Re: proxy class and __add__ method

[Gabriel Genellina]

En Tue, 29 Jul 2008 13:13:51 -0300, Magnus Schuster
<ma************@yahoo.comescribi�:

Hello,
I have written the following small proxy class which I expect to pass all
function calls to the 'original' object:

--- BEGIN ---
class proxy(object):
def __init__( self, subject ):
self.__subject = subject
def __getattr__( self, name ):
return getattr( self.__subject, name )

prx_i=proxy(1)
print hasattr(prx_i,'__add__')
j=prx_i.__add__(1)
k=prx_i+1
--- END ---

Actually the "hasattr(prx_i,'__add__')" returns "True" as expected, and
"j=prx_i.__add__(1)" sets j=2.

But "k=prx_i+1" raises a
<type 'exceptions.TypeError'>: unsupported operand type(s) for +: 'proxy'
and 'int'.

How is this addition different from the previous line "j=..."? And how
can I
modify the proxy class so that all methods are passed on, which are not
explicitly overloaded?

__magic__ methods on new style classes are searched in the class, *not* in
the instance. prx_i+1 looks for __add__ in type(prx_i), that is, in the
proxy class. Try implementing a similar __getattr__ method in a metaclass.

--
Gabriel Genellina

[Rhamphoryncus]

On Jul 29, 10:23*pm, "Gabriel Genellina" <[e-mail]>
wrote:

En Tue, 29 Jul 2008 13:13:51 -0300, Magnus Schuster *
<[e-mail] :

Hello,
I have written the following small proxy class which I expect to pass all
function calls to the 'original' object:

--- BEGIN ---
class proxy(object):
* * def __init__( self, subject ):
* * * * self.__subject = subject
* * def __getattr__( self, name ):
* * * * return getattr( self.__subject, name )

prx_i=proxy(1)
print hasattr(prx_i,'__add__')
j=prx_i.__add__(1)
k=prx_i+1
--- END ---

Actually the "hasattr(prx_i,'__add__')" returns "True" as expected, and
"j=prx_i.__add__(1)" sets j=2.

But "k=prx_i+1" raises a
<type 'exceptions.TypeError'>: unsupported operand type(s) for +: 'proxy'
and 'int'.

How is this addition different from the previous line "j=..."? And how *
can I
modify the proxy class so that all methods are passed on, which are not
explicitly overloaded?

__magic__ methods on new style classes are searched in the class, *not* in *
the instance. prx_i+1 looks for __add__ in type(prx_i), that is, in the *
proxy class.

This much is true.

Try implementing a similar __getattr__ method in a metaclass.

But I don't think they use __getattr__.. they bypass it. Effectively
they catch the assignment to __add__ and cache it. You'll have to
always define it in the class and have it be ineffectual in some cases.

[Gabriel Genellina]

En Wed, 30 Jul 2008 14:54:51 -0300, Rhamphoryncus <rh****@gmail.com>
escribió:

On Jul 29, 10:23*pm, "Gabriel Genellina" <[e-mail]>
wrote:

>En Tue, 29 Jul 2008 13:13:51 -0300, Magnus Schuster *
<[e-mail] :

I have written the following small proxy class which I expect to pass

all

function calls to the 'original' object:

--- BEGIN ---
class proxy(object):
* * def __init__( self, subject ):
* * * * self.__subject = subject
* * def __getattr__( self, name ):
* * * * return getattr( self.__subject, name )

But "k=prx_i+1" raises a
<type 'exceptions.TypeError'>: unsupported operand type(s) for +:

'proxy'

and 'int'.

How is this addition different from the previous line "j=..."? And
how *
can I
modify the proxy class so that all methods are passed on, which are not
explicitly overloaded?

Try implementing a similar __getattr__ method in a metaclass.

But I don't think they use __getattr__.. they bypass it. Effectively
they catch the assignment to __add__ and cache it. You'll have to
always define it in the class and have it be ineffectual in some cases.

Ouch, yes, thanks, I noticed the fact after some testing.

--
Gabriel Genellina