Le Vendredi 23 Juin 2006 16:03, Carl Banks a écrit :
> Don't follow? The actual source code won't be much easier. Here's an
> example.
>
> class MetaThing(type):
> def __new__(metacls,name,bases,clsdict,extra_information):
> # use extra_information
> return type.__new__(metacls,name,bases,clsdict)
>
> class Thing(object):
> def __new__(cls,extra_information):
> clsdict = {'__new__':object.__new__}
> my_very_own_class = MetaThing(
> "Subthing",(Thing,),clsdict,extra_information)
> return object.__new__(my_very_own_class)
Hmmm, rigourously speaking, metaclasses in OOP are classes whose instances are
class.
Something like that :
In [114]: class MetaClass(object) :
.....: def __new__(cls, name, bases=(), **some_attributes) :
.....: return type('newtype %s' % name, bases, some_attributes)
.....:
.....:
Let's play with it :
In [115]: Concrete1 = MetaClass('conc1', (), classprop=1, method=lambda
s : "fun")
In [116]: Concrete2 = MetaClass('conc1', (), classprop=1, method=lambda
s : "fun")
In [117]: isinstance(Concrete1(), Concrete2)
Out[117]: False
In [118]: isinstance(Concrete1(), Concrete1)
Out[118]: True
In [119]: Concrete1().method()
Out[119]: 'fun'
In [120]: Concrete1.classprop
Out[120]: 1
In [121]: class Abstract(object) :
.....: def __init__(self) : self._attr = self._attr_type()
.....:
.....:
In [122]: Concrete = MetaClass('concrete_with_list', (Abstract,),
_attr_type=list)
In [123]: Concrete()._attr
Out[123]: []
In [124]: Concrete = MetaClass('concrete_with_int', (Abstract,),
_attr_type=int)
In [125]: Concrete()._attr
Out[125]: 0
In [126]: type(Concrete)
Out[126]: <type 'type'>
In [127]: type(Concrete())
Out[127]: <class '__main__.newtype concrete_with_int'>
regards,
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Maric Michaud
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