Create new columns and fill with calculated values from same dataframe

You can do this easily manually for each column like this:

df['A_perc'] = df['A']/df['sum']

If you want to do this in one step for all columns, you can use the div method (http://pandas.pydata.org/pandas-docs/stable/basics.html#matching-broadcasting-behavior):

ds.div(ds['sum'], axis=0)

And if you want this in one step added to the same dataframe:

>>> ds.join(ds.div(ds['sum'], axis=0), rsuffix='_perc')
          A         B         C         D       sum    A_perc    B_perc  \
1  0.151722  0.935917  1.033526  0.941962  3.063127  0.049532  0.305543   
2  0.033761  1.087302  1.110695  1.401260  3.633017  0.009293  0.299283   
3  0.761368  0.484268  0.026837  1.276130  2.548603  0.298739  0.190013   

     C_perc    D_perc  sum_perc  
1  0.337409  0.307517         1  
2  0.305722  0.385701         1  
3  0.010530  0.500718         1