Derived classes sometimes need to delegate portions of the work in overridden
methods to methods in their base classes. This was traditionally done with
explicit calls in python, e.g.,
class Derived(Left, Right):
def __init__(self, myarg, larg, rarg):
Left.__init__(self, larg)
Right.__init__(self, rarg)
self.data = myarg
print 'derived'
This worked great. It was possible to grab the appropriate arguments and send
them off to the right point. However, there was a problem.
class Base:
def __init__(self):
print 'base'
class Left(Base):
def __init__(self, arg):
Base.__init__(self)
print 'left'
class Right(Base):
def __init__(self, arg):
Base.__init__(self)
print 'right'
Now, when Derived(Left, Right) is initialized, Base.__init__ is called twice.
Sometimes that's O.K. Usually, it's a bad thing. Along came new-style
classes and 'super'. Unfortunately, super-based delegation doesn't play
nicely with traditional classes.
http://www.ai.mit.edu/people/jknight/super-harmful/
Moreover, it undermines any attempts to control which subset of arguments go
to which base class. This second problem is serious. In real life, base
classes differ from each other: I need to be able to send the right arguments
to each.
What I really want to do is explicitly delegate tasks to base classes,
choosing the arguments to send to each, but avoid double-calls resulting from
reconverging paths in the inheritance directed acyclic (pray it's acyclic)
graph.
I think the appended code may solve this problem, play nicely with traditional
classes, and allow the coder to send the right arguments to the right base
classes.
However, I'm new to python so I need some help.
1) Is my thinking reasonable or is there an better way to solve the
reconvergent path problem in python without undermining control over
arguments?
2) What's the proper way to rewrite the appended code. I'm sure it's
dreadfully inefficient. There are also probably ways to make its use more
intuitive, but I'm new to the language so I don't know the tricks yet.
Thanks for any tips,
-Robert Dick-
----
'''See the example at the bottom.'''
import inspect
def flatten_tree(tree):
'''Flatten a tree represented by nested lists'''
if isinstance(tree, list):
return [j for i in tree for j in flatten_tree(i)]
else:
return (tree,)
# Cache for delegation decisions.
call_cache = set()
def should_call(self, pos, supr):
'''Examines the inheritance DAG (might work for DCGs, too... haven't
checked) for 'self' to determine whether 'pos' is the leftmost derived
for 'supr'. Returns bool. Caches results for performance.'''
if (self.__class__, pos, supr) in call_cache: return True
ct = flatten_tree(inspect.getclasstree(inspect.getmro(s elf.__class__),
True))
# ct is a list of (class, (base classes)) tuples
# Find the first instance of the supr as a base class
do_call = pos is [cls for cls, bases in ct if supr in bases][0]
if do_call: call_cache.add((self.__class__, pos, supr))
return do_call
def delegate(self, pos, s_call, *pargs, **kargs):
'''If 'pos' is the leftmost derived for 's_call' in the 'self' inheritance
DAG, call s_call with 'pargs' and 'kargs'.'''
if inspect.ismethoddescriptor(s_call):
supr = s_call.__objclass__
else:
supr = s_call.im_class
if should_call(self, pos, supr):
s_call(self, *pargs, **kargs)
if __name__ == '__main__':
class Base(object):
def __init__(self):
delegate(self, Base, object.__init__)
print 'base'
class Left(Base):
def __init__(self):
delegate(self, Left, Base.__init__)
print 'left'
class Right(Base):
def __init__(self):
delegate(self, Right, Base.__init__)
print 'right'
class Der(Left, Right):
def __init__(self):
delegate(self, Der, Left.__init__)
delegate(self, Der, Right.__init__)
print 'der'
der = Der()
