substitution of a method by a callable object

[netimen]

Can I substitute a method of a class by a callable object (not a
function)? I can very easy insert my function in a class as a method,
but an object - can't.

I have the following:

class Foo(object):
pass

class Obj(object):
def __call__(self, obj_self):
print 'Obj'

def func(self):
print 'func'

f = Foo()
Foo.meth = func
f.meth() # all goes OK
Foo.meth = Obj()
f.meth() # I get TypeError: __call__() takes exactly 2 arguments (1
given)

[Terry Reedy]

Re: substitution of a method by a callable object

netimen wrote:

Can I substitute a method of a class by a callable object (not a
function)? I can very easy insert my function in a class as a method,
but an object - can't.

Yes you can and did.

class Foo(object):
pass
>
class Obj(object):
def __call__(self, obj_self):
print 'Obj'
>
def func(self):
print 'func'
>
f = Foo()
Foo.meth = func
f.meth() # all goes OK
Foo.meth = Obj()
f.meth() # I get TypeError: __call__() takes exactly 2 arguments (1
given)

So either remove the unused obj_self parameter from __call__ or pass
something -- anything -- to be bound to it.
f.meth(1) for instance, works fime (in 3.0 at least)

[George Sakkis]

Re: substitution of a method by a callable object

On Oct 22, 12:13 pm, netimen <neti...@gmail. comwrote:

Can I substitute a method of a class by a callable object (not a
function)? I can very easy insert my function in a class as a method,
but an object - can't.
>
I have the following:
>
class Foo(object):
    pass
>
class Obj(object):
    def __call__(self, obj_self):
        print 'Obj'
>
def func(self):
    print 'func'
>
f = Foo()
Foo.meth = func
f.meth() # all goes OK
Foo.meth = Obj()
f.meth() # I get TypeError: __call__() takes exactly 2 arguments (1
given)

You have to wrap it as an (unbound) instance method explicitly:

from types import MethodType
Foo.meth = MethodType(Obj( ), None, Foo)
f.meth()

For normal functions this seems to be done implicitly (of course you
can do it explicitly if you want):

>>Foo.meth = func
>>print Foo.meth

<unbound method Foo.func>

George

[netimen]

Re: substitution of a method by a callable object

On 22 §à§Ü§ä, 20:46, Bruno Desthuilliers
<bdesth.quelque ch...@free.quel quepart.frwrote :

netimen a ¨¦crit :
>

Can I substitute a method of a class by a callable object (not a
function)? I can very easy insert my function in a class as a method,
but an object - can't.

>
functions implement the descriptor protocol so when looked up as class
attributes, the lookup invoke their __get__ method, which in turn
returns a method object (which is a thin wrapper around the function,
the class and the instance).
>
You can either build the method manually (as Georges explained), or make
your own Obj class a proper descriptor:
>
from types import MethodType
>
class Obj(object):
__name__ = "Obj" # for Method.__repr_
>
def __call__(self, obj_self):
print 'Obj'
>
def __get__(self, instance, cls):
return MethodType(self , instance, cls)
>
HTH

thanks!

[Terry Reedy]

Re: substitution of a method by a callable object

George Sakkis wrote:

On Oct 22, 12:13 pm, netimen <neti...@gmail. comwrote:
>

>Can I substitute a method of a class by a callable object (not a
>function)? I can very easy insert my function in a class as a method,
>but an object - can't.
>>
>I have the following:
>>
>class Foo(object):
> pass
>>
>class Obj(object):
> def __call__(self, obj_self):
> print 'Obj'
>>
>def func(self):
> print 'func'
>>
>f = Foo()
>Foo.meth = func
>f.meth() # all goes OK
>Foo.meth = Obj()
>f.meth() # I get TypeError: __call__() takes exactly 2 arguments (1
>given)

>
You have to wrap it as an (unbound) instance method explicitly:

Nope. As the error message says, the method was called with nothing
provided to be bound to the extraneous parameter obj_self. Either
provide an arg, such as with f.meth(1), *or* delete obj_self and 'Obj'
is printed, with both 2.5 and 3.0.

[netimen]

Re: substitution of a method by a callable object

On 23 ÏËÔ, 00:34, Terry Reedy <tjre...@udel.e duwrote:

George Sakkis wrote:

On Oct 22, 12:13 pm, netimen <neti...@gmail. comwrote:

>

Can I substitute a method of a class by a callable object (not a
function)? I can very easy insert my function in a class as a method,
but an object - can't.

>

I have the following:

>

class Foo(object):
š š pass

>

class Obj(object):
š š def __call__(self, obj_self):
š š š š print 'Obj'

>

def func(self):
š š print 'func'

>

f = Foo()
Foo.meth = func
f.meth() # all goes OK
Foo.meth = Obj()
f.meth() # I get TypeError: __call__() takes exactly 2 arguments (1
given)

>

You have to wrap it as an (unbound) instance method explicitly:

>
Nope. šAs the error message says, the method was called with nothing
provided to be bound to the extraneous parameter obj_self. šEither
provide an arg, such as with f.meth(1), *or* delete obj_self and 'Obj'
is printed, with both 2.5 and 3.0.

OK, I have implemented Bruno Desthuilliers example. But there is
another question: can I having a method determine if it is an instance
of given class. So:

class Obj(object):
__name__ = "Obj" # for Method.__repr_

def __call__(self, obj_self):
print 'Obj'

def __get__(self, instance, cls):
return MethodType(self , instance, cls)

class Foo(object):
pass

Foo.meth = Obj()

## in some another place of code
if isinstance(Foo. meth, Obj): # doesn't work because type(Foo.meth) is
now 'instancemethod '
...

Can I determine that?

[netimen]

Re: substitution of a method by a callable object

On 23 §à§Ü§ä, 00:26, Bruno Desthuilliers
<bdesth.quelque ch...@free.quel quepart.frwrote :

netimen a ¨¦crit :
(snip)
>

OK, I have implemented Bruno Desthuilliers example. But there is
another question: can I having a method determine if it is an instance
of given class. So:

>
As the name imply, a method is usually, well, an instance of type
'method' !-)
>
>
>

class Obj(object):

(snip)
>

class Foo(object):
pass

>

Foo.meth = Obj()

>

## in some another place of code
if isinstance(Foo. meth, Obj): # doesn't work because type(Foo.meth) is
now 'instancemethod '

>
Indeed. I suppose what you want is to know if the callable wrapped by
the method is an instance of Obj ?
>
>
>

Can I determine that?

>
Yeps. Test the im_func attribute of the method:
>
if isinstance(Foo. meth.im_func, Obj): print "yadda"
>
But you'd better put this in a try/except block, because a callable
attribute of a class is not necessarily a method - and so it may not
have an im_func attribute.
>
Also, may I ask why you want to check this ? Not that it's necessarily
wrong, but real use case for typechecking are pretty rare.

Thanks! I have wasted much time playing with different Foo.meth trying
to get to Obj through them. But im_func was one of the fields I
haven't checked )

Indeed, I have managed already without that. The task was to
substitute __str__ method by a complex formatter. It called a low-
level formatter, a function wich returned simple str(self). That
function could be called with objects with substituted formatters and
with simple objects. So to avoid recursion I wanted to check wether
the formatter of my object was substituted.