title: Subtract two columns in dataframe
body: <p>My df looks as follows:</p>
  
  <pre><code>Index    Country    Val1  Val2 ... Val10
  1        Australia  1     3    ... 5
  2        Bambua     12    33   ... 56
  3        Tambua     14    34   ... 58
  </code></pre>
  
  <p>I'd like to substract Val10 from Val1 for each country, so output looks like:</p>
  
  <pre><code>Country    Val10-Val1
  Australia  4
  Bambua     23
  Tambua     24
  </code></pre>
  
  <p>So far I've got:</p>
  
  <pre><code>def myDelta(row):
      data = row[['Val10', 'Val1']]
      return pd.Series({'Delta': np.subtract(data)})
  
  def runDeltas():
      myDF = getDF() \
          .apply(myDelta, axis=1) \
          .sort_values(by=['Delta'], ascending=False)
      return myDF
  </code></pre>
  
  <p>runDeltas results in this error:</p>
  
  <pre><code>ValueError: ('invalid number of arguments', u'occurred at index 9')
  </code></pre>
  
  <p>What's the proper way to fix this?</p>