title: How to merge two dataframes with pandas Python
body: <p>here are 2 dataframes :</p>
  <pre><code>d1 = pd.DataFrame({'a': [19, 65, 7, 49, 66, 89, 545],
                    'b': [4, 6, 6, 90, 5, 77, 767],
                    'c': [34, 78, 65, 666, '', '', '']})
  
  d2 = pd.DataFrame({'c': [34, 78, 65, '', ''],
                    'd': [4, 6, 6, 90, 767]})
  </code></pre>
  <p>I would like to make a merge between them with &quot;c&quot; column as jointure.</p>
  <p>In my case, I use this :</p>
  <pre><code>df = pd.merge(d1, d2, how='left')
  </code></pre>
  <p>But the result is not good. In fact, I have some doublons, plus the final result should be a dataframe with the same length of d1. In my case It is not true.</p>
  <p>Here is the result I would like to have :</p>
  <pre><code>df = pd.DataFrame({'a': [19, 65, 7, 49, 66, 89, 545],
                    'b': [4, 6, 6, 90, 5, 77, 767],
                    'c': [34, 78, 65, 666, '', '', ''],
                    'd': [4, 6, 6, 90, 767, '', '']})
  </code></pre>