[Python-3000] PEP 3124 - Overloading, Generic Functions, Interfaces, etc. (original) (raw)

Phillip J. Eby pje at telecommunity.com
Wed May 2 06:04:35 CEST 2007

At 02:50 PM 5/2/2007 +1200, Greg Ewing wrote:

Phillip J. Eby wrote: > The PEP lists five built-in decorators, all of which support this behavior:: > > @overload, @when, @before, @after, @around

This seems massively over-designed. All you need is the ability to call the next method, and you can get all of these behaviours. If you call it first, then you get after behaviour; if you call it last, you get before behaviour; etc.

Yep, that was my theory too, until I actually used generic functions. As it happens, it's:

  1. a lot more pleasant not to write the extra boilerplate all the time, and

  2. having @before or @after tells you right away the intent of the method, without having to carefully inspect the body to see when and whether it is calling the next method, and whether it is modifying the arguments or return values in some way.

In other words, the restricted behavior of @before and @after methods makes them easier to write and easier to read.

By the way, if you look at the PEP, you'll find motivating examples for each of the decorators, as well as an explanation and examples of when and how you might want to create even more such decorators.

IIRC, CLOS has about 8 more kinds of method combinators that come standard, including ones that we'd probably spell something like @sum, @product, @min, @max, @list, @any, and @all, if it weren't for most of those names already being builtins that mean something else. :) The PEP doesn't propose implementing all of those, but it does show how easily you can create things like that if you want to.

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