Re: proxy class and __add__ method

[Gabriel Genellina]

En Tue, 29 Jul 2008 13:13:51 -0300, Magnus Schuster
<magnusschuster @yahoo.comescri bi�:

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.Typ eError'>: 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]

Re: proxy class and __add__ method

On Jul 29, 10:23 pm, "Gabriel Genellina" <gagsl-...@yahoo.com.a r>
wrote:

En Tue, 29 Jul 2008 13:13:51 -0300, Magnus Schuster  
<magnusschus... @yahoo.comescri bi :
>
>
>

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.Typ eError'>: 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]

Re: proxy class and __add__ method

En Wed, 30 Jul 2008 14:54:51 -0300, Rhamphoryncus <rhamph@gmail.c om>
escribió:

On Jul 29, 10:23 pm, "Gabriel Genellina" <gagsl-...@yahoo.com.a r>
wrote:

>En Tue, 29 Jul 2008 13:13:51 -0300, Magnus Schuster  
><magnusschus.. .@yahoo.comescr ibi :
>>

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.Typ eError'>: 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