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Date: Wed, 27 Mar 2013 16:33:08 -0400
Subject: Re: Decorator help
From: Jason Swails <jason.swails@gmail.com>
To: "Joseph L. Casale" <jcasale@activenetwerx.com>
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On Wed, Mar 27, 2013 at 3:49 PM, Joseph L. Casale <jcasale@activenetwerx.com
> wrote:

> I have a class which sets up some class vars, then several methods that
> are passed in data
> and do work referencing the class vars.
>
>
> I want to decorate these methods, the decorator needs access to the class
> vars, so I thought
> about making the decorator its own class and allowing it to accept args.
>
>
> I was hoping to do all the work on in_data from within the decorator,
> which requires access
> to several MyClass vars. Not clear on the syntax/usage with this approach
> here, any guidance
> would be greatly appreciated!
>

My guess is that you don't quite 'get' decorators yet (since I remember
similar types of questions when trying to learn them myself).  Decorators
execute when the class type itself is being built (e.g., when a module is
first imported at runtime).  So decorators will never take instance
variables as arguments (nor should they, since no instance can possibly
exist when they execute).  Bear in mind, a decorator should take a callable
as an argument (and any number of 'static' parameters you want to assign
it), and return another callable.

I provide an example decorator using the format the I typically adopt below
(where the decorator is a simple function, not a class):

def my_decorator(fcn):
   """ Decorator for a function """
   def new_fcn(self, *args, **kwargs):
      """ This is the new function that we will return. """
      # You can access any instance variables here
      returnval = fcn(self, *args, **kwargs)
      # Do anything else here with instance variables
      return returnval # or any other return value you want

   return new_fcn

Notice here I define a new_fcn callable function that takes self and an
arbitrary argument/keyword-argument list, and I return this function (which
does not get called) to replace the function I passed in.  You can use
instance variables inside new_fcn since new_fcn is called by instances of
MyClass.  This is a very simple type of decorator, but hopefully helps
illustrate what decorators are.  There is a particularly good thread on SO
with information about decorators here:
http://stackoverflow.com/questions/739654/understanding-python-decorators

Hope this helps,
Jason

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<font color=3D"#000000"><br></font><br><div class=3D"gmail_quote">On Wed, M=
ar 27, 2013 at 3:49 PM, Joseph L. Casale <span dir=3D"ltr">&lt;<a href=3D"m=
ailto:jcasale@activenetwerx.com" target=3D"_blank">jcasale@activenetwerx.co=
m</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex">I have a class=A0which=A0sets up some class =
vars, then several methods that are passed in data<br>
and do work=A0referencing the class vars.<br>
<br>
<br>
I want to decorate these methods, the decorator needs access to the class v=
ars, so I thought<br>
about making the decorator its own class and allowing it to accept args.<br=
>
<br>
<br>
I was hoping to do all the work on=A0in_data from within the decorator, whi=
ch requires access<br>
to several=A0MyClass vars. Not clear on the syntax/usage with this approach=
 here, any guidance<br>
would be greatly appreciated!<br></blockquote><div><br></div><div>My guess =
is that you don&#39;t quite &#39;get&#39; decorators yet (since I remember =
similar types of questions when trying to learn them myself). =A0Decorators=
 execute when the class type itself is being built (e.g., when a module is =
first imported at runtime). =A0So decorators will never take instance varia=
bles as arguments (nor should they, since no instance can possibly exist wh=
en they execute). =A0Bear in mind, a decorator should take a callable as an=
 argument (and any number of &#39;static&#39; parameters you want to assign=
 it), and return another callable.</div>
<div><br></div><div>I provide an example decorator using the format the I t=
ypically adopt below (where the decorator is a simple function, not a class=
):</div><div><br></div><div>def my_decorator(fcn):</div><div>=A0 =A0&quot;&=
quot;&quot; Decorator for a function &quot;&quot;&quot;</div>
<div>=A0 =A0def new_fcn(self, *args, **kwargs):</div><div>=A0 =A0 =A0 &quot=
;&quot;&quot; This is the new function that we will return. &quot;&quot;&qu=
ot;</div><div>=A0 =A0 =A0 # You can access any instance variables here</div=
><div>=A0 =A0 =A0 returnval =3D fcn(self, *args, **kwargs)</div>
<div>=A0 =A0 =A0 # Do anything else here with instance variables</div><div>=
=A0 =A0 =A0 return returnval # or any other return value you want</div><div=
>=A0 =A0</div><div>=A0 =A0return new_fcn</div><div><br></div><div>Notice he=
re I define a new_fcn callable function that takes self and an arbitrary ar=
gument/keyword-argument list, and I return this function (which does not ge=
t called) to replace the function I passed in. =A0You can use instance vari=
ables inside new_fcn since new_fcn is called by instances of MyClass. =A0Th=
is is a very simple type of decorator, but hopefully helps illustrate what =
decorators are. =A0There is a particularly good thread on SO with informati=
on about decorators here:=A0<a href=3D"http://stackoverflow.com/questions/7=
39654/understanding-python-decorators">http://stackoverflow.com/questions/7=
39654/understanding-python-decorators</a></div>
<div><br></div><div>Hope this helps,<br>Jason</div></div>

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