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

Guido van Rossum guido at python.org
Thu May 21 20:17:29 CEST 2015

Dang it. :-) I just want to encourage you to continue pursuing this idea,
one way or another.

On Thu, May 21, 2015 at 7:01 AM, Ben Hoyt <benhoyt at gmail.com> wrote:

> 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?
> -Ben
> 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)

--Guido van Rossum (python.org/~guido)
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