Here is a problem I am having trouble with and I hope someone in this group
will suggest a solution. First, some code that works. 3 classes that are
derived from each other (A->B->C), each one implementing only 2 methods,
__init__ and setConfig.
-------------------------------------------------------
#!/usr/bin/python
class A (object):
def __init__(self):
super(A, self).__init__()
self.x = 0
def setConfig(self, config):
self.x += config['x']
class B (A):
def __init__(self):
super(B, self).__init__()
self.y = 0
def setConfig(self, config):
super(B, self).setConfig(config)
self.y += config['y']
class C (B):
def __init__(self):
super(C, self).__init__()
self.z = 0
def setConfig(self, config):
super(C, self).setConfig(config)
self.z += config['z']
config = {'x':1, 'y':2, 'z':3}
alpha = A()
alpha.setConfig(config)
print alpha.x
beta = B()
beta.setConfig(config)
print beta.x, beta.y
beta.setConfig(config)
print beta.x, beta.y
gamma = C()
gamma.setConfig(config)
print gamma.x, gamma.y, gamma.z
gamma.setConfig(config)
print gamma.x, gamma.y, gamma.z
--------------------------------------------------
The output from that code is:
1
1 2
2 4
1 2 3
2 4 6
So far, so good! But let's assume that I want to change the __init__
methods so that they take a configuration as an argument so the objects are
created and configured in one step, like this:
alpha = A(config)
How can the code be changed to implement this? It is tempting to modify the
__init__ methods like this:
class A (object):
def __init__(self, config):
super(A, self).__init__(config)
self.x = 0
A.setConfig(self, config)
However, if implemented this way, the output is:
1
2 2
3 4
3 4 3
4 6 6
This shows that setConfig gets called more than once because both __init__
and setConfig are cooperative methods.
I have been thinking about this for a day now and I cannot find a good
solution. I imagine this would be a common problem and that there would be
a recipe somewhere, but I couldn't find one (I looked in the Python
Cookbook). I've thought of creating 2 separate methods instead of
setConfig, one that does the "local" configuration and another one that
takes care of invoking super. But I don't like the idea of creating 2
methods in each class and I haven't been able to think of a way to solve the
problem with just one change in the base class that would be inherited by
all the subclasses.
Anyone has any ideas? Thanks.
Dan
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