strange __call__

[Rahul]

Consider the following:
def a(x):
return x+1

def b(f):
def g(*args,**kwargs):
for arg in args:
print arg
return f(*args,**kwargs)
return g

a.__call__ = b(a.__call__)

now calling a(1) and a.__call__(1) yield 2 different results!!
i.e. for functions a(1) doesnt seem to be translated to a.__call__ if
you assign a new value to a.__call__?
i am using python 2.3.3
somebody please clear this confusion

 

[Michael Hoffman]

Consider the following:
def a(x):
return x+1

def b(f):
def g(*args,**kwargs):
for arg in args:
print arg
return f(*args,**kwargs)
return g

a.__call__ = b(a.__call__)

now calling a(1) and a.__call__(1) yield 2 different results!!
i.e. for functions a(1) doesnt seem to be translated to a.__call__ if
you assign a new value to a.__call__?

I don't know why this happens, but setting the __call__ attribute of a
is a pretty strange thing to do. Why not just set a instead? The
original function a(x) will still be stored as a closure in what is
returned from b().

If this is of purely academic interest then the answer is I don't know.

 

[Andreas Kostyrka]

Just a guess, but setting "__X__" special methods won't work in most cases
because these are usually optimized when the class is created.

It might work if a.__call__ did exist before (because class a: contained
a __call__ definition).

Andreas

 

[Rahul]

Hi.
well if you do dir(a) just after defining 'a' then it does show
'__call__'.
the reason i wanted to do it is that i wanted to see if theres a
uniform way to wrap a function and callable objects so that for
example i can get some message printed whenever a function or a
function-like-object is called. then i could simply do :

def wrapper(obj):
g = obj.__call__
def f(*args,**kwargs):
for arg in argsrint arg
return g(*args,**kwargs)
obj.__call__=f
but it seems this will not work for functions

 

[Steven Bethard]

def wrapper(obj):
g = obj.__call__
def f(*args,**kwargs):
for arg in argsrint arg
return g(*args,**kwargs)
obj.__call__=f
but it seems this will not work for functions

def wrap(obj):
def f(*args, **kwargs):
for arg in args:
print arg
return obj(*args, **kwargs)
return f

@wrap
def func(a, b, c):
...

class C(object):
...
C = wrap(C)

STeVe

 

[Rahul]

If you do C = wrap(C) C no longer remains a class..it becomes a
function.

 

[Reinhold Birkenfeld]

If you do C = wrap(C) C no longer remains a class..it becomes a
function.

Does that matter?

Reinhold

 

[Steven Bethard]

def wrap(obj):
def f(*args, **kwargs):
for arg in args:
print arg
return obj(*args, **kwargs)
return f

@wrap
def func(a, b, c):
...

class C(object):
...
C = wrap(C)

Rahul top-posted:

> If you do C = wrap(C) C no longer remains a class..it becomes a
> function.

And if you do
func = wrap(func)
which is the equivalent of
@wrap
def func(...):
...
then func no longer has the same signature. But as Reinhold suggests,
does that really matter? In the case of the class, you can still call
it to create class instances. In the case of the function, you can
still call it to retrieve return values. Why do you care about the type
of the object?

In the case that it does matter, e.g. you want to be able to invoke your
methods from the class instead of the instance, you can wrap the
specific function that you need wrapped, e.g.

class C(object):
@wrap
def __new__(cls, *args):
super(C, cls).__new__(cls, *args)
...

STeVe

 

[Rahul]

Hi.
I understood your point.
thanks...
rahul

Rahul top-posted:

And if you do
func = wrap(func)
which is the equivalent of
@wrap
def func(...):
...
then func no longer has the same signature. But as Reinhold suggests,
does that really matter? In the case of the class, you can still call
it to create class instances. In the case of the function, you can
still call it to retrieve return values. Why do you care about the type
of the object?

In the case that it does matter, e.g. you want to be able to invoke your
methods from the class instead of the instance, you can wrap the
specific function that you need wrapped, e.g.

class C(object):
@wrap
def __new__(cls, *args):
super(C, cls).__new__(cls, *args)
...

STeVe