one-time initialization of class members

[Gabriel Genellina]

En Wed, 13 Jun 2007 22:03:50 -0300, Steven D'Aprano  
<steve at REMOVE.THIS.cybersource.com.au> escribió:

> On Wed, 13 Jun 2007 23:55:02 +0000, James Turk wrote:
>>> James Turk wrote:
>>>
>>> > I have a situation where I have some class members that should only  
>>> be
>>> > done once.  Essentially my problem looks like this:
>>>
>> ChildClass doesn't take the parameter in it's constructor, it supplies
>> it for the BaseClass.  Every ChildClass of the same type should use
>> the same dataset.
>
> Then each type of ChildClass should be a sub-class, and provide it's own
> dataset:
>
>
> class BaseClass:
>     dataset = None
>     # blah blah blah...
>
>
> class ChildClass1(BaseClass):
>     dataset = SomethingUseful
> class ChildClass2(BaseClass):
>     dataset = SomethingElse

But the OP also stated that creating the dataset is expensive. The  
original code does what you say (each ChildClass is a subclass and  
provides its own dataset) so I think is an acceptable solution:

py> def build_dataset(x):
...     print "building dataset:",x
...     return [x]
...
py> class Base(object):
...     dataset = None
...     def __init__(self, param):
...         if type(self).dataset is None:
...             type(self).dataset = build_dataset(param)
...
py> class ChildClass1(Base):
...     def __init__(self):
...         Base.__init__(self, "Params for ChildClass1")
...
py> class AnotherChildClass(Base):
...     def __init__(self):
...         Base.__init__(self, "Params for AnotherChildClass")
...
py> c1 = ChildClass1()
building dataset: Params for ChildClass1
py> c2 = AnotherChildClass()
building dataset: Params for AnotherChildClass
py> c3 = ChildClass1()
py> print Base.dataset
None
py> print ChildClass1.dataset
['Params for ChildClass1']
py> print AnotherChildClass.dataset
['Params for AnotherChildClass']
py> print c1.dataset
['Params for ChildClass1']
py> print c3.dataset
['Params for ChildClass1']
py> print c1.dataset is c3.dataset
True
py>


-- 
Gabriel Genellina