[Python-Dev] Re: Decorators: vertical bar syntax

[David Abrahams]

Bill Janssen <janssen at parc.com> writes:

>> In the discussion on decorators months ago, solutions involving
>> special syntax inside the block were ruled out early on.  Basically,
>> you shouldn't have to peek inside the block to find out important
>> external properties of the function.
>
> Guido, could you expand on this principle a bit?  Just stated as it
> is, it sounds as pointless (to my untutored mind, at least :-), as the
> arguments about "@ is ugly, so it shouldn't be used".  After all,
> isn't "what the function does" an important external property of the
> function, and don't you have to peek inside the block to determine
> that?  For instance, how do you know whether a function is a generator
> unless you look for yield statements inside it?

I'm not Guido (obviously), but:

  You look at the documentation.  The fact that Python doesn't divorce
  interface declarations from implementation is convenient for
  authors, but in some ways a trap for readers/maintainers (this is
  not a design criticism -- it was probably the right design choice).
  I often have to look inside Python functions to understand them, and
  find that their authors expect me to do so.  I think a style that
  cleanly delimits interface and implementation as much as possible
  should be encouraged. The fact that docstrings sit inside the
  function block make that harder to achieve; that's why I'm glad to
  hear that prefix decorators are being considered for docstrings.

That said, I'm not sure that "immediately following the opening colon"
is such a bad place for interface-y things.  It's nice that you can
find the whole function's declaration by looking forward from "def
..." today [except for the decorators, of course ;-(].

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com