[Python-Dev] Policy for making changes to the AST (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sun Apr 3 07:25:04 CEST 2011

On Sun, Apr 3, 2011 at 2:24 PM, Eugene Toder <eltoder at gmail.com> wrote:

However, I'm not sure we can do a general-purpose AST transformation that handles both new nodes and changes to existing nodes correctly for all applications. As long as both versions contain the same information we can write a transformation that does a near-perfect job. E.g. for my changes I can write a convertor that produces AST in almost the same form as the current one, the only change being the new 'docstring' attribute set to None. (That's for converting AST before optimizations, after optimizations it can contain nodes that couldn't be represented before). I believe it's similar for Try change that Benjamin mentioned above. Also, if written in Python, conversion can at least serve as a template even if it doesn't work out of the box.

If it's do-able, your option 2 is probably the way to go. Out of the box, it may just need to raise an exception if asked to down-convert code that uses new constructs that can't readily be expressed using the old AST (I'm specifically thinking of the challenge of converting PEP 380's yield-from).

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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