Improve error reporting when `yapf` encounters unsupported Python syntax

[i-zaitsev]

I've encountered a problem with yapf==0.32 when processing code with multiple context managers. Here is an example.

# context.py 

from contextlib import contextmanager

@contextmanager
def manager1(x):
    yield x + 1
    print("manager1 yielded")

@contextmanager
def manager2(y):
    yield y ** 2
    print("manager2 yielded")

def main():
    with (
        manager1(1) as first,
        manager2(2) as second,
    ):
        print(f"{first=}, {second=}")

if __name__ == "__main__":
    main()

See the main function. The tool doesn't like context managers separated with commas.

However, my question is not about unsupported language features, but rather about error reporting. This is what I see as the output. (Truncated to make it more readable.)

> yapf context.py
Traceback (most recent call last):
...
    raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=1, value='as', context=(' ', (16, 20))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
...
    raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=1, value='as', context=(' ', (16, 20))

During handling of the above exception, another exception occurred:
...
    return '{}:{}:{}: {}'.format(e.args[1][0], e.args[1][1], e.args[1][2], e.msg)
IndexError: tuple index out of range

During handling of the above exception, another exception occurred:
...

    return '{}:{}:{}: {}'.format(e.args[1][0], e.args[1][1], e.args[1][2], e.msg)
IndexError: tuple index out of range

Unfortunately, none of these lines tell you where exactly this parsing error happened.

I've stumbled upon this kind of error multiple times already, with different language features being unsupported by a version of yapf used at that time. Updating the version usually helps, but it would be nice to see files that break the tool. We use yapf as a pre-commit hook, and it might be difficult to find what exactly fails when there are multiple files changed in a pull request. Seeing the origin of the error would help to roll back to supported syntax until the tool is updated to a newer version.