How do I use matplotlib autopct?

You can do:

plt.pie(values, labels=labels, autopct=lambda p : '{:.2f}%  ({:,.0f})'.format(p,p * sum(values)/100))

autopct enables you to display the percent value using Python string formatting. For example, if autopct='%.2f', then for each pie wedge, the format string is '%.2f' and the numerical percent value for that wedge is pct, so the wedge label is set to the string '%.2f'%pct.

import matplotlib.pyplot as plt
plt.figure()
values = [3, 12, 5, 8] 
labels = ['a', 'b', 'c', 'd'] 
plt.pie(values, labels=labels, autopct='%.2f')
plt.show()

yields Simple pie chart with percentages

You can do fancier things by supplying a callable to autopct. To display both the percent value and the original value, you could do this:

import matplotlib.pyplot as plt

# make the pie circular by setting the aspect ratio to 1
plt.figure(figsize=plt.figaspect(1))
values = [3, 12, 5, 8] 
labels = ['a', 'b', 'c', 'd'] 

def make_autopct(values):
    def my_autopct(pct):
        total = sum(values)
        val = int(round(pct*total/100.0))
        return '{p:.2f}%  ({v:d})'.format(p=pct,v=val)
    return my_autopct

plt.pie(values, labels=labels, autopct=make_autopct(values))
plt.show()

Pie chart with both percentages and absolute numbers.

Again, for each pie wedge, matplotlib supplies the percent value pct as the argument, though this time it is sent as the argument to the function my_autopct. The wedge label is set to my_autopct(pct).