title: Adding column(s) from one dataframe to another python pandas
body: <p>I have been searching online and found similar questions but still couldn't find the answer to what I'm looking for. 
  I have 2 excel files:</p>
  
  <p>data1 </p>
  
  <pre><code>ColumnA    columnB    columnC   columnD
    A          B          C          D
    A          B          C          D
    A          B          C          D
  </code></pre>
  
  <p>data2</p>
  
  <pre><code>ColumnE    columnF    columnG   
    E          F          G          
    E          F          G          
    E          F          G    
  </code></pre>
  
  <p>I want to add the column F from data2 to data1:</p>
  
  <pre><code>ColumnA    columnB    columnC   columnD  columnF 
    A          B          C          D       F
    A          B          C          D       F
    A          B          C          D       F
  </code></pre>
  
  <p>I tried </p>
  
  <pre><code>data2['columnF'] = data1['columnF']  #doesn't work
  </code></pre>
  
  <p>also tried</p>
  
  <pre><code>data1['columnF'] = ''   #adding a columnF to data1
  merg_left = pd.merge(left=data1,right=data2, how='left',      
  left_on='columnF', right_on='columnF')  
  #gave me a weird output file 
  </code></pre>