Exceptions from the decimal module are quite unfriendly:

    >>> Decimal(42) / Decimal(0)
    ...
    decimal.DivisionByZero: [<class 'decimal.DivisionByZero'>]

    >>> huge = Decimal('9' * 99)
    >>> huge.quantize(Decimal('0.1'))
    ...
    decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]


compared to the pure Python implementation:

    decimal.DivisionByZero: x / 0

    decimal.InvalidOperation: quantize result has too many digits for current context


If I'm reading http://bugs.python.org/issue21227 right, the exception argument is a of signals, and indicates the complete set of signals raised by the operation. However, this is (AFAICS) not documented, and not portable (since it's not present in the Python version).

I believe this behavior should be
- either dropped in favor of friendlier error messages (or possibly moved to a more internal attribute of the exception, and documented as an optional CPython feature),
- or documented, and implemented in _pydecimal as well.

Which of those actions would be right seems to be a question of whether the exception argument is part of the API.