Mitchell L Model wrote:
From: Scott David Daniels <scott.dani...@acm.org>
Date: Tue, 30 Jun 2009 16:49:18 -0700
Message-ID: <ieudnfm_j89ep9fxnz2dnuvz_g6dn...@pdx.net>
Subject: Re: invoking a method from two superclasses
Mitchell L Model wrote:
In Python 3, how should super() be used to invoke a method defined in C
that overrides its two superclasses A and B, in particular __init__?
...
I've discovered the surprising fact described in the documentation of super
<http://docs.python.org/3.1/library/functions.html#super>
that specifying a class as the first argument of super means to skip that class
when
scanning the mro so that ....
This seems weird. Would someone please give a clear example and explanation of
the recommended way of initializing both superclasses in a simple multiple
inheritance
situation?
OK, in Diamond inheritance in Python (and all multi-inheritance is
diamond-shaped in Python), the common ancestor must have a method
in order to properly use super. The mro is guaranteed to have the
top of the split (C below) before its children in the mro, and the
join point (object or root below) after all of the classes from
which it inherits.
So, the correct way to do what you want:
class A:
def __init__(self):
super().__init__()
print('A')
class B:
def __init__(self):
super().__init__()
print('B')
class C(A, B):
def __init__(self):
super().__init__()
print('C')
C()
And, if you are doing it with a message not available in object:
class root:
def prints(self):
print('root') # or pass if you prefer
class A(root):
def prints(self):
super().prints()
print('A')
class B(root):
def prints(self):
super().prints()
print('B')
class C(A, B):
def prints(self):
super().prints()
print('C')
C().prints()
--Scott David Daniels
scott.dani...@acm.org
Great explanation, and 1/2 a "duh" to me. Thanks.
What I was missing is that each path up to and including the top of the diamond
must include a definition of the method, along with super() calls to move the
method
calling on its way up. Is this what the documentation means by
"cooperative multiple inheritance"?
I expect so.
... Again, many thanks for the quick and clear response.
Since you know exactly what is confusing right now, and
what the resolution is, could I lean on you to take a stab
at improving the explanation? I think I know what the
solution is, but I don't have a good handle on what needs
explaining and what is easily understood. If you do something,
others can fix it where it doesn't work.
--Scott David Daniels
scott.dani...@acm.org
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