[Python-ideas] PEP 511: Add a check function to decide if a "language extension" code transformer should be used or not

[Andrew Barnert]

On Jan 27, 2016, at 12:20, Brett Cannon <brett at python.org> wrote:
> 
>> On Wed, 27 Jan 2016 at 08:49 Andrew Barnert via Python-ideas <python-ideas at python.org> wrote:
>> 
>> If someone builds a transformer that only runs on files with a different extension, he already needs an import hook, so he might as well just call his transformer from the input hook, same as he does today.
> 
> And the import hook is not that difficult.

Unless it has to work in 2.7 and 3.3 (or, worse, 2.6 and 3.2). :)

> You can reuse everything from importlib without modification except for needing to override a single method in some loader to do your transformation

Yes, as of 3.4, the design is amazing. In fact, hooking any level--lookup, source, AST, bytecode, or pyc--is about as easy as it could be.

My only complaint is that it's not easy enough to find out how easy import hooks are. When I tell people "you could write a simple import hook to play with that idea", they get a look of fear and panic that's completely unwarranted and just drop their cool idea. (I wonder if having complete examples of a simple global-transformer hook and a simple special-extension hook at the start of the docs would be enough to solve that problem?)

And I'm a bit worried that if Victor tries to make things like MacroPy and Hy easier, it still won't be enough for real-life cases, so all it'll do is discourage people from going right to writing import hooks and seeing how easy that already is.
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