Swapping superclass from a module

[Terry Reedy]

Peter Otten wrote:
> Terry Reedy wrote:
> 

>> If the names of superclasses is resolved when classes are instantiated,
>> the patching is easy.  If, as I would suspect, the names are resolved
>> when the classes are created, before the module becomes available to the
>> importing code, then much more careful and extensive patching would be
>> required, if it is even possible.  (Objects in tuples cannot be
>> replaced, and some attributes are not writable.)
> 
> It may be sufficient to patch the subclasses:

I was not sure if __bases__ is writable or not.  There is also __mro__ 
to consider.

> $ cat my_file.py
> class Super(object):
>     def __str__(self):
>         return "old"
> 
> class Sub(Super):
>     def __str__(self):
>         return "Sub(%s)" % super(Sub, self).__str__()
> 
> class Other(object):
>     pass
> 
> class SubSub(Sub, Other):
>     def __str__(self):
>         return "SubSub(%s)" % super(SubSub, self).__str__()
> 
> if __name__ == "__main__":
>     print Sub()
> 
> $ cat main2.py
> import my_file
> OldSuper = my_file.Super
> 
> class NewSuper(OldSuper):
>     def __str__(self):
>         return "new" + super(NewSuper, self).__str__()
> 
> my_file.Super = NewSuper
> for n, v in vars(my_file).iteritems():
>     if v is not NewSuper:
>         try:
>             bases = v.__bases__
>         except AttributeError:
>             pass
>         else:
>             if OldSuper in bases:
>                 print "patching", n
>                 v.__bases__ = tuple(NewSuper if b is OldSuper else b
>                                     for b in bases)
> 
> 
> print my_file.Sub()
> print my_file.SubSub()
> $ python main2.py
> patching Sub
> Sub(newold)
> SubSub(Sub(newold))
> 
> Peter
>