[Python-Dev] Enable access to the AST for Python code

Ben Hoyt benhoyt at gmail.com
Thu May 21 16:01:04 CEST 2015

Thanks. Good point about python-ideas -- I was thinking that after I sent
it too. I'll repost there soon.

Out of interest, what specifically were you referring to as "definitely
cool" here: LINQ-style generator expressions that build SQL ala PonyORM, or
the more general feature of enabling AST access?


On Wed, May 20, 2015 at 10:31 PM, Guido van Rossum <guido at python.org> wrote:

> Hey Ben, this is probably a better topic for python-ideas. I'll warn you
> that a hurdle for ideas like this is that ideally you don't want to support
> this just for CPython. It's definitely cool though! (Using movie poster
> style quotes you can turn this into a ringing endorsement: "definitely
> cool" -- The BDFL. :-)
> On Wed, May 20, 2015 at 7:26 PM, Ben Hoyt <benhoyt at gmail.com> wrote:
>> Hi Python devs,
>> Enabling access to the AST for compiled code would make some cool
>> things possible (C# LINQ-style ORMs, for example), and not knowing too
>> much about this part of Python internals, I'm wondering how possible
>> and practical this would be.
>> Context: PonyORM (http://ponyorm.com/) allows you to write regular
>> Python generator expressions like this:
>>     select(c for c in Customer if sum(c.orders.price) > 1000)
>> which compile into and run SQL like this:
>>     SELECT "c"."id"
>>     FROM "Customer" "c"
>>     LEFT JOIN "Order" "order-1" ON "c"."id" = "order-1"."customer"
>>     GROUP BY "c"."id"
>>     HAVING coalesce(SUM("order-1"."total_price"), 0) > 1000
>> I think the Pythonic syntax here is beautiful. But the tricks PonyORM
>> has to go to get it are ... not quite so beautiful. Because the AST is
>> not available, PonyORM decompiles Python bytecode into an AST first,
>> and then converts that to SQL. (More details on all that from author's
>> EuroPython talk at http://pyvideo.org/video/2968)
>> I believe PonyORM needs the AST just for generator expressions and
>> lambda functions, but obviously if this kind of AST access feature
>> were in Python it'd probably be more general.
>> I believe C#'s LINQ provides something similar, where if you're
>> developing a LINQ converter library (say LINQ to SQL), you essentially
>> get the AST of the code ("expression tree") and the library can do
>> what it wants with that.
>> What would it take to enable this kind of AST access in Python? Is it
>> possible? Is it a good idea?
>> -Ben
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> --
> --Guido van Rossum (python.org/~guido)
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