On Sun, 23 May 2010 15:40:13 -0400
Alex Hall <[email protected]> wrote:
> Hello all,
> I know Python reasonably well, but I still run into basic questions
> which those over on the other python list request I post here instead.
> I figure this would be one of them:
> Why would this not work:
>
> class c(object):
> def __init__(self, arg1, arg2):
> self.arg1=arg1
> self.arg2=arg2
>
> def doSomething(self, arg3=self.arg1):
> ...
>
> The above results in an error that "name 'self' is not defined". Why
> can I not set the default values of a method's arguments to class vars
> like that? Thanks!
Python gives you the answer (I'm not kidding): "name 'self' is not defined".
"self" is not a magic name in python, unlike in some other languages,
automagically receiving a value. It is just a name you can use for anything,
like "self=1".
A *convention* tells that this name is used for the *target* of a message. For
instance:
l = [3,1,2]
l.sort()
calls sort on the target l, which becomes the object on which sort applies.
Right? Then, python magically (this is the only magic) adds l as first argument
to the method sort of the list class, at call time. If sort were defined in
python, its header could look like:
def sort(target_list, compare_function=None):
The convention requires "target_list" to be rather called "self".
In your code, nowhere is a variable called "self" defined, before you try to
use it. Since this happens in a class def, it should be first defined outside
(probably in the global scope). If you write "self=1" somewhere before the
class def, python will stop complaining about "self"... but it will complain
about "arg1"!
A solution for what I guess you're trying to do is:
def doSomething(self, arg3=None):
if arg3 is None then arg3=self.arg1
The reason is that your default value, not only is not a constant, but is a
variable depending on the actual instance, at call time. The only handle
available to address an instance is precisely as target of a method call. So,
you can only define something (here a default value) that depends on the
instance from *inside* a method body, where the handle exists (magically
brought by python).
Hope it's clear.
Denis
________________________________
vit esse estrany ☣
spir.wikidot.com
_______________________________________________
Tutor maillist - [email protected]
To unsubscribe or change subscription options:
http://mail.python.org/mailman/listinfo/tutor
