[Numpy-discussion] Adding more detailed exception types to numpy

[Eric Wieser]

PR #12593 <https://github.com/numpy/numpy/pull/12593> adds a handful of new
exception types for . Some consequences of this change are that:

   1. The string formatting is moved to python, allowing us to give better
   error messages without a lot of work
   2. The formatting is dispatched lazily, meaning that users trying
   ufuncs, but catching unsupported loop types don’t have to pay the cost of
   string formatting
   3. Users can catch a specific exception type. This might expose more of
   our internals than we’re willing to though.
   4. We need to expose these new exception names in the public API.

3 & 4 raise some questions, which I’d like some feedback on:

   -

   Should we actually expose the detailed exception types to the user, or
   should they be kept an implementation detail? One way to hide the
   implementation details would be

   class TypeError(builtins.TypeError):
       """actually UFuncCastingError"""
   _UFuncCastingError = TypeError  # for internal use when raising
   _UFuncCastingError.__module__ = None
   class TypeError(builtins.TypeError):
       """actually UFuncLoopError"""
   _UFuncLoopError= TypeError  # for internal use when raising
   _UFuncLoopError  .__module__ = None
   del TypeError

   This gives us all the advantages of 1 & 2 without the user noticing that
   they’re receiving anything other than TypeError, which their tracebacks
   will continue to show.
   -

   If we do expose them, where should these exceptions go? In the past, we
   also added AxisError and TooHardError - it would be nice to start being
   consistent about where to expose these things
   1. np.UFuncCastingError (expands the global namespace even further)
      2. np.core._exceptions.UFuncCastingError (shouldn’t really be
      private, since users won’t know how to catch it)
      3. np.core.UFuncCastingError
      4. np.core.umath.CastingError
      5. A dedicated namespace for exceptions:
         - np.errors.UFuncCastingError (matches pandas)
         - np.exceptions.UFuncCastingError
         - np.exc.UFuncCastingError (matches sqlalchemy)

Eric
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