Optimizing constants in loops

[Michael Hoffman]

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=Fals e):
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=Fals e):
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]

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=Fals e):
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=Fals e):
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]

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