On Sun, 31 Jan 2010 20:58:50 -0800, keakon wrote:
> I've found strange performance issue when using default value, the test
> code is list below:
>
> from timeit import Timer
>
> def f(x):
> y = x
> y.append(1)
> return y
>
> def g(x=[]):
> y = []
> y.append(1)
> return y
>
> def h(x=[]):
> y = x
> y.append(1)
> return y
>
> def f2(x):
> y = x
> y.append(1)
> return y + []
>
> def g2(x=[]):
> y = []
> y.append(1)
> return y + []
>
> def h2(x=[]):
> y = x
> y.append(1)
> return y + []
>
> TIMES = 10000
> print Timer('f([])','from __main__ import f, g, h').timeit(TIMES) print
> Timer('g()','from __main__ import f, g, h').timeit(TIMES) print
> Timer('h([])','from __main__ import f, g, h').timeit(TIMES) print
> Timer('h()','from __main__ import f, g, h').timeit(TIMES) print
> Timer('f2([])','from __main__ import f2, g2, h2').timeit(TIMES) print
> Timer('g2()','from __main__ import f2, g2, h2').timeit(TIMES) print
> Timer('h2([])','from __main__ import f2, g2, h2').timeit(TIMES) print
> Timer('h2()','from __main__ import f2, g2, h2').timeit(TIMES)
>
>
> I tested it with Python 2.5.4, 2.6.4 and 3.1.1 on Windows XP, and get
> almost the same result:
> 0.00449247041174
> 0.00439608944712
> 0.00455867994396
> 0.00327471787615
> 0.00791581052899
> 0.00684919452053
> 0.00734311204357
> 0.30974942346
You've only run those results once, haven't you?
That is not trustworthy. In a multi-tasking operating system, including
Windows, some other process might have run for some milliseconds,
throwing the results off.
More importantly, if you had run it multiple times, you'd see this:
>>> for i in range(7):
... Timer('h2()','from __main__ import f2, g2, h2').timeit(TIMES)
...
0.0190200805664
0.315117120743
0.941411972046
1.56618499756
2.18553495407
2.79832291603
3.44865894318
h2 (and probably h, but I haven't tested it) get slower and slower and
slower every time you run it.
Why? I'm pretty sure this is in the FAQs, but default values are created
once, not every time the function is called. So you create a function
with the default value of []. Call the function, and that list gets 1
appended to it, so the default value is now [1]. Call the function again,
and it becomes [1, 1]. And so on, and so on, and so on.
The other functions create a new empty list each time the function is
called, so you're comparing the time it takes to grow an ever-bigger list
with the time it takes to create a tiny list.
--
Steven
--
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