decorators and multimethods

[Howard Stearns]

Hey, that's pretty nice. I'll have to get some coffee and study this. What 
do I read about? I can't find anything about @ or 'decorator' in the 2.3 
documentation. What should I be looking for?

Michele Simionato wrote:
> Decorators can generate endless debate about syntax, but can also
> be put to better use ;)
> 
> Actually I was waiting for decorators to play a few tricks that were
> syntactically too ugly to be even imaginable for Python 2.3.
> 
> One trick is to use decorators to implement multimethods. A while ago
> Howard Stearns posted here a recipe to implement generic functions
> a.k.a multimethods.
> 
> I have not studied his recipe, so don't ask me how it works. All I
> did was to add an "addmethod" function and save his code in a module
> called genericfunctions.
> 
> Decorators allowed me to use the following syntax:
> 
> # BEGIN generic functions in Python, example
> # use code and examples from Howard Stearns
> 
> from genericfunctions import Generic_Function, addmethod
> 
> foo = Generic_Function()
> 
> @addmethod(object, object, object)
> def foo(_, x, y, z):
>     return 'default'
> 
> @addmethod(int, int, int)
> def foo(call_next, x, y, z): 
>     return 'all ints , ' + call_next(x, y, z)
> 
> @addmethod(object, object)
> def foo( _, x, y):
>     return 'just two'
> 
> print foo # the multimethod table as a dictionary
> print foo(1, 2, 'three') # => default
> print foo(1, 2, 3) # => all ints, default
> print foo(1, 'two')  #> just two
> print foo('oops') #=> genericfunctions.NoNextMethod error
> 
> # END generic functions in Python, example
> 
> Howard Stearns' code is posted here
> http://groups.google.it/groups?hl=it&lr=&ie=UTF-8&threadm=40C3BC71.5050206%40charter.net&rnum=1&prev=/groups%3Fq%3Dhoward%2Bstearns%2Bgroup:comp.lang.python.*%26hl%3Dit%26lr%3D%26ie%3DUTF-8%26group%3Dcomp.lang.python.*%26selm%3D40C3BC71.5050206%2540charter.net%26rnum%3D1
> 
> I just added the following function:
> 
> import sys
> 
> def addmethod(*types):
>      """sys._getframe hack; works when the generic function is defined
>      in the globals namespace."""
>      caller_globs = sys._getframe(1).f_globals
>      def function2generic(f):
>           generic = caller_globs[f.func_name]
>           generic[types] = f
>           return generic
>      return function2generic
> 
> Decorators did all the rest ;) Just to add an use case I haven't
> seen before.
> 
>        Michele Simionato