On 06/10/2013 01:42 PM, Rui Maciel wrote:
Peter Otten wrote:
Have you read the code in the interpreter session I posted?
If you do not agree that the demonstrated behaviour is puzzling I'll have
to drop my claim...
I don't see how it should be puzzling. You've deleted the attribute, so it
ceassed to exist.
Clearly you didn't reason through all the code in Peter's example.
Likewise if you can show a benefit of the
position = []
line.
I wrote the code that way to declare intent and help document the code. In
this case that the class Point is expected to have an attribute named
position which will point to a list.
So why do you also have an instance attribute of the same name? If you
really want all instances to have the same value for position, you'd
better change the __init__() function so it doesn't mask that single value.
Could it be that you really meant that instances of class Point are each
expected to have an attribute named Position? Could it be that you want
each instance's position to be independent of the others?
By having a class attribute with the same name as the instance
attribute, you're occasionally going to use the class version when you
meant the instance one.
I suspect you didn't realize the distinction between class attributes
and instance attributes, and that a comment would be a much better way
to communicate than creating a misleading value.
In one of your other messages, you asked:
> How do you guarantee that any object of a class has a
> specific set of attributes?
Answer is to define those INSTANCE attributes in the __init__() method
(or occasionally in the __new__() method), and to make sure you don't
ever delete them. The attributes of the object are instance attributes,
while the attributes defined inside the class are class attributes.
Occasionally, it can be useful to let a class attribute be a 'backup' to
the instance attributes, but you've got to think through your use case.
If memory is really tight, and if nearly all of the instances want the
same value, then you could omit the instance attribute except for the
exceptional ones, and let the class attribute fill in. But as Peter
says, if it's mutable, you then open yourself to somebody changing them
all, thinking it was only changing the one. But there are good reasons
not to use this trick to save memory.
--
DaveA
--
http://mail.python.org/mailman/listinfo/python-list
