title: pandas: Dataframe sum and add
body: <pre><code>         total pts resistance 1 
  0          0.5         0.872
  1          2.0         1.770
  2          0.0         0.246
  3          2.0         3.500
  4          0.5         1.069
  5          1.0         0.793
  6          0.5           nan
  7          1.5         1.394
  </code></pre>
  
  <p>I use command like below</p>
  
  <pre><code>&gt;&gt;&gt; df_new['total pts'].add(df_new['resistance 1 '],fill_value=0)
  </code></pre>
  
  <p>and it return error </p>
  
  <pre><code>Traceback (most recent call last):
  
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/local/lib/python2.7/dist-packages/pandas/core/ops.py", line 686, in flex_wrapper
      return self._binop(other, op, level=level, fill_value=fill_value)
    File "/usr/local/lib/python2.7/dist-packages/pandas/core/series.py", line 1459, in _binop
      result = func(this_vals, other_vals)
  TypeError: unsupported operand type(s) for +: 'float' and 'str'
  </code></pre>
  
  <p>But it work perfectly with the below command</p>
  
  <pre><code>&gt;&gt;&gt; df_new[['total pts','resistance 1 ']].sum(axis=1)
  </code></pre>
  
  <p>The issue is i can use sum to replace add but i need to do subtraction, division and multiplication. I want to know why these operator not working.</p>