Take into account __future__ imports and pass flags to compile()

[flyte]

Fixes #53 by parsing the source for future imports, then supplying the correct flags to compile().

This has the side-effect of changing the function signature for reformatter.Reformat() by allowing the unformatted_source to be passed in. This may not be ideal, so please let me know if you can see a more suitable way to get the compiler flags from where they're extracted from the source to the compile() calls in verifier.VerifyCode().

I have accepted the CLA.

[bwendling]

Hi Flyte,

Thanks for the code. I looked over it and I'm not sure that we need all of this for what we're doing. Or at the least it should be done in a different place. :-)

The idea of the "verifier" module is that it will catch catastrophic errors. Note that there's a better way to validate that the semantics haven't changed (generating before and after ASTs and comparing them). So with that in mind, this seems like a bit of overkill. An easier way to do this is to generate all of the supported "from future import ..." statements and try to compile / parse them in the verifier module. We do something similar with the print statement already.

[flyte]

Understood. I shouldn't have stormed ahead with it without discussing it here first anyway.. I'll probably turn the code into a library since I couldn't find any to do this. Thanks for your responses.

[flyte]

Just if you're interested, I looked into the AST you mentioned and have used it to simplify my code massively. I did turn it into a library but it's hardly worth it now that it's so easy!
https://github.com/flyte/static-future/blob/develop/static_future/analysis.py

[bwendling]

A side note, we're thinking about disabling the verification step for all but development (and maybe not even that).