yapf cannot handle 3.x metaclass syntax when run under 2.7 about yapf

[eliben]

To format Python 3 code, run yapf under Python 2, and vice versa. This is because yapf tries to verify it didn't mess up the semantic meaning of the code. The sample you pasted is invalid in Python 2

[DRMacIver]

I don't think the way this is currently handled is a good call. It's fine (well, not fine, but acceptable) if yapf can't format code running on a different version, but crashing is a really bad way to signal this and severely limits the utility. If you can't parse code as valid python syntax when you're given it, silently doing nothing (or better yet, giving a warning) is better than crashing.

One of the problems here is that code bases for libraries can be and often are mixed python 2 / python 3. I'd love to be able to drop python 2 support, but I basically can't, and until the point when I can the entire code base is highly likely to have some bits which don't parse correctly on python 2 and some bits that don't parse correctly on python 3 (as it happens I've avoided this so far, but I'm not going to be able to for much longer), so unless I'm very very careful about how I run yapf it's just going to crash.