Re: bug or feature?

beza1e1:

Coming back from a bug hunt, i am not sure what to think of this python
behaviour. Here is a demo program:

class A:
   def __init__(self, lst=[]):
      self.lst = lst

a = A()
b = A()
b.lst.append("hallo")
print a.lst # output: ["hallo"]

The point seems to be, that lst=[] creates a class attribute (correct
name?), which is shared by all instances of A. So a.lst ist the same
object as b.lst, despite the fact, that object a is different to object
b.

Fredrik Lundh wrote: 
Steve Holden wrote:


Interestingly I couldn't find this in the FAQ, though it *is* a
frequently-asked question [note: my not finding it doesn't guarantee
it's not there].



it's there:

    http://www.python.org/doc/faq/general.html#why-are-default-values-shared-between-objects

(maybe "default values" should be changed to "default argument values")

I couldn't believe it wasn't, but you're right: it should be easier to find, and a change of wording may do that.

regards
 Steve
--
Steve Holden       +44 150 684 7255  +1 800 494 3119
Holden Web LLC                     www.holdenweb.com
PyCon TX 2006                  www.python.org/pycon/

.

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