get a self data in a method call

[Bengt Richter]

On 23 Dec 2002 07:23:15 GMT, bokr at oz.net (Bengt Richter) wrote:
[... several examples, one with a bad sentinel ...]
><you can skip this>
>Well, I don't really like referring to SomeClass.anything
>from a method within SomeClass. It's an ugly dependency
>on a global binding IMO. With new classes there is probably
>a way around it. Ok, here's a go at getting around a global binding,
>in this case of a sentinel:
>
> >>> class MyClass(object):
> ...     _sentinel = None
                      ^^^^
This is a goof, sorry ;-/
_sentinel should be bound to a unique object, not None, which
is the same object for every specified reference to None ;-/

Pretty silly, having warned about using None ;-/ Sorry about that.
Just typed in None and didn't test very well. Noticed when looking
back at it now. Instead, this should be better:

 >>> class MyClass(object):
 ...     _sentinel = object()
 ...     default = 'A class variable available via any instance'
 ...     class __metaclass__(type):
 ...         def __init__(cls, name, bases, dct):
 ...             def func(self, data=cls._sentinel):
 ...                 if data is cls._sentinel: data = getattr(self,'data','<no data>')
 ...                 return (data, self.default) # or otherwise use them
 ...             setattr(cls, 'func', func)
 ...             def __init__(self, data=cls._sentinel):
 ...                 if data is cls._sentinel: return # don't set it if not specified
 ...                 self.data = '%s [%s]' % (data, self.default[0]) # silly mod using class data  

 ...             setattr(cls, '__init__', __init__)
 ...     def ordinary(self): return 'ordinary, returning self.data if avail: %s' % self.data
 ...
 >>> mc=MyClass()
 >>> mc.default
 'A class variable available via any instance'
 >>> mc.func()
 ('<no data>', 'A class variable available via any instance')
 >>> vars(mc)
 {}
 >>> mc.func('func arg')
 ('func arg', 'A class variable available via any instance')
 >>> vars(mc)
 {}
 >>> mc.func(None)
 (None, 'A class variable available via any instance')
 >>> mc=MyClass('specified')
 >>> vars(mc)
 {'data': 'specified [A]'}
 >>> mc.data
 'specified [A]'
 >>> mc.func('func arg')
 ('func arg', 'A class variable available via any instance')
 >>> mc.func()
 ('specified [A]', 'A class variable available via any instance')
 >>> del mc.data
 >>> mc.func()
 ('<no data>', 'A class variable available via any instance')
 >>> mc=MyClass(None)
 >>> vars(mc)
 {'data': 'None [A]'}
 >>> mc.data
 'None [A]'
 >>> mc.func('func arg')
 ('func arg', 'A class variable available via any instance')
 >>> mc.func()
 ('None [A]', 'A class variable available via any instance')
 >>> del mc.data
 >>> mc.func()
 ('<no data>', 'A class variable available via any instance')
 >>> mc.data = 'added directly'
 >>> mc.func()
 ('added directly', 'A class variable available via any instance')
 >>> mc.func(MyClass._sentinel) # should fool it to do same as mc.Func()
 ('added directly', 'A class variable available via any instance')
 >>> mc.func('x')
 ('x', 'A class variable available via any instance')

Regards,
Bengt Richter