A performance issue when using default value

alex23 wuwei23 at gmail.com
Mon Feb 1 06:16:43 CET 2010

keakon <kea... at gmail.com> 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 you're seeing is due to the increasing
size of x _per timing iteration_. h2([]) will pass a new list each
time, while h2() will append to the same list _every_ time. The
difference between h & h2 is due to the concatenation of a new list to
the built one: the longer the default list grows, the longer this will
take, as extending a list takes O(k) time, with k being the number of

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