;&gt;&gt; right.index
  RangeIndex(start=0, stop=138, step=1)
  &gt;&gt;&gt; left.index
  RangeIndex(start=0, stop=2114906, step=1)
  &gt;&gt;&gt; x = left.merge(right, on='isin', how='left')
  C:\ANACON~1\envs\R-RETI~1\lib\site-packages\pandas\core\reshape\merge.py:618: UserWarning: merging between different levels can give an unintended result (2 levels on the left, 1 on the right)
    warnings.warn(msg, UserWarning)
  </code></pre>