pytest.raises() doesn't always return Exception instance in py26

[pytestbot]

Originally reported by: Eric Siegerman (BitBucket: eks, GitHub: eks)

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Typically, the ExceptionInfo.value returned by pytest.raises() (when the exception is indeed raised) is an Exception instance. But under certain conditions, it isn't. It looks as though it contains the Exception's .args, rather than the Exception itself. This happens when all of the following are true:

• Python is 2.6 (I've tested with 2.6.6 and 2.6.9)
• The exception is one of Python's internal ones (I've tested with ZeroDivisionError, KeyError, and IOError; not sure whether this applies to others)
• The exception was raised by the interpreter -- if I explicitly raise KeyError("msg"), for example, the test passes

The attached tests demonstrate the problem -- and the more useful behaviour when you use vanilla try/except instead of pytest.raises(). Under Python 2.6, the three pytest.raises() cases fail; under Python 2.7 and 3.4, they pass. (The try/except tests pass under all three Python versions.)

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• Bitbucket: https://bitbucket.org/pytest-dev/pytest/issue/767

[nicoddemus]

@eks, FYI: #788. Thanks for the report!