[Python] While executing the class definition which object is referenced by the first argument of the class method, Y r Object attributes not allowed as default arguments

En Thu, 06 Mar 2008 22:48:42 -0200, Krishna <Krishna.00.K at gmail.com>
escribi?:
Function default arguments are evaluated when the function is defined
(when the class is defined, in this case) so "self" itself has not a
value. Try this instead:

def func(self, k=None):
if k is None:
k = self.a
print k

If None is an allowed argument, use a special marker instead:

_marker=object()
...

def func(self, k=_marker):
if k is _marker:
k = self.a
...

Thanks for the reply. I am currently using the approach suggested by
you. But, I am more interested in knowing about the first argument
('self'), what does it hold to allow the evaluation of the method,
take the example you gave, 'self.a' as Rvalue inside the method, how
and why is this allowed, when the same 'self.a' is not allowed as the
default argument, considering the fact that I have already specified
'self' as first argument, only after whose evaluation, I believe would
the next statement (k = self.a, in def func(self, k = self.a) ) gets
evaluated

Thanks,
Krishna

Is there enough information at that point in the statement to assign
to k as specified by this language?

No.

Does there exist a possible language in which there is?

Yes.

"self" is treated as an argument like any other argument, except that
when you call a method on an instance Python automatically provides the
self for you.

Consider the following piece of code:
... def squawk(self, n):
... return "spam " * n
...
'spam spam spam '
'spam spam spam '


In the first call, I call the squawk() method from the instance, and
Python automatically fills in the self argument.

In the second call, I call the squawk() method from the class. Since the
class doesn't know what instance I'm using, I have to manually provide
the self argument.

But inside the method, self is just a name in a namespace, like any other
name. It's not special. You can do anything you like to it. You can even
re-assign to it, or delete it:
... def spam(self):
... print "I am", self
... self = "foo"
... print "Now I am", self
...
I am <__main__.Spam object at 0xb7f5192c>
Now I am foo
<__main__.Spam object at 0xb7f5192c>


It isn't that self.a is "not allowed". Python doesn't contain any code
that says "if the default value contains "self", raise an error. You can
prove that for yourself:
... return x
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'self' is not defined
... return x
...
2


You believe wrong. You've already been told that the function default
"k=self.a" is evaluated when the method is compiled, not at runtime.
Since "self" doesn't exist at compile time, it is an error.

There is no way to create a reference to a class instance before the
class is even defined.