Hi all
I have a small problem. I have come up with a solution, but I don't
know if it is a) safe, and b) optimal.
I have a class with a number of attributes, but for various reasons I
cannot assign values to all the attributes at __init__ time, as the
values depend on attributes of other linked classes which may not have
been created yet. I can be sure that by the time any values are
requested, all the other classes have been created, so it is then
possible to compute the missing values.
At first I initialised the values to None, and then when I needed a
value I would check if it was None, and if so, call a method which
would compute all the missing values. However, there are a number of
attributes, so it got tedious. I was looking for one trigger point
that would work in any situation. This is what I came up with.
>>> class A(object):
... __slots__ = ('x','y','z')
... def __init__(self,x,y):
... self.x = x
... self.y = y
... def __getattr__(self,name):
... print 'getattr',name
... if name not in self.__class__.__slots__:
... raise AttributeError,name
... self.z = self.x * self.y
... return getattr(self,name)
>>> a = A(3,4)
>>> a.x
3
>>> a.y
4
>>> a.z
getattr z
12
>>> a.z
12
>>> a.q
getattr q
Attribute Error: q
In other words, I do not declare the unknown attributes at all. This
causes __getattr__ to be called when any of their values are
requested, and __getattr__ calls the method that sets up the
attributes and computes the values.
I use __slots__ to catch any invalid attributes, otherwise I would get
a 'maximum recursion depth exceeded' error.
Is this ok, or is there a better way?
Thanks
Frank Millman
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