How do I use matplotlib autopct

Question

I want to create a matplotlib pie chart that contains the value of each wedge written on top of the wedge.
Reference doc: auto pact: [ None | format string | format function ] If not None, is a string or function used to label the wedges with their numeric value. The label will be placed inside the wedge. If it is a format string, the label will be fmt%pct. If it is a function, it will be called.
Using this basic example below, how can I display each numerical value on top of its wedge?

plt.figure()
values = [3, 12, 5, 8] 
labels = ['a', 'b', 'c', 'd'] 
plt.pie(values, labels=labels) #autopct??
plt.show()

Please can someone help?