Velocity Reviews > Optimizing constants in loops

# Optimizing constants in loops

Michael Hoffman
Guest
Posts: n/a

 06-13-2007
The peephole optimizer now takes things like

if 0:
do_stuff()

and optimizes them away, and optimizes away the conditional in "if 1:".

What if I had a function like this?

def func(debug=False):
for index in xrange(1000000):
if debug:
print index
do_stuff(index)

Could the "if debug" be optimized away on function invocation if debug
is immutable and never reassigned in the function? When performance
really matters in some inner loop, I usually move the conditional
outside like this:

def func(debug=False):
if debug:
for index in xrange(1000000):
print index
do_stuff(index)
else:
for index in xrange(1000000):
do_stuff(index)

It would be nice if this sort of thing could be done automatically,
either by the interpreter or a function decorator.
--
Michael Hoffman

Thomas Heller
Guest
Posts: n/a

 06-13-2007
Michael Hoffman schrieb:
> The peephole optimizer now takes things like
>
> if 0:
> do_stuff()
>
> and optimizes them away, and optimizes away the conditional in "if 1:".
>
> What if I had a function like this?
>
> def func(debug=False):
> for index in xrange(1000000):
> if debug:
> print index
> do_stuff(index)
>
> Could the "if debug" be optimized away on function invocation if debug
> is immutable and never reassigned in the function? When performance
> really matters in some inner loop, I usually move the conditional
> outside like this:
>
> def func(debug=False):
> if debug:
> for index in xrange(1000000):
> print index
> do_stuff(index)
> else:
> for index in xrange(1000000):
> do_stuff(index)
>
> It would be nice if this sort of thing could be done automatically,
> either by the interpreter or a function decorator.

Just use the builtin __debug__ variable for that purpose.
__debug__ is 'True' if Python is run normally, and 'False'
if run with the '-O' or '-OO' command line flag.
The optimizer works in the way you describe above (which
it will not if you use a custom variable).

Thomas

Michael Hoffman
Guest
Posts: n/a

 06-13-2007
Thomas Heller wrote:

> Just use the builtin __debug__ variable for that purpose.
> __debug__ is 'True' if Python is run normally, and 'False'
> if run with the '-O' or '-OO' command line flag.
> The optimizer works in the way you describe above (which
> it will not if you use a custom variable).

Thanks, I didn't know that __debug__ was optimized like this. But that
was really just a specific example of the general case.
--
Michael Hoffman