[Python-Dev] astgen.py vs. ASDL for ASTs

Brett Cannon brett at python.org
Wed Sep 25 15:35:02 CEST 2013

cc'ing Jeremy Hylton who made the decision to use Zephyr.

On Wed, Sep 25, 2013 at 9:15 AM, Eli Bendersky <eliben at gmail.com> wrote:

> Hello,
> Some history (as best as I could collect it) followed by a question:
> Before Python 2.5, the ast stdlib module was auto generated by a script
> named astgen.py from a textual AST definition in Tools/compiler/ast.txt.
> Since 2.5 (http://www.python.org/dev/peps/pep-0339/) ASTs are part of the
> normal compilation flow by the Python compiler itself, and the ast module
> uses the same ASTs generated from Parser/Python.asdl
> My question is, when the switch was made in 2.5 - why didn't the existing
> AST-generating code was used and the path moved to ASDL instead? What
> advantages does ASDL have over the previous approach? One reason I could
> think of is that ASDL nodes are typed and that's maybe better for the
> generated C code to handle.
> [My interest here is personal. One of my projects (pycparser) uses a
> astgen.py-like approach, and in a new project I'm considering the options
> again and remembered ASDL. ADSL's documentation it extremely scarce online
> - seems like CPython is one of its only somewhat-visible users these days]
> Thanks in advance,
> Eli
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