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

Eli Bendersky eliben at gmail.com
Wed Sep 25 15:15:19 CEST 2013


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,
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