[issue15767] add ModuleNotFoundError

[Eric Snow]

Eric Snow added the comment:

Any chance we could revive ModuleNotFoundError?  It's nice to be able to distinguish between the failure to *find* the module during import from other uses of ImportError.  I'd definitely expect it to work the way Guido explained.  Basically only importlib._bootstrap._find_and_load_unlocked() would raise ModuleNotFoundError (when _find_spec() returns None).

I've found the exception to be very useful while working on the importlib backport (https://bitbucket.org/ericsnowcurrently/importlib2).  My desire for adding ModuleNotFoundError is unrelated to its internal use in importlib that motivated the original request (see msg182332).

Here's the signature:

  ModuleNotFoundError(*args, name=None), inherits from ImportError

For reference, here's ImportError:

  ImportError(*args, name=None, path=None)

ModuleNotFoundError would need to be exposed somewhere sensible since once people see in tracebacks they'll want to catch it. :)  I'd expect that to be either in builtins or as importlib.ModuleNotFoundError.  We may be able to get away with not adding it to builtins, but maybe it would still make sense.

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