Dennis Lee Bieber wrote:
> On Sat, 15 Nov 2008 01:40:04 -0800 (PST), Rick Giuly
> <[EMAIL PROTECTED]> declaimed the following in comp.lang.python:
>
>> Why is python designed so that b and c (according to code below)
>> actually share the same list object? It seems more natural to me that
>> each object would be created with a new list object in the points
>> variable.
>>
> This is a FAQ... default arguments are evaluation only ONCE, during
> the "compilation" of the function.
>
>> class Blob:
>> def __init__(self, points=[]):
>> self._points = points
>>
> The preferred/recommended form is to use (very explicit, one test,
> one "assignment")
>
> def __init__(self, points=None):
> if points:
> self._points = points
> else:
> self._points = []
>
> or (shorter; one test, potentially two "assignments")
>
> def __init__(self, points=None):
> if not points: points = []
> self._points = points
>
I hesitate to beat the thoroughly obvious to death with a stick, but
this is a *very* bad way to make the test. If you are using None as a
sentinel to indicate that no argument was provided to the call then the
correct test is
if points is None:
points = []
The code shown fails to distinguish between passing an empty list and
not passing an argument at all.
regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/
--
http://mail.python.org/mailman/listinfo/python-list
