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
keakon wrote:
> The default value is mutable, and can be reused by all each call.
> So each call it will append 1 to the default value, that's very
> different than C++.
Being different from C++ is one of the many reasons some of us choose
Python ;)
This tends to bite most newcomers, so it's men
On 2月1日, 下午1时20分, alex23 wrote:
> alex23 wrote:
> > keakon wrote:
> > > def h2(x=[]):
> > > y = x
> > > y.append(1)
> > > return y + []
>
> > Are you aware that 'y = x' _doesn't_ make a copy of [], that it
> > actually points to the same list as x?
>
> Sorry, I meant to suggest trying the
alex23 wrote:
> keakon wrote:
> > def h2(x=[]):
> > y = x
> > y.append(1)
> > return y + []
>
> Are you aware that 'y = x' _doesn't_ make a copy of [], that it
> actually points to the same list as x?
Sorry, I meant to suggest trying the following instead:
def h2(x=None):
if x is None:
On Sun, Jan 31, 2010 at 8:58 PM, 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
keakon wrote:
> def h2(x=[]):
> y = x
> y.append(1)
> return y + []
> h2() is about 42 times slower than h2([]), but h() is a litter faster
> than h([]).
Are you aware that 'y = x' _doesn't_ make a copy of [], that it
actually points to the same list as x?
My guess is that the slowdown yo
