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

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Steven D'Aprano wrote:

> On Sun, 10 May 2015 08:43 pm, Peter Otten wrote:
> 
>> A significant part of the extra time is apparently spent on stack
>> inspection:
> 
> I don't know what you mean by "stack inspection", or how you come to that
> conclusion.
> 
> 
>> $ python3 -m timeit -s 'f = (lambda: 42); code = f.__code__; ns = {}'
>> 'f()' 10000000 loops, best of 3: 0.179 usec per loop
>> 
>> $ python3 -m timeit -s 'f = (lambda: 42); code = f.__code__; ns = {}'
>> 'eval(code)'
>> 1000000 loops, best of 3: 0.852 usec per loop
>> 
>> $ python3 -m timeit -s 'f = (lambda: 42); code = f.__code__; ns = {}'
>> 'eval(code, ns)'
>> 1000000 loops, best of 3: 0.433 usec per loop
> 
> Curious.
> 
> If I'm reading that correctly, supplying an explicit namespace cuts the
> time by a factor of two. That surprises me, as your function doesn't do
> any name lookups in the body of the function, so I would have thought that
> would be irrelevant.

I had a quick look at the implementation in bltinmodule.c, and the only 
piece of code that I could prevent from being run was:

    if (globals == Py_None) {
        globals = PyEval_GetGlobals();
        if (locals == Py_None) {
            locals = PyEval_GetLocals();
            if (locals == NULL)
                return NULL;
        }
    }

When there was an actual speed-up I also had a look at 
PyEval_GetGlobals/Locals() which in turn call 

PyEval_GetFrame() 

and

PyEvalPyFrame_FastToLocalsWithError()

whatever these do. (The first function reminded me of sys._getframe() hence 
the mention of stack inspection)

>> $ python3 -m timeit -s 'f = (lambda: 42); code = f.__code__; ns = {}'
>> 'eval; ns; f()'
>> 1000000 loops, best of 3: 0.263 usec per loop
> 
> So, roughly speaking, calling eval(code, ns) takes about 1.6 times as long
> as calling the function; calling eval(code) without providing a namespace
> takes about 3.2 times as long. That's roughly consistent with the results
> I'm getting.
 

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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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