is there any way to tell the class the base class during runtime?
a.
a.
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Re: dynamic inheritance
On Thu, 08 Jun 2006 21:14:48 -0400,
alf <ask@me> wrote:
[color=blue]
> is there any way to tell the class the base class during runtime?[/color]
[color=blue][color=green][color=darkred]
>>> class A(object):
>>> pass
>>> class B(object):
>>> pass
>>> o = A()
>>> o.__class__[/color][/color][/color]
<class '__main__.A'>[color=blue][color=green][color=darkred]
>>> o.__class__ = B
>>> o.__class__[/color][/color][/color]
<class '__main__.B'>
I don't know if that's a good idea. Maybe this one:
class A(object):
pass
class B(object):
pass
if some_function() :
C = A
else:
C = B
class D(C):
pass
Why do you want to change a class' base class during runtime?
HTH,
Dan
--
Dan Sommers
<http://www.tombstoneze ro.net/dan/>
"I wish people would die in alphabetical order." -- My wife, the genealogist
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Re: dynamic inheritance
alf wrote:[color=blue]
> is there any way to tell the class the base class during runtime?
>
> a.[/color]
Example:
[color=blue][color=green][color=darkred]
>>> class A(object):pass
>>> class B(A):pass
>>> B.mro()[/color][/color][/color]
[<class '__main__.B'>, <class '__main__.A'>, <type 'object'>]
See also Micheles nice article about the semantics of the "mro" (
method resolution order).
Regards,
Kay
Comment
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Re: dynamic inheritance
alf wrote:[color=blue]
> is there any way to tell the class the base class during runtime?
>[/color]
Technically, yes - the solution depending on your definition of "during
runtime"
FWIW, the class statement is evaled at import/load time, which is
"during runtime".... So if you want to use one or other (compatible)
classes depending on configuration or system or like, you can use a
conditionnal at the top level, *before* the class statement is eval'd. ie:
import os
if os.name == 'posix':
import posixmodule as basemodule
elif os.name == 'nt':
import ntmodule as basemodule
# etc...
class MyClass(basemod ule.baseclass):
# class def here
If you want to dynamically change the base class (or one of the base
classes) during execution (ie: after the class statement has been
eval'd), read Kay Schluehr's answer.
*But* you'd probably better tell us about the problem you're trying to
solve. Since in Python, inheritance is mostly about implementation (ie:
not needed for subtyping), your problem would probably be best solved
with composition/delegation, for which Python offers a good support:
class MyClass(object) :
def __init__(self, delegate):
self._delegate = delegate
def __getattr__(sel f, name):
return getattr(self._d elegate, name)
or, if you don't want to explicitely pass the delegate at instanciation
time:
import os
if os.name == 'posix':
import posixmodule as basemodule
elif os.name == 'nt':
import ntmodule as basemodule
# etc...
class MyClass(object) :
_delegate_class = basemodule.Some Class
def __init__(self):
self._delegate = self._delegate_ class()
# etc
there are of course some variants of the above solutions, but one can't
tell you which one to use without knowing more about your actual problem.
HTH
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"
Comment
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Re: dynamic inheritance
bruno at modulix wrote:[color=blue]
>
> *But* you'd probably better tell us about the problem you're trying to
> solve. Since in Python, inheritance is mostly about implementation (ie:
> not needed for subtyping), your problem would probably be best solved
> with composition/delegation, for which Python offers a good support:
>[/color]
I did not think about any particular problem, just thought it would be
cool to abstract out the base class. In fact you can do that in C++ (to
some extend) using templates and parameterizing the base class.
regards,
a.
Comment
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Re: dynamic inheritance
alf wrote:[color=blue]
> I did not think about any particular problem, just thought it would be
> cool to abstract out the base class. In fact you can do that in C++ (to
> some extend) using templates and parameterizing the base class.[/color]
Python is ways cooler than C++. This is a sensible use case where you
may
want to change the base class at runtime:
[color=blue][color=green][color=darkred]
>>> class Base(object):[/color][/color][/color]
.... pass
[color=blue][color=green][color=darkred]
>>> class BasePlusDebugMe thods(Base):[/color][/color][/color]
.... pass
....
[color=blue][color=green][color=darkred]
>>> class C(Base):[/color][/color][/color]
.... pass
[color=blue][color=green][color=darkred]
>>> C.__bases__ = (BasePlusDebugM ethods,)[/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> C.mro()[/color][/color][/color]
[<class '__main__.C'>,
<class '__main__.BaseP lusDebugMethods '>,
<class '__main__.Base' >,
<type 'object'>]
(i.e. in a running program with a problem you can add debug methods and
possibily
even fix the problem without restarting the program).
Michele Simionato
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Re: dynamic inheritance
Michele Simionato wrote:[color=blue]
> alf wrote:
> Python is ways cooler than C++.[/color]
I switched to Python from C++ over year ago and do not see a way back.
C++ just sucks at each corner.
[color=blue]
> This is a sensible use case where you may
> want to change the base class at runtime:[/color]
Thx for the example.
A.
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