Answer: Python Puzzler #1348

The answer is C. The program prints:

Checked by running the program for real.

Python works from the inside out. First double(4) returns 8. Then that answer goes into double again, which returns 16.

A trace table follows the program step by step. Here’s this one:

step	value
double(4)	8
double(8)	16

def double(number):
    return number * 2

print(double(double(4)))

Try it: change one number in the program and predict the new output before you run it.