[Python-Dev] PEP 318 -- a couple use cases (original) (raw)

Edward Loper edloper at gradient.cis.upenn.edu
Thu Apr 1 11:56:39 EST 2004

I saw both of the following use cases mentioned on the list, and they seemed interesting, so I went ahead and wrote up implementations:

def property_getter(func): """ A decorator that can be used to incrementally construct properties. For example, the following code constructs a property 'x' from an fget, an fset, and an fdel function:

     >>> class A:
     ...     def x(self) [property_getter]:
     ...         return self.__x
     ...     def x(self, val) [property_setter]:
     ...         self.__x = val
     ...     def x(self) [property_deleter]:
     ...         del self.__x

 In particular, this decorator checks if a property named
 'func' is defined in the enclosing frame.  If so, then it
 updates that property's fget to be 'func'.  If not, then
 it creates a new property whose fget is 'func'.  The
 property's docstring is taken from the first decorated
 property function to define a docstring.
 """

def generic(*type_signature): """ A decorator-generator that can be used to incrementally construct a generic function that delegates to individual functions based on the type signature of the arguments. For example, the following code defines a generic function that uses two different actual functions, depending on whether its argument is a string or an int:

     >>> def f(x) [generic(int)]:
     ...     print x, 'is an int'
     >>> def f(x) [generic(str)]:
     ...     print x, 'is a string'

 In particular, the decorator returned by this function checks if a
 Generic function-delegation object with the same name as the
 decorated function is defined in the enclosing frame.  If so, then
 it registers the function with that generic object, under
 'type_signature'.  If not, then it creates a new Generic
 function-delegation object, and registers the function.
 """

Full code is at <http://www.cis.upenn.edu/~edloper/pydecorators.html>.

But I still don't feel like I have a good handle on what "acceptable" uses of decorators are.. So, for each of these 2 use cases, is it... - an excellent example that should be included in the stdlib - perfectly acceptable, and belongs in the python cookbook - somewhat hackish, but ok under the right circumstances - an abhorition of nature

(If one of the first 3, then maybe they should be added to the pep?) Thanks for the feedback.

-Edward

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