Understanding decorator and class methods


A

axis.of.weasel

can someone please explain why the following works, in contrast to the second example?

def decorator(func):
def on_call(*args):
print args
return func(args)
return on_call

class Foo:
@decorator
def bar(self, param1):
print 'inside bar'

f=Foo()
f.bar(4) # from where is the decorator getting the Foo instance?



I understand why the following works/does not work

class decorator2:
def __init__(self, func):
self.func=func
def __call__(self, *args):
self.func(*args)

class Foo2:
@decorator2
def bar2(self, param): pass


f2 = Foo2()
Foo2.bar2(f2, 4) # works, Foo2 instance and param are passed to decorator2 call
f2.bar2(4) # does not work, Foo2 instance is missing, decorator2 cannot invoke method bar
 
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R

Rotwang

can someone please explain why the following works, in contrast to the second example?

def decorator(func):
def on_call(*args):
print args
return func(args)
return on_call

class Foo:
@decorator
def bar(self, param1):
print 'inside bar'

f=Foo()
f.bar(4) # from where is the decorator getting the Foo instance?



I understand why the following works/does not work

class decorator2:
def __init__(self, func):
self.func=func
def __call__(self, *args):
self.func(*args)

class Foo2:
@decorator2
def bar2(self, param): pass


f2 = Foo2()
Foo2.bar2(f2, 4) # works, Foo2 instance and param are passed to decorator2 call
f2.bar2(4) # does not work, Foo2 instance is missing, decorator2 cannot invoke method bar

From http://docs.python.org/3/reference/datamodel.html:

Instance methods

An instance method object combines a class, a class instance and
any callable object (normally a user-defined function).

[...]

User-defined method objects may be created when getting an
attribute of a class (perhaps via an instance of that class), if
that attribute is a user-defined function object or a class method
object.

[...]

Note that the transformation from function object to instance
method object happens each time the attribute is retrieved from the
instance. In some cases, a fruitful optimization is to assign the
attribute to a local variable and call that local variable. Also
notice that this transformation only happens for user-defined
functions; other callable objects (and all non-callable objects)
are retrieved without transformation.


Notice the last sentence in particular. After being decorated by
decorator2 Foo2.bar2 is not a user-defined function (i.e. an instance of
types.FunctionType), so is not transformed into a method upon being
accessed through an instance. I suppose you could create a class that
mimics the behaviour of methods, though I don't know why you would want
to. The following is tested with 3.3.0; I expect someone who knows more
than I will probably be along soon to point out why it's stupid.

class decorator3:
def __init__(self, func):
self.func = func
def __call__(self, *args, **kwargs):
print('Calling func(self, *%r, **%r)' % (args, kwargs))
return self.func(self.__self__, *args, **kwargs)
def __get__(self, instance, owner):
self.__self__ = instance
return self

class Foo3:
@decorator3
def bar3(self, param):
return self, param
Calling func(self, *('param',), **{})
(<__main__.Foo3 object at 0x0000000002BDF198>, 'param')
 
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T

Terry Reedy

can someone please explain why the following works, in contrast to the second example?

Because function attributes of classes become instance methods, with
special behavior, when accessed via an instance of the class.
def decorator(func):
def on_call(*args):
print args
return func(args)

This has to be func(*args) (as in second example) or one gets
TypeError: bar() missing 1 required positional argument: 'param1'
Did you re-type instead of pasting?
return on_call

class Foo:
@decorator
def bar(self, param1):
print 'inside bar'

f=Foo()
f.bar(4) # from where is the decorator getting the Foo instance?

from args.
f.bar(4) == Foo.bar(f, 4) == on_call(*args), which prints args (tuple
f said:
I understand why the following works/does not work

class decorator2:
def __init__(self, func):
self.func=func
def __call__(self, *args):
self.func(*args)

class Foo2:
@decorator2
def bar2(self, param): pass

Using a class decorator to decorate an instance method of another class
is asking for trouble. As explained below, the result is no longer an
instance method.
f2 = Foo2()
Foo2.bar2(f2, 4) # works, Foo2 instance and param are passed to decorator2 call
f2.bar2(4) # does not work, Foo2 instance is missing, decorator2 cannot invoke method bar

Remember that
@deco
def f(): pass
is essentially equivalent to
def f(): pass
f = deco(f)

Decorator decorator replaces a function with a function. So the wrapped
bar is still seen as an instance method, so f.bar(x) gets the magic
instance method translation to Foo.bar(f, x). Decorator2 replaces
function bar with a callable instance of itself, which is *not* a
'function' and which therefore is not seen as an instance method, but
merely a callable attribute of Foo2. So f.bar == Foo.bar, and you would
need f2.bar(f2, 4).
 

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