[Python-Dev] defmacro (was: Anonymous blocks)

Samuele Pedroni pedronis at strakt.com
Mon Apr 25 19:08:35 CEST 2005

Michael Chermside wrote:

>Jim Jewett writes:
>>As best I can tell, the anonymous blocks are used to take
>>care of boilerplate code without changing the scope -- exactly
>>what macros are used for.
>Folks, I think that Jim is onto something here.
>I've been following this conversation, and it sounds to me as if we
>are stumbling about in the dark, trying to feel our way toward something
>very useful and powerful. I think Jim is right, what we're feeling our
>way toward is macros.
>The problem, of course, is that Guido (and others!) are on record as
>being opposed to adding macros to Python. (Even "good" macros... think
>lisp, not cpp.) I am not quite sure that I am convinced by the argument,
>but let me see if I can present it:
>  Allowing macros in Python would enable individual programmers or
>  groups to easily invent their own "syntax". Eventually, there would
>  develop a large number of different Python "dialects" (as some
>  claim has happened in the Lisp community) each dependent on macros
>  the others lack. The most important casualty would be Python's
>  great *readability*.
>(If this is a strawman argument, i.e. if you know of a better reason
>for keeping macros OUT of Python please speak up. Like I said, I've
>never been completely convinced of it myself.)
The typical argument in defense of macros is that macros are just like 
functions, you go to the definition
and see what they does.

But depending on how much variation they offer over the normal grammar 
even eye parsing them may be difficult.

They make it easy to mix to code that is evaluated immediately and code 
that will be evalutated, maybe even repeatedely, later, each
macro having its own rules about this. In most cases the only way  to 
discern this and know what is what is indeed looking at the macro 

You can get flame wars about whether introducing slightly different 
variations of if is warranted. <.5 wink>

My personal impression is that average macro definitions (I'm thinking 
about Common Lisp or Dylan and similar) are much less
readable that average function definitions. Reading On Lisp may give an 
idea about this. That means that introducing macros in Python, because 
of the importance that readability has in Python, would need a serious 
design effort to make the macro definitions themself readable. I think 
that's a challenging design problem.

Also agree about the technical issues that Guido cited about referencing 
and when macros definition enter in effect etc.

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