title: Add column with constant value to pandas dataframe
body: <p>Given a DataFrame:</p>
  
  <pre><code>np.random.seed(0)
  df = pd.DataFrame(np.random.randn(3, 3), columns=list('ABC'), index=[1, 2, 3])
  df
  
            A         B         C
  1  1.764052  0.400157  0.978738
  2  2.240893  1.867558 -0.977278
  3  0.950088 -0.151357 -0.103219
  </code></pre>
  
  <p>What is the simplest way to add a new column containing a constant value eg 0?</p>
  
  <pre><code>          A         B         C  new
  1  1.764052  0.400157  0.978738    0
  2  2.240893  1.867558 -0.977278    0
  3  0.950088 -0.151357 -0.103219    0
  </code></pre>
  
  <hr>
  
  <p>This is my solution, but I don't know why this puts NaN into 'new' column?</p>
  
  <pre><code>df['new'] = pd.Series([0 for x in range(len(df.index))])
  
            A         B         C  new
  1  1.764052  0.400157  0.978738  0.0
  2  2.240893  1.867558 -0.977278  0.0
  3  0.950088 -0.151357 -0.103219  NaN
  </code></pre>