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

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Steven D'Aprano schrieb am 10.05.2015 um 11:58:
> 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")
> 
> # Best of 10 trials.
> print (min(t1.repeat(repeat=10)))
> print (min(t2.repeat(repeat=10)))
> 
> # === cut ===
> 
> 
> Note that both tests include a name lookup for eval, so that as much as
> possible I am comparing the two pieces of code on an equal footing.
> 
> Here are the results I get:
> 
> 
> [steve@ando ~]$ python2.7 eval_test.py
> 0.804041147232
> 1.74012994766
> [steve@ando ~]$ python3.3 eval_test.py
> 0.7233301624655724
> 1.7154695875942707
> 
> 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
> object.

Well, yes, but it does so directly in C code. What you are essentially
doing here is replacing a part of the fast C code path for executing a
Python function by some mostly equivalent but more general Python code. So,
you're basically replacing a function call by another function call to
eval(), plus some extra generic setup overhead.

Python functions know exactly what they have to do internally in order to
execute. eval() cannot make the same streamlined assumptions.

Stefan

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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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