If you need multiple factories you can do so but to do what you're asking requires both a factory and an instance method initializer, just like __new__ is a class method and __init__ is an instance method. One creates and one initializes what is already created.
On Dec 12, 2007, at 12:49 PM, Yoav Goldberg wrote: > A lot of my code supplement/replace the constructor with class > factory methods. > > So, instead of: > > a= A(...) > > I would write: > > a = A.from_file(...) > > a = A.create(...) > etc. > > This is implemented as follows: > > class A: > def __init__(self, ...): pass > > @classmethod > def from_file(cls, ....): > inst = cls() > .... > return inst > > Derived classes of A can of course use these factories to create > instances of the derived class as well. > > The problem arise when the derived class needs a different > implementation of the factory. > If it was a constructor (or any method for that matter), this would > be achieved as: > > class B(A): > def __init__(self, ...): > A.__init__(self, ...) > .... > > However, this can not be done with the factory: > > class B(A): > @classmethod > def from_file(cls, ...) > inst = A.from_file(...) > .... > return inst > > will result with the type of the created object being A and not B. > > This can be overcome in two ways. The first (neat hack) is due to a > colleague of mine, using static methods and default arguments: > > class A: > @staticmethod > def from_file(cls=None, ...) > if not cls: cls = A > inst = cls() > ... > > class B(A): > @staticmethod > def from_file(cls=None, ...) > if not cls: cls = B > inst = A.from_file(cls, ...) > > > The second is by delegating the work of the factory to an instance > method: > > class A: > @classmethod > def from_file(cls,...) > inst = cls() > inst._from_file(...) > return inst > > and then overriding the instance method and never touching the > factory method. > > > Now, my questions are as follow: > > 1/ Am I missing a third, more "standard" way of achieving > inheritance of class methods? > 2/ If not, I think future python's should have a syntax for this -- > the first method I proposed is a neat hack, and the second is a > workaround, but I wouldn't want the first method to be the official > way of doing things, and I suspect there are some use cases that > can not be implemented as nicely with the second method. > > > Yoav > -- > http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
