Optimizing constants in loops

Thomas Heller theller at ctypes.org
Wed Jun 13 17:40:06 CEST 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




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