FAQ
class Test(object):
... def __init__(self):
... self.a= 2
... def func(self, k = self.a):
... print k
...
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 4, in Test
NameError: name 'self' is not defined
>>>

In the 'definition of the class', what would the first argument 'self'
in the methods evaluate to; when we have an object defined, it is
bound to the object reference, but what happens while the class
definition is executed, which I believe happens when the module
containing the class definition is imported

Thanks,
Kr

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  • Gabriel Genellina at Mar 7, 2008 at 1:04 am
    En Thu, 06 Mar 2008 22:48:42 -0200, Krishna <Krishna.00.K at gmail.com>
    escribi?:
    class Test(object):
    ... def __init__(self):
    ... self.a= 2
    ... def func(self, k = self.a):
    ... print k
    ...
    Traceback (most recent call last):
    File "<stdin>", line 1, in ?
    File "<stdin>", line 4, in Test
    NameError: name 'self' is not defined
    In the 'definition of the class', what would the first argument 'self'
    in the methods evaluate to; when we have an object defined, it is
    bound to the object reference, but what happens while the class
    definition is executed, which I believe happens when the module
    containing the class definition is imported
    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
    ...

    --
    Gabriel Genellina
  • Krishna at Mar 7, 2008 at 5:49 pm

    On Mar 6, 5:04 pm, "Gabriel Genellina" wrote:
    En Thu, 06 Mar 2008 22:48:42 -0200, Krishna <Krishna.0... at gmail.com>
    escribi?:


    class Test(object):
    ... def __init__(self):
    ... self.a= 2
    ... def func(self, k = self.a):
    ... print k
    ...
    Traceback (most recent call last):
    File "<stdin>", line 1, in ?
    File "<stdin>", line 4, in Test
    NameError: name 'self' is not defined
    In the 'definition of the class', what would the first argument 'self'
    in the methods evaluate to; when we have an object defined, it is
    bound to the object reference, but what happens while the class
    definition is executed, which I believe happens when the module
    containing the class definition is imported
    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
    ...

    --
    Gabriel Genellina
    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
  • Castironpi at Mar 7, 2008 at 7:14 pm

    On Mar 7, 11:49?am, Krishna wrote:
    On Mar 6, 5:04 pm, "Gabriel Genellina" wrote:




    En Thu, 06 Mar 2008 22:48:42 -0200, Krishna <Krishna.0... at gmail.com>
    escribi?:
    class Test(object):
    ... ? ? def __init__(self):
    ... ? ? ? ? ? ? self.a= 2
    ... ? ? def func(self, k = self.a):
    ... ? ? ? ? ? ? print k
    ...
    Traceback (most recent call last):
    ? File "<stdin>", line 1, in ?
    ? File "<stdin>", line 4, in Test
    NameError: name 'self' is not defined
    In the 'definition of the class', what would the first argument 'self'
    in the methods evaluate to; when we have an object defined, it is
    bound to the object reference, but what happens while the class
    definition is executed, which I believe happens when the module
    containing the class definition is imported
    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
    ? ? ? ? ?...
    --
    Gabriel Genellina
    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- Hide quoted text -

    - Show quoted text -
    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.
  • Steven D'Aprano at Mar 7, 2008 at 9:44 pm

    On Fri, 07 Mar 2008 09:49:58 -0800, Krishna wrote:

    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,
    "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:
    class Parrot(object):
    ... def squawk(self, n):
    ... return "spam " * n
    ...
    instance = Parrot()
    instance.squawk(3)
    'spam spam spam '
    Parrot.squawk(instance, 3)
    '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:
    class Spam(object):
    ... def spam(self):
    ... print "I am", self
    ... self = "foo"
    ... print "Now I am", self
    ...
    x = Spam()
    x.spam()
    I am <__main__.Spam object at 0xb7f5192c>
    Now I am foo
    x
    <__main__.Spam object at 0xb7f5192c>


    when the same 'self.a' is not allowed as the default argument,
    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:
    def foo(x=self):
    ... return x
    ...
    Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    NameError: name 'self' is not defined
    self = 2
    def foo(x=self):
    ... return x
    ...
    foo()
    2


    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
    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.




    --
    Steven

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postedMar 7, '08 at 12:48a
activeMar 7, '08 at 9:44p
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