Since Python 2.7.15 import bs4 (BeautifulSoup4) fails when using the -3 option on the Python binary.

...
    'You are trying to run the Python 2 version of Beautiful Soup under Python 3. This will not work.'<>'You need to convert the code, either by installing it (`python setup.py install`) or by running 2to3 (`2to3 -w bs4`).'
                                                                                                       ^
SyntaxError: unknown parsing error

With 2.7.14 this works fine and I get the expected deprecation warning.

A workaround is to import the package without -3 first, so the .pyc file is generated and used in subsequent imports.

The Python interpreter itself does not have references to Beautiful Soup. This error is generated by third-party code. Report a bug on the Beautiful Soup bug tracker.

Yes, it's third party code, but that worked up until Python 2.7.14 and only broke with 2.7.15, which is why I reported it here.

The expected behaviour is not a SyntaxError, but a DeprecationWarning, which is what you get pre Python 2.7.15. So this is a regression.

Ah, I was fooled by the message about Beautiful Soup. It is not a message of an error, it is a part of source line, printed for a SyntaxError. Seems sources of Beautiful Soup intentionally contain a code invalid in Python 3.

There is a bug in Python. The tokenizer returns error E_OK, but it should never return such error code. The AST parser is confused and raises a SyntaxError with the message "unknown parsing error".

This bug is reproduced when run Python wish both options -3 and -We and parse the "<>" operator.

$ ./python -3 -We -c '[] <> []'
error=10
  File "<string>", line 1
    [] <> []
        ^
SyntaxError: unknown parsing error

But it is reproduced with 2.7.14 too.

You are right, this actually happens with Python 2.7.14 as well. I was fooled by a warnings.filterwarnings matching to a path in my codebase. That one did match for my Python 2.7.15 testing but didn't for my Python 2.7.14 testing, because those were done in a temporary path which didn't match. I should have tried to narrow it down further first.

I'm glad you found the necessary -We option.

Thanks for looking into this and for the patch!

I'm currently unable to test the patch though.

New changeset d5e7556e522f4662ad34b35924b6c76895df340e by Serhiy Storchaka in branch '2.7':
bpo-33645: Fix an "unknown parsing error" in the parser. (GH-7119)
https://github.com/python/cpython/commit/d5e7556e522f4662ad34b35924b6c76895df340e

I'm not sure that this will fix your case (there may be other bug in BeautifulSoup4 or other changes in your environment), but it fixed a bug in Python. Thank you for your report.