[Python-ideas] Syntax for defining parametric decorators

[Mathias Panzenböck]

On 07/08/2012 10:22 PM, Mike Graham wrote:
> A common stumbling block for new users is writing decorators that take
> arguments. To create a decorator like
>
> @timesn(n)
> def f(y):
>      ...
>
> We write code like
>
> def timesn(n)
>      def decorator(f):
>          def inner(y):
>              return n * f(y)
>          return inner
>       return decorator
>
> which confuses many users and can be a handful to type. I wonder if it
> would be clearer for people to write
>
> def timesn(n)(f):
>      def inner(y):
>          return n * f(y)
>      return inner
>

Why not write this?:

def timesn(n)(f)(y):
     return n * f(y)

This would be a currified function. One could implement something like that like this:

def curry(f):
	for i in range(f.func_code.co_argcount-1):
		f = (lambda f: lambda *args: partial(f,*args))(f)
	return f

@curry
def timesn(n,f,y):
	return n * f(y)

> which is more concise and looks a lot more like a non-parametric
> decorator someone might have written already. The syntax is mostly
> self-explaining and could potentially be useful in other contexts.
>
> There exist tools like the decorator library to try to simplify this
> already, but in my experience they mostly serve to confuse people
> using decorators for the first time more.
>
> One thing I didn't specify was whether `n` was nonlocal or not and the
> behavior of something that keeps and reuses timesn(some_specific_n)
> multiple times.
>
> Does anyone think a feature like this may be useful?
>
> Regards,
> Mike