title: Python - concatenate two same columns and leave other unchanged
body: <p>i have some data like</p>
  
  <pre><code>&gt;&gt;&gt; df1
    Col1     Col2
  0   Hi    Hello
  1  How     Fine
  2  Why  Because
  3  You       Me
  4   We     They
  &gt;&gt;&gt; df2
        Col1   Col2
  0       Mr    Mrs
  1  Welcome    Bye
  2      Wow    Ohh
  3      Sit  Stand
  </code></pre>
  
  <p>I want to concatenate only the first Column <code>Col1</code> of these two dataframes and update the <code>df1</code> leaving <code>Col2</code> unchanged but i am unable to do it</p>
  
  <p>i tried many ways but failed doing this</p>
  
  <p><strong>Output I want</strong></p>
  
  <pre><code>&gt;&gt;&gt; df2
    Col1     Col2
  0   Hi    Hello
  1  How     Fine
  2  Why  Because
  3  You       Me
  4  We     They
  5  Mr
  6  Welcome
  7  Wow
  8  Sit
  </code></pre>
  
  <p>It doesnt matter if index is different or there comes NaN instead of Empty Cells</p>
  
  <p><strong>Things i Tried</strong></p>
  
  <p><strong>1</strong></p>
  
  <pre><code>&gt;&gt;&gt; r=pd.concat([df1["Col1"],df2["Col1"]])
  &gt;&gt;&gt; df1["Col1"] = r.reset_index(drop=True)
  &gt;&gt;&gt; df1
    Col1     Col2
  0   Hi    Hello
  1  How     Fine
  2  Why  Because
  3  You       Me
  4   We     They
  </code></pre>
  
  <p><strong>2</strong></p>
  
  <pre><code>&gt;&gt;&gt; pd.concat([df1["Col1"],df2])
       0     Col1   Col2
  0   Hi      NaN    NaN
  1  How      NaN    NaN
  2  Why      NaN    NaN
  3  You      NaN    NaN
  4   We      NaN    NaN
  0  NaN       Mr    Mrs
  1  NaN  Welcome    Bye
  2  NaN      Wow    Ohh
  3  NaN      Sit  Stand
  </code></pre>
  
  <p><strong>3</strong></p>
  
  <p><code>I Tried append(), merge() and some more techniques but failed</code> </p>