[Tutor] [Fwd: Re: Intercepting methods calls]

[Andreas Kostyrka]

On Friday 06 June 2008 18:19:23 you wrote:
> On Thu, June 5, 2008 9:39 am, Andreas Kostyrka wrote:
> > On Thursday 05 June 2008 00:18:55 Marilyn Davis wrote:
> >> You listed __init__ and I'm not sure I know what you mean.
> >
> > Well, __init__ can assign attributes to the instance that are callable.
>
> Oh, well, Python is such a wide-open system.   It keeps blowing my mind.

Well, there is basically nothing that you cannot force the object system of Python to do, almost.

E.g. transplanting methods from class A to class B:

B.__dict__["method"] = A.__dict__["method"]

Or deriving a class from itself (although a better name would be "from a class that happened to be named the same).

class A:
	pass
class A(A):
	pass
print A.__bases__

Now, as an observation, you will notice that most experienced Python developers usually stay away from all this "magic" stuff. Rules of thumb include "explicit is good, implicit is bad", and so on. 
Clearly stuff tagged as "you really should know what you are doing".

>
> I'm thinking that if you can provide an __init__ you can intercept methods
> the regular way, by providing them the regular way, and overriding them.

Nope, I mentioned it, because this has been way back, when Python was 
young and slow, this was a technique to speed up method lookup.

Actually, it seems to work with py2.5 too, for oldstyle classes:

andreas at andi-lap:~/Videos$ python -m timeit -s 'class A:' -s '   def m(self): pass'
10000000 loops, best of 3: 0.0675 usec per loop
andreas at andi-lap:~/Videos$ python -m timeit -s 'class A:' -s '   def m(self): pass' -s 'a=A()' 'a.m()'
1000000 loops, best of 3: 0.747 usec per loop
andreas at andi-lap:~/Videos$ python -m timeit -s 'class A:' -s '   def m(self): pass' -s 'a=A()' -s 'a.m=a.m' 'a.m()'
1000000 loops, best of 3: 0.575 usec per loop
andreas at andi-lap:~/Videos$ python -m timeit -s 'class A(object):' -s '   def m(self): pass' -s 'a=A()' 'a.m()'
1000000 loops, best of 3: 0.671 usec per loop
andreas at andi-lap:~/Videos$ python -m timeit -s 'class A(object):' -s '   def m(self): pass' -s 'a=A()' -s 'a.m=a.m' 'a.m()'
1000000 loops, best of 3: 0.641 usec per loop

The reason for this is, that the instance dictionary is the first place that Python looks when looking for a.m.

>
> Thank you Andreas.  This has been really interesting and instructive.

Yeah, but please consider my comment. I think every developer should
have seen the possibilities, but in most cases forget about that ugly stuff. You will get a headache, especially 
if you try to use the full "power" that Python provides. One can usually be quite productive without relying on
these mechanisms.

Andreas
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