why a main() function?

[Peter Otten]

Diez B. Roggisch wrote:

> bearophileHUGS at lycos.com wrote:
> 
>> Others have already told you the most important things.
>> 
>> There is another secondary advantage: the code inside a function runs
>> faster (something related is true for C programs too). Usually this
>> isn't important, but for certain programs they can go 20%+ faster.
> 
> I totally fail to see why that should be the case - for python as well as
> for C.
> 
> So - can you explain that a bit more, or provide resources to read up on
> it?

A trivial example (for Python):

$ cat main.py
def main():
    x = 42
    for i in xrange(1000000):
        x; x; x; x; x; x; x; x; x; x
        x; x; x; x; x; x; x; x; x; x
        x; x; x; x; x; x; x; x; x; x

main()
$ time python main.py

real    0m0.874s
user    0m0.864s
sys     0m0.009s
$ cat nomain.py
x = 42
for i in xrange(1000000):
    x; x; x; x; x; x; x; x; x; x
    x; x; x; x; x; x; x; x; x; x
    x; x; x; x; x; x; x; x; x; x
$ time python nomain.py

real    0m2.154s
user    0m2.145s
sys     0m0.009s
$

Now let's verify that global variables are responsible for the extra time:

$ cat main_global.py
def main():
    global i, x
    x = 42
    for i in xrange(1000000):
        x; x; x; x; x; x; x; x; x; x
        x; x; x; x; x; x; x; x; x; x
        x; x; x; x; x; x; x; x; x; x

main()
$ time python main_global.py

real    0m2.002s
user    0m1.995s
sys     0m0.007s


Peter