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> I'm sorry, but this seems like it should be an importlib internal
> affair. The new exception is too much in everyone's face, because
> the exception name gets printed on every traceback.
That's the crux of the issue. If there isn't much utility outside importlib to distinguishing between module-not-found and other causes of ImportError, then there isn't much point to a new exception. It just boils down to what the other potential causes of ImportError are and how much people care about them.
I keep thinking about PEP 3151 (IOError/OSError hierarchy rework) and the lessons we've learned about exception attributes vs. subclasses. For readability and write-ability, I'd rather write this:
 try:
 from _collections import OrderedDict
 except ModuleNotFoundError:
 pass
than this:
 try:
 from _collections import OrderedDict
 except ImportError as e:
 if e.reason is not importlib.machinery.ImportReason.MODULE_NOT_FOUND:
 raise
But the relevant question is, what is the benefit (outside importlib) of either over this:
 try:
 from _collections import OrderedDict
 except ImportError:
 pass