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Re: Calling a function is faster than not calling it?

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On 5/10/2015 5:58 AM, Steven D'Aprano wrote:
> Why is calling a function faster than bypassing the function object and
> evaluating the code object itself? And not by a little, but by a lot?
>
> Here I have a file, eval_test.py:
>
> # === cut ===
> from timeit import Timer
>
> def func():
>      a = 2
>      b = 3
>      c = 4
>      return (a+b)*(a-b)/(a*c + b*c)
>
>
> code = func.__code__
> assert func() == eval(code)
>
> t1 = Timer("eval; func()", setup="from __main__ import func")
> t2 = Timer("eval(code)", setup="from __main__ import code")

eval has 3 parameters. I believe omitting the last two results in 
globals() and locals() calls, but at least one of them.  If {} is 
passed, access to builtins is added.

> # Best of 10 trials.
> print (min(t1.repeat(repeat=10)))
> print (min(t2.repeat(repeat=10)))

Adding
g = globals()
t3 = Timer("eval(code, g)", setup="from __main__ import code, g")
print (min(t3.repeat(repeat=10)))

 >>>
0.3992733933018515
0.6967548323372563
0.49210603735894587

2/3s of the extra time disappears, but there is still extra time needed 
for processing the two extra arguments. Passing g twice has no effect

> [steve@ando ~]$ python2.7 eval_test.py
> 0.804041147232
> 1.74012994766

> Directly eval'ing the code object is easily more than twice as expensive
> than calling the function, but calling the function has to eval the code

Calling the function executes the code via its .__call__ method.
Perhaps eval wraps the code object in a function object with .__call__. 
  I don't know the comparative internals.


-- 
Terry Jan Reedy

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Thread

Calling a function is faster than not calling it? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-10 19:58 +1000
  Re: Calling a function is faster than not calling it? Christian Gollwitzer <auriocus@gmx.de> - 2015-05-10 12:34 +0200
    Re: Calling a function is faster than not calling it? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-11 01:04 +1000
  Re: Calling a function is faster than not calling it? Peter Otten <__peter__@web.de> - 2015-05-10 12:43 +0200
    Re: Calling a function is faster than not calling it? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-11 00:49 +1000
      Re: Calling a function is faster than not calling it? Peter Otten <__peter__@web.de> - 2015-05-10 18:14 +0200
      Re: Calling a function is faster than not calling it? Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-10 10:25 -0600
  Re: Calling a function is faster than not calling it? Terry Reedy <tjreedy@udel.edu> - 2015-05-10 12:37 -0400
  Re: Calling a function is faster than not calling it? BartC <bc@freeuk.com> - 2015-05-10 22:08 +0100
    Re: Calling a function is faster than not calling it? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-11 13:58 +1000
      Re: Calling a function is faster than not calling it? BartC <bc@freeuk.com> - 2015-05-11 10:50 +0100
        Re: Calling a function is faster than not calling it? Skip Montanaro <skip.montanaro@gmail.com> - 2015-05-11 09:12 -0500
          Re: Calling a function is faster than not calling it? BartC <bc@freeuk.com> - 2015-05-11 16:01 +0100
            Re: Calling a function is faster than not calling it? Skip Montanaro <skip.montanaro@gmail.com> - 2015-05-11 10:13 -0500
      Re: Calling a function is faster than not calling it? Tony the Tiger <tony@tiger.invalid> - 2015-05-15 01:35 +0000
  Re: Calling a function is faster than not calling it? Stefan Behnel <stefan_ml@behnel.de> - 2015-05-11 08:11 +0200
  Re: Calling a function is faster than not calling it? Piet van Oostrum <piet@vanoostrum.org> - 2015-06-22 23:49 +0200

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