beza1e1 wrote:
> 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.
>
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]. The nearest I could get was in
http://www.python.org/doc/faq/programming.html#my-program-is-too-slow-how-do-i-speed-it-up
which says:
"""Default arguments can be used to determine values once, at compile
time instead of at run time."""
The point is that the value of the keyword argument is determined when
the def statement is executed (which is to say when the function body is
being bound to its name).
If the default argument is (a reference to) a mutable object (such as a
list instance) then if one call to the function modifies that mutable
object, subsequent calls see the mutated instance as the default value.
regards
Steve
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