title: Pandas add a column and fill values by using list comprehension if else
body: <p>I want to add a column name <code>A</code> in <code>df</code>, and fill the values. If column <code>a1</code> has value, fill df.A with df.a1, else fill df.A with df.a2.</p>
  
  <pre><code>tm = pd.DataFrame({'a1':['astr1',np.nan,'astr2',np.nan],'a2':[np.nan,np.nan,np.nan,'astr3']})
  tm
  
      a1      a2
  0   str1    NaN
  1   NaN     NaN
  2   str2    NaN
  3   NaN     str2
  </code></pre>
  
  <p>I want this.</p>
  
  <pre><code>    a1      a2       A
  0   str1    NaN      str1
  1   NaN     NaN      NaN
  2   str2    NaN      str2
  3   NaN     str2     str2
  </code></pre>