title: merging columns when column name is an integer
body: <p>this is my dataframe:</p>
  
  <pre><code>A   B     1    2    5
  
  1   1     26    5   5     
  2   6     4     2   2    
  2   3     8     7   7     
  2   4     3    10   19
  </code></pre>
  
  <p>So here you can see that columns are 'A' 'B' '1' '2' '5'
  I want to create a function that accepts one argument as int and then creates a new dataframe that has 'A' 'B' columns and column which was passed as arugment.
  so it would go like this, however im not sure how to get the column when its name is an int any ideas ? Thanks!:</p>
  
  <pre><code>def merge(arg):
      new_df=pd.concat([df.A, df.B, df.arg], axis=1 )
  merge(5)
  
  A   B     5
  
  1   1     5   
  2   6     2  
  2   3     7   
  2   4     19
  </code></pre>