No macros in Python

[Mike Meyer]

Peter Hansen <peter at engcorp.com> writes:
> Mike Meyer wrote:
> > 
> > Python doesn't have macros. On the other hand, looking at the world
> > through OO glasses rather than S-expression glasses means you don't
> > need macros as often. But the topic comes up at irregular intervals,
> > so someone may eventually come up with a way of doing macros in Python
> > that is right - at which point it'll be added.
> In what way would macros (as I understand them... from C for example)
> improve Python, other than possibly a small improvement in performance?

C macros are a very poor example of the species. I was referring to
lisp-like macros, where you get the full functionality of the language
to manipulate the code that the macro is dealing with.

This works particularly well in lisp, where the code is handed to the
macro preparsed and stored in one of the fundamental data structures
of the language. That makes writing macros easier. Since all code is
written in that data structure, they also fit naturally into the
language when they are invoked.

Any macro facility added to python has to deal with both these
problems. There needs to be some way to avoid the macro author having
to write a python parser to manipulate the code, and some way for them
to be invoked that fits well with the language.

As for what this lets you do, the answer is "lots of things". Much of
what is part of the syntax of Python can be implemented as macros in
lisp - short-circuit boolean operators come readily to mind. For a
programmatic example, there's the ever-popular "with-io-redirection"
macro family. A proper macro facility in python should let me write:

        with_output_to_file "filename":
            code that will be executed
            with print statement output
            going into filename
        code that will have
        print statement output
        going to where ever it was
        going before the macro

It should also allow me to add conditional expressions to the
language, ala the C ?: operator.

Which brings up the primary argument people have against macros. By
changing the syntax of the language, they make it impossible to
understand a program by local examination. You have to know what
macros are in scope and thus might change the programs behavior. Then
again, the ability to redefine operators does much the same
thing. Which is why I don't buy the "they'll never be added because
they are inherently evil" argument. The two are equally evil, for the
same reasons. We've got one. Why not the other?

        <mike
-- 
Mike Meyer <mwm at mired.org> http://www.mired.org/home/mwm/ Independent
WWW/Perforce/FreeBSD/Unix consultant, email for more information.