dinamically altering a function
Steven Bethard
steven.bethard at gmail.com
Sat Mar 12 16:48:23 EST 2005
vegetax wrote:
> I i need a decorator that adds a local variable in the function it
> decorates, probably related with nested scopes, for example:
>
> def dec(func):
> def wrapper(obj = None):
> if not obj : obj = Obj()
> <bind obj to func>
> return func()
>
> return wrapper()
>
> @dec()
> def fun(b):
> obj.desc = 'marked'
> obj.b = b
> return obj
>
> so the call to fun : fun(obj = myobj,'b argument')
> or fun('b argument')
>
> So the function "fun" assumes it has an obj instance and other instance
> objects computed by the decorator, this decorator will be like a generic
> factory for this kind of functions,which depends on the decorator to work.
For a byte-code hack that does something similar, see the recent thread:
http://mail.python.org/pipermail/python-list/2005-March/270324.html
It can do something like:
py> class Object(object):
... pass
...
py> @presets.presets(obj=Object())
... def fun(b):
... obj.desc = "marked"
... obj.b = b
... return obj
...
py> fun(1)
<__main__.Object object at 0x01162BD0>
py> fun(1).b
1
But note that you then only have a single instance for all calls to the
function:
py> fun(1) is fun(2)
True
Have you considered using OO here? You might find that this is more
easily written as:
py> class Object(object):
... pass
...
py> class fun(object):
... def __new__(self, *args):
... if len(args) == 2:
... obj, b = args
... elif len(args) == 1:
... obj, [b] = Object(), args
... else:
... raise TypeError
... obj.desc = "marked"
... obj.b = b
... return obj
...
py> myobj = Object()
py> fun(myobj, 2)
<__main__.Object object at 0x01162E30>
py> myobj.b
2
py> obj = fun(1)
py> obj.b
1
This doesn't use any bytecode hacks, but I'm still not certain it's
really the way to go. What is it you're trying to write this way?
STeVe
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