title: pandas, multiply all the numeric values in the data frame by a constant
body: <p>How to multiply all the numeric values in the data frame by a constant without having to specify column names explicitly? Example:</p>
  
  <pre><code>In [13]: df = pd.DataFrame({'col1': ['A','B','C'], 'col2':[1,2,3], 'col3': [30, 10,20]})
  
  In [14]: df
  Out[14]: 
    col1  col2  col3
  0    A     1    30
  1    B     2    10
  2    C     3    20
  </code></pre>
  
  <p>I tried <code>df.multiply</code> but it affects the string values as well by concatenating them several times.</p>
  
  <pre><code>In [15]: df.multiply(3)
  Out[15]: 
    col1  col2  col3
  0  AAA     3    90
  1  BBB     6    30
  2  CCC     9    60
  </code></pre>
  
  <p>Is there a way to preserve the string values intact while multiplying only the numeric values by a constant? </p>