michele.simionato at gmail
Aug 7, 2004, 12:40 AM
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decorators and multimethods
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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
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decorators and multimethods
