Why does Python code run faster in a function?

Question

def main():

for i in xrange(10**8):

pass main()

This piece of code in Python runs in (Note: The timing is done with the time function in BASH in Linux.)

real 0m1.841s 

user 0m1.828s 

sys  0m0.012s

However, if the for loop isn't placed within a function,

for i in xrange(10**8): 

pass

then it runs for a much longer time:

real 0m4.543s 

user 0m4.524s 

sys 0m0.012s

Why is this?

Answer 1

The reason for a Python code run faster in a function is because,  when a function is compiled, the local variables are stored in a fixed-size array (not a dictionary) and variable names are assigned to indexes. This is possible because we can't dynamically add local variables to a function. While retrieving a local variable is literally a pointer lookup into the list and a refcount increase on the PyObject which is trivial.