how can I use a callable object as a method

[Piotr Sobolewski]

Hello,

I would like to use a callable object as a method of a class. So, when I
have such normal class:

class f:
version = 17
def a(self):
return self.version

f1 = f()
print f1.a()
I want to change it to something like that:

class add:
def __call__(self, another_self):
return another_self.version

class f:
version = 17
a = add()

f1 = f()
print f1.a()

However, the second version does not work. I think I understand why. That's
because "a" inside f1 is not a function (but an object). So f1.a is not a
method. So when I do f1.a(), the implicit argument self is not passed.

Q1: Am I right? Is this the problem?
Q2: What can I do to make it work?

[Marco Wahl]

Piotr Sobolewski <NI********@gazeta.plwrites:

I would like to use a callable object as a method of a class. So, when I
have such normal class:

I want to change it to something like that:

class add:
def __call__(self, another_self):
return another_self.version

class f:
version = 17
a = add()

f1 = f()
print f1.a()

However, the second version does not work. I think I understand why. That's
because "a" inside f1 is not a function (but an object). So f1.a is not a
method. So when I do f1.a(), the implicit argument self is not passed.

Q1: Am I right? Is this the problem?
Q2: What can I do to make it work?

Use the right argument for the call.

Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.

>>class add:

def __call__(self, another_self):
return another_self.version

.... ... ... >>class f:
version = 17
a = add()

.... ... ... >>f1 = f()

>>f1

<__main__.f instance at 0x00A805D0>

>>f1.a

<__main__.add instance at 0x00A80DA0>

>>f1.a()

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __call__() takes exactly 2 arguments (1 given)

>>f1.a(f1)

17

>>>

HTH

[Hrvoje Niksic]

Piotr Sobolewski <NI********@gazeta.plwrites:

However, the second version does not work. I think I understand
why. That's because "a" inside f1 is not a function (but an object).

An object that defines __call__ is perfectly usable as a function.
Your problem is that it doesn't know how to convert itself to a
method, so that f1.a() knows how to pass f1 as another_self to
add.__call__. To do that, add needs to be a bit smarter:

>>class add(object):

.... def __call__(self, another_self):
.... return another_self.version
.... def __get__(self, obj, type=None):
.... return lambda: self(obj)
....

>>class f(object):

.... version = 17
.... a = add()
....

>>f1 = f()
f1.a()

17

If you can't modify add, you can probably use an adaptor that defines
__get__ in a similar way.

[Piotr Sobolewski]

Hrvoje Niksic wrote:

>However, the second version does not work. I think I understand
why. That's because "a" inside f1 is not a function (but an object).

An object that defines __call__ is perfectly usable as a function.
Your problem is that it doesn't know how to convert itself to a
method, so that f1.a() knows how to pass f1 as another_self to
add.__call__. To do that, add needs to be a bit smarter:

A! Now it works, thanks a lot!