Emulate @classmethod using decorator and descriptor

[WaterWalk]

Hello, I was recently learning python decorator and descriptor and
emulated a @classmethod decorator:
class EmuClassMethod( object):
def __init__(self, f=None):
self.f = f
def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
def wrapped(*args):
return self.f(klass, *args)
return wrapped

class Test(object):
@EmuClassMethod
def t(cls):
print "I'm %s" % cls.__name__

It worked, and seems that a function decorator works as follows:
# decf is a decorator
@decf
def func():
print 'func'

will be "converted" to:

def func():
print 'func'
func = decf(func)

Is this really the case? Or correct me if i'm wrong. Thanks in advance.

[Dustan]

Re: Emulate @classmethod using decorator and descriptor


WaterWalk wrote:

Hello, I was recently learning python decorator and descriptor and
emulated a @classmethod decorator:
class EmuClassMethod( object):
def __init__(self, f=None):
self.f = f
def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
def wrapped(*args):
return self.f(klass, *args)
return wrapped
>
class Test(object):
@EmuClassMethod
def t(cls):
print "I'm %s" % cls.__name__
>
It worked, and seems that a function decorator works as follows:
# decf is a decorator
@decf
def func():
print 'func'
>
will be "converted" to:
>
def func():
print 'func'
func = decf(func)
>
Is this really the case? Or correct me if i'm wrong. Thanks in advance.

Yes, the two are equivalent. The documentation is found here:

http://docs.python.org/ref/function.html

But I personally found the explanation for decorators a bit confusing.

[Gabriel Genellina]

Re: Emulate @classmethod using decorator and descriptor

On 12 dic, 08:46, "WaterWalk" <toolmas...@163 .comwrote:

Hello, I was recently learning python decorator and descriptor and
emulated a @classmethod decorator:
class EmuClassMethod( object):
def __init__(self, f=None):
self.f = f
def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
def wrapped(*args):
return self.f(klass, *args)
return wrapped
>
class Test(object):
@EmuClassMethod
def t(cls):
print "I'm %s" % cls.__name__

Basically you're right. But note that @classmethod does some additional
work to keep the details: the function name is now "wrapped", no
docstring, no argument names, no defaults, wrong type...

>>class Test2(object):

.... @EmuClassMethod
.... def t2(cls, arg1, arg2=2):
.... "t2 docstring"
.... print "I'm %s arg1=%s arg2=%s" % (cls.__name__, arg1, arg2)
....

>>print Test2.t2

<function wrapped at 0x00AB30B0>

>>print Test2.t2.func_n ame

wrapped

>>print Test2.t2.__doc_ _

None

>>print Test2.t2.func_d efaults

None

>>Test2.t2(10 0)

I'm Test2 arg1=100 arg2=2

>>Test2.t2(arg1 =100)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: wrapped() got an unexpected keyword argument 'arg1'

--
Gabriel Genellina