I have 2 classes each implementing a specific behavior and using parent classes.
Now I have a new class that needs to use either behavior but which one can only be determined during/after construction.
That is currently being done by using multiple inheritance.
However as the base classes were not written taking this into account the contained super()
call will call into the wrong sub-tree
Consider the following MWE:
class A:
def __init__(self, use_b):
self.init_b = use_b # Maybe complicated
def foo(self):
print("A")
class B(A):
def foo(self):
super().foo()
print("B")
class C1(A):
def foo(self):
super().foo()
print("C1")
class C2(C1):
def foo(self):
super().foo()
print("C2")
class D(B, C2):
def __init__(self, *args):
super().__init__(*args)
self.use_b = self.init_b
def foo(self):
if self.use_b:
B.foo(self)
else:
C2.foo(self)
d = D(True)
d.foo()
print("NEXT")
d = D(False)
d.foo()
Somewhere in A
a property is initialized that is used in D
to determine the appropriate class to use. Both of the candidates B
and C2
inherit from the base class as they are/used to be independent implementations
It works for calling C2.foo
but calling B.foo
ends up calling foo
in C2
and C1
too, which I do not want.
I do know WHY this happens (MRO) but have no good idea to resolve this.
I might be able to have D
only inherit from A
, initialize this and then decide which subclass to use, i.e.:
class D(A):
def __init__(self, *args):
super().__init__(*args)
if self.init_b:
self.impl = B(*args)
else:
self.impl = C2(*args)
def foo(self):
self.impl.foo()
But this has 3 issues:
- It initializes at least
A
multiple times. That is bad for performance and might fail ifA.__init__
modifies the ref-type args (e.g. lists) - It might fail if there are some args only
B
andC2
understand soA
can’t handle them - I’ll need to implement and wrap all methods of
A
that could be called. If I forget one that was overwritten by either of theB
orC
classes the behavior will be silently wrong. Similar for properties
Is there a clean and safe way to handle this scenario?