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.

If it doesn't take too much effort, I would like the see the exception messages made more user friendly and kept more in-sync with the pure Python version.

Yes, ideally the exceptions should be in sync. I don't find the list
of signals entirely uninteresting, but implementing it in _pydecimal
would require #8613 to be solved, which is unlikely to happen.


In _decimal the exceptions come directly from libmpdec, so in order
to have the same error messages as _pydecimal I'd need to implement
detailed error codes and store them somewhere in the context.

Just noticed this myself when testing with Python 3.5-3.7:

    >>> from decimal import Decimal
    >>> d = Decimal('foo')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    decimal.InvalidOperation: [<class 'decimal.ConversionSyntax'>]


With Python 2.7 I get this error instead:

    decimal.InvalidOperation: Invalid literal for Decimal: 'foo'


I'm writing type conversion code and was planning to include the error message by Python along with some higher level explanation when reporting errors. `[<class 'decimal.ConversionSyntax'>]` would be such a strange message that I need to make a special case with decimal.

The thing is that Python explicitly supports any object as the "message".

I was quite pleased when I discovered that because it provides a way
to include both signals and the condition list in the "message", which can be used in programs for dispatching on the condition, not only
on the signal.

Signals directly map to the exceptions, but there can be more than one
condition that caused the signal.

pypy3 actually took over the terse exceptions, but without the conditions.  They'd also need error codes from libmpdec now.

>>>> huge = Decimal('9' * 99)
>>>> huge.quantize(Decimal('0.1'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/stefan/tmp/pypy3-v6.0.0-linux64/lib_pypy/_decimal.py", line 842, in quantize
    workctx._ctx, status_ptr)
  File "/home/stefan/tmp/pypy3-v6.0.0-linux64/lib_pypy/_decimal.py", line 1631, in __exit__
    self.context._add_status(status)
  File "/home/stefan/tmp/pypy3-v6.0.0-linux64/lib_pypy/_decimal.py", line 1263, in _add_status
    raise exception
decimal.InvalidOperation

Some other non-matching exceptions are unfriendly, but do not come from libmpdec and could be made to match without, I think, causing problems.  For example:

    >>> format(_decimal.Decimal(1), "invalid")
    ...
    ValueError: invalid format string
    >>> format(_pydecimal.Decimal(1), "invalid")
    ...
    ValueError: Invalid format specifier: invalid

Would it make sense to make a patch to make these match?