[C++-sig] Inheritance and __init__
Mark.English at rbccm.com
Wed Oct 31 12:07:33 CET 2007
> -----Original Message-----
> From: c++-sig-bounces at python.org
> [mailto:c++-sig-bounces at python.org] On Behalf Of Bue Vedel-Larsen
> Sent: 31 October 2007 10:46
> To: c++-sig at python.org
> Subject: Re: [C++-sig] Inheritance and __init__
> >> class Base:
> >> def foo(self, str):
> >> print str
> >> class Derived(Base):
> >> def __init__(self):
> >> self.foo('__init__')
> >> d = Derived()
> >> // This doesn't work
> >> run_script(
> >> "class DerivedB(Base):\n"
> >> "\tdef __init__(self):\n"
> >> "\t\tself.foo('In Python: DerivedB.__init__()')\n"
> >> "dB = DerivedB()\n"
> >> );
> > Does it make a difference if in the __init__ methods you call
> > "super(KlassName, self).__init__()" first ?
> > E.g.
> > "class DerivedB(Base):\n"
> > "\tdef __init__(self):\n"
> > "\t\tsuper(DerivedB, self).__init__()
> > "\t\tself.foo('In Python: DerivedB.__init__()')\n"
> Amazing, adding "super(DerivedB, self).__init__()" or
> "Base.__init__ (self)" before self.foo('In Python:
> DerivedB.__init__()') makes it work.
> But why is this necessary? The pure Python example works
> without explicitly calling the __init__ method of the Base
> class. Do my class_ definition of the module lack something?
As far as I know, the Python code working is undefined behaviour.
Whenever overriding __init__() (especially for new style classes which
Boost Python class_ instances implicitly are), call the base class
__init__() passing through arguments as necessary.
If you don't do this, the __init__ of the base class won't get called
which for Boost Python is bad news.
self.data = 1
print 'I am derived'
>>> d = Derived() #Happy
I am derived
>>> d.data #Unhappy
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