dinamically altering a function

Steven Bethard steven.bethard at gmail.com
Sat Mar 12 22:48:23 CET 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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