On 12/11/2016 5:29 PM, Gregory Ewing wrote:
Juan C. wrote:
The instructor said that the right way to call a class attribute is to
use
'Class.class_attr' notation, but on the web I found examples where people
used 'self.class_attr' to call class attributes. I believe that using the
first notation is better ('Class.class_attr'), this way the code is more
explicit, but is there any rules regarding it?
Yes. Use the form appropriate to the situation. In other words, use
open-eyed rule, not a closed-eye rule. This applies to much of Python
programming and programming is general. Greg nicely explains the
application of this rule.
It depends on how the class attribute is being used.
If you're only reading the attribute, either way will work.
Which one is more appropriate depends on what the attribute
is used for. Often a class attribute is used as a default
value for an instance attribute, in which case accessing it
via the instance is entirely appropriate.
The use of a (constant) class attribute as default instance attribute
might be an optimization added after the first version of the class, or
one that could disappear in the future.
> On the other
hand, if it's truly mean to be an attribute of the class
itself, accessing it via the class is probably clearer.
If the attribute is being written, you don't have any
choice. If you want to rebind the attribute in the class,
you have to access it via the class. This is the case
for this line in your example:
Box.serial += 1
If instead you did 'self.serial += 1' it would create
a new instance attribute shadowing the class attribute,
and the class attribute would remain bound to its
previous value.
I agree with the other post suggesting using 'next_serial' as the class
attribute, as that is what the class attribute is. I would access it as
Box.serial.
Instance methods should normal be accessed through an instance, though
there are exceptions.
--
Terry Jan Reedy
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