On 06/12/11 15:57, Paul Moore wrote:
I want to set up an inheritance hierarchy. The base class will define
a string value which should include the class name, but I don't want
people who inherit from my class to have to remember to override the
value.
If I do this using an instance variable, it's reasonably easy:
class Base:
... def __init__(self):
... self.key = 'Key_for_' + self.__class__.__name__
... def display(self):
... print self.key
...
class Inherited(Base):
... pass
...
b = Base()
i = Inherited()
b.display()
Key_for_Base
i.display()
Key_for_Inherited
Rather than having the key for every instance, I'd like to use a class
variable, but I can't see how I'd make that work (a class variable
which is inherited but has a different value in derived classes). I
could use a classmethod,but that feels like even more overkill than an
instance attribute.
Is there a way of doing this via class variables or something, or more
relevantly, I guess, what would be the idiomatic way of doing
something like this?
Thanks,
Paul
You can use a metaclass for this:
>>> class BaseMeta(type):
... def __new__(mcs, name, bases, dict):
... dict['key'] = 'Key_for_%s' % name
... return type.__new__(mcs, name, bases, dict)
...
>>> class Base:
... __metaclass__ = BaseMeta
...
>>> class Inherited(Base):
... pass
...
>>> Base.key
'Key_for_Base'
>>> Inherited.key
'Key_for_Inheritor'
You can find more info on metaclasses here:
http://http://docs.python.org/reference/datamodel.html#customizing-class-creation
Regards
Matt
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