On 1/8/2014 2:56 PM, axis.of.wea...@gmail.com wrote:
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,4) and calls func(*args) == Foo.<real bar>(f, 4) which prints 'inside bar'
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).
--
Terry Jan Reedy
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