title: How to add columns to an empty pandas dataframe?
body: <p>I have an empty <code>dataframe</code>.</p>
  
  <pre><code>df=pd.DataFrame(columns=['a'])
  </code></pre>
  
  <p>for some reason I want to generate df2, another empty dataframe, with two columns 'a' and 'b'.</p>
  
  <p>If I do </p>
  
  <pre><code>df.columns=df.columns+'b'
  </code></pre>
  
  <p>it does not work (I get the columns renamed to 'ab')
  and neither does the following </p>
  
  <pre><code>df.columns=df.columns.tolist()+['b']
  </code></pre>
  
  <p>How to add a separate column 'b' to df, and <code>df.emtpy</code> keep on being <code>True</code>?</p>
  
  <p>Using .loc is also not possible</p>
  
  <pre><code>   df.loc[:,'b']=None
  </code></pre>
  
  <p>as it returns </p>
  
  <pre><code>  Cannot set dataframe with no defined index and a scalar
  </code></pre>