Loop performance disappearance

[Bjorn Pettersen]

Unfortunately (or fortunately, depending on how you view it), Python is
too dynamic for this to be easy.  You would have to be able to detect
someone doing eg:

	def range(n):
		return [n]

in a module you are importing.  This isn't impossible, but it's probably
far more work than you think it is...

-- bjorn

Chris Ryland wrote:
> 
> I always assumed (perhaps being too much of a newbie) that
> 
> for i in range(n):
>    :
> 
> would automatically be recognized by the compiler and turned into a simple
> loop control using i alone (like turning it into an xrange, I guess). Seems
> simple enough and common enough that it should be automatic, no? (Forgive
> any massive display of ignorance here.)
> 
> --
> Cheers!
> / Chris Ryland, President / Em Software, Inc. / www.emsoftware.com
> "Remco Gerlich" <scarblac-spamtrap at pino.selwerd.nl> wrote in message
> news:slrn8cva9a.aj8.scarblac-spamtrap at flits104-37.flits.rug.nl...
> > Mikael Johansson wrote in comp.lang.python:
> > > I was just wondering what the reason for the huge performance decrease
> > > in a loop execution when the number of steps exceeds some critical value
> > > is. To give an example:
> > >
> > > for i in range(loops):
> > >     pass
> > >
> > > If loops=500 000 (space added for clarity) the "program" executes in ~2
> > > secs on my machine. But if loops is set to 5 000 000, the execution time
> > > rises to substantially more than tenfold (I terminated it after one
> > > minute). However the CPU-load is quite small, most of the time is spent
> > > disk swapping like crazy!
> >
> > That's because range(loops) is a list. It actually has to construct a list
> > that size, and then walks through it. 5 million doesn't fit in your memory
> > and it starts swapping.
> >
> > > Any ideas of the reason for this, work-arounds?
> >
> > Use xrange instead of range. xrange doesn't return a list, but an object
> > that generates values on demand, simulating a list. Slightly slower, but
> > doesn't need the memory.
> >
> > --
> > Remco Gerlich,  scarblac at pino.selwerd.nl
> >   Murphy's Rules, "Which is why you get 'em so cheap":
> >    In SPI's Universe, the sword is prohibited from use at any combat
> >    range.
> 
> --
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