Ethan Furman a écrit :
Bruno Desthuilliers wrote:
Bruno Desthuilliers a écrit :
Ethan Furman a écrit :
Bruno Desthuilliers wrote:
Duncan Booth a écrit :
(snip)
Or you could create the default as a class attribute
from the OP:
"""
I have a class (FuncDesigner oofun) that has no attribute "size", but
it is overloaded in __getattr__, so if someone invokes
"myObject.size", it is generated (as another oofun) and connected to
myObject as attribute.
"""
so this solution won't obviously work in this case !-)
Also and FWIW, I wouldn't advocate this solution if the "default"
class attribute is of a mutable type.
Well, it is Monday, so I may be missing something obvious, but what
is the effective difference between these two solutions?
If you meant "what is the difference between creating the "whatever"
attribute with a default value in the initializer and creating it on
demand in the __getattr__ hook", the main difference is that in the
first case, the instance is garanteed to have this attribute, so you
get rid of "hasattr" checks (and the unwanted side effects) or, worse,
direct check of the instance __dict__. Except for a couple of corner
case, client code shouldn't have to worry about this kind of things -
this breaks encapsulation.
Yay Tuesday! :D
What I meant was what is the difference between:
[Bruno Desthuilliers]
> DEFAULT_WHATEVER = Whathever()
> class MyClass(object):
> def __init__(self, x, y):
> self.size = DEFAULT_WHATEVER
and
[Duncan Booth]
> class MyClass(object):
> size = Whatever()
> def __init__(self, x, y):
> ...
in both cases the object ends up with a size attribute and no further
need of __getattr__. Of course, the warning about having a mutable
object as a class attribute stands.
Indeed.
To phrase it another way, why does your solution (Bruno) work, but
Duncan's "obviously won't"?
As it is written (and assuming the name "Whatever" is bound to a
callable !-)), Duncan's code would work, even if it might not be the
safest solution in the case of a mutable type.
The problem here is that the OP stated that the "size" attribute was to
be of the same type as the host class, so the code would look something
like:
class MyClass(object):
size = MyClass()
which will raise a NameError, since MyClass is not yet defined when
"size=MyClass()" is executed.
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