[Python-Dev] A Hygienic Macro System in Python?

[Tim Peters]

[Guido van Rossum]
> Just throwing out some thoughts.
>
> There are several possible use cases for new customizable syntax:
>
> - resource allocation (lock.acquire(); try-finally: lock.release())
>
> - defining functions with special treatment (e.g. classmethods,
>   properties)
>
> - defining class-like things (e.g. Zope-style interface declarations)

With that last I assume (or hope) that you're trying to subsume what Jim
Fulton was trying to get at in the "Context binding" thread on the Zope list
a few weeks back (which was itself a repeat of an earlier proposal nobody
responded to at the time -- wanted to, but couldn't make time).

> Maybe it's possible to invent a new statement that covers all of
> these?  Certainly a macro system should support doing all these
> easily...

I'd like to go back to Jim's original statement of "the problem" (since it
was more helpful than the proposed solution <wink>):

    It would be cool, IMO, if people could define their own
    block statements in Python.

I think all the use cases you had in mind fit this too, and it's not
vacuous.  For example, it rules out "expression macros", like trying to
cater to defining

    x implies y

as expanding to

    ((not (x)) or (y))

Python's better off without anything like that; new block statements aren't
nearly as disgusting <wink>.

one-sickly-step-at-a-time-ly y'rs  - tim