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Newsgroups: comp.lang.python
Date: Sat, 30 Mar 2013 20:06:25 -0700 (PDT)
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Subject: Re: Decorator help
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Xref: csiph.com comp.lang.python:42356

Jason Swails=E6=96=BC 2013=E5=B9=B43=E6=9C=8828=E6=97=A5=E6=98=9F=E6=9C=9F=
=E5=9B=9BUTC+8=E4=B8=8A=E5=8D=884=E6=99=8233=E5=88=8608=E7=A7=92=E5=AF=AB=
=E9=81=93=EF=BC=9A
> On Wed, Mar 27, 2013 at 3:49 PM, Joseph L. Casale <jca...@activenetwerx.c=
om> wrote:
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> I have a class=C2=A0which=C2=A0sets up some class vars, then several meth=
ods that are passed in data
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> and do work=C2=A0referencing the class vars.
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>=20
>=20
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> I want to decorate these methods, the decorator needs access to the class=
 vars, so I thought
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> about making the decorator its own class and allowing it to accept args.
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>=20
>=20
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> I was hoping to do all the work on=C2=A0in_data from within the decorator=
, which requires access
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> to several=C2=A0MyClass vars. Not clear on the syntax/usage with this app=
roach here, any guidance
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> would be greatly appreciated!
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> My guess is that you don't quite 'get' decorators yet (since I remember s=
imilar types of questions when trying to learn them myself). =C2=A0Decorato=
rs execute when the class type itself is being built (e.g., when a module i=
s first imported at runtime). =C2=A0So decorators will never take instance =
variables as arguments (nor should they, since no instance can possibly exi=
st when they execute). =C2=A0Bear in mind, a decorator should take a callab=
le as an argument (and any number of 'static' parameters you want to assign=
 it), and return another callable.
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>=20
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> I provide an example decorator using the format the I typically adopt bel=
ow (where the decorator is a simple function, not a class):
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>=20
> def my_decorator(fcn):

I might add default parameters here  if I am programming=20
in python to save the troubles of subclassing
similar decorators.

But that is only the stylish problem in python.

I might need to translate the decorator part into cython=20
or c/c++ in the future.


> =C2=A0 =C2=A0""" Decorator for a function """
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> =C2=A0 =C2=A0def new_fcn(self, *args, **kwargs):
> =C2=A0 =C2=A0 =C2=A0 """ This is the new function that we will return. ""=
"
> =C2=A0 =C2=A0 =C2=A0 # You can access any instance variables here
> =C2=A0 =C2=A0 =C2=A0 returnval =3D fcn(self, *args, **kwargs)
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> =C2=A0 =C2=A0 =C2=A0 # Do anything else here with instance variables
> =C2=A0 =C2=A0 =C2=A0 return returnval # or any other return value you wan=
t
> =C2=A0 =C2=A0
> =C2=A0 =C2=A0return new_fcn
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> Notice here I define a new_fcn callable function that takes self and an a=
rbitrary argument/keyword-argument list, and I return this function (which =
does not get called) to replace the function I passed in. =C2=A0You can use=
 instance variables inside new_fcn since new_fcn is called by instances of =
MyClass. =C2=A0This is a very simple type of decorator, but hopefully helps=
 illustrate what decorators are. =C2=A0There is a particularly good thread =
on SO with information about decorators here:=C2=A0http://stackoverflow.com=
/questions/739654/understanding-python-decorators
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> Hope this helps,
> Jason