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multiple inheritance with builtins

Hello,

I noticed that bultin types like list, set, dict, tuple don't seem to adhere to
the convention of using super() in constructor to correctly allow
diamond-shaped inheritance (through MRO). For instance:

class A(object): .... def __init__(self):
.... print "A.__init__ "
.... super(A, self).__init__( )
.... class B(A, list): .... def __init__(self):
.... print "B.__init__ "
.... super(B, self).__init__( )
.... B.__mro__ (<class '__main__.B'>, <class '__main__.A'>, <type 'list'>, <type 'object'>) B() B.__init__
A.__init__
[] class C(list, A): .... def __init__(self):
.... print "C.__init__ "
.... super(C, self).__init__( )
.... C.__mro__ (<class '__main__.C'>, <type 'list'>, <class '__main__.A'>, <type 'object'>) C()

C.__init__
[]

It seems weird to me that I have to swap the order of bases to get the expected
behaviour. Is there a reason for this, or is it simply a bug that should be
fixed?
--
Giovanni Bajo
Jul 18 '05 #1
1 1052
Giovanni Bajo wrote:
Hello,

I noticed that bultin types like list, set, dict, tuple don't seem to adhere to
the convention of using super() in constructor to correctly allow
diamond-shaped inheritance (through MRO). For instance:
class A(object):
... def __init__(self):
... print "A.__init__ "
... super(A, self).__init__( )
...
class B(A, list):
... def __init__(self):
... print "B.__init__ "
... super(B, self).__init__( )
...
B.__mro__
(<class '__main__.B'>, <class '__main__.A'>, <type 'list'>, <type 'object'>)
B()
B.__init__
A.__init__
[]
class C(list, A):
... def __init__(self):
... print "C.__init__ "
... super(C, self).__init__( )
...
C.__mro__
(<class '__main__.C'>, <type 'list'>, <class '__main__.A'>, <type 'object'>)
C()


C.__init__
[]

It seems weird to me that I have to swap the order of bases to get the expected
behaviour. Is there a reason for this, or is it simply a bug that should be
fixed?


The documentation explicitly states that only one of the built-in types
can be used as a base class: they aren't desinged to be mixed with each
other.

regards
Steve
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/
Jul 18 '05 #2

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