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Re: Decorator help

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  Re: Decorator help Jason Swails <jason.swails@gmail.com> - 2013-03-27 16:33 -0400
    Re: Decorator help 88888 Dihedral <dihedral88888@googlemail.com> - 2013-03-30 20:06 -0700
    Re: Decorator help 88888 Dihedral <dihedral88888@googlemail.com> - 2013-03-30 20:06 -0700

#42041 — Re: Decorator help

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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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#42355

Jason Swails於 2013年3月28日星期四UTC+8上午4時33分08秒寫道：
> On Wed, Mar 27, 2013 at 3:49 PM, Joseph L. Casale <jca...@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):

I might add default parameters here  if I am programming 
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 
or c/c++ in the future.


>    """ 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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#42356

Jason Swails於 2013年3月28日星期四UTC+8上午4時33分08秒寫道：
> On Wed, Mar 27, 2013 at 3:49 PM, Joseph L. Casale <jca...@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):

I might add default parameters here  if I am programming 
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 
or c/c++ in the future.


>    """ 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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