Issue #15767: Introduce ModuleNotFoundError, a subclass of

ImportError.

The exception is raised by import when a module could not be found.
Technically this is defined as no viable loader could be found for the
specified module. This includes ``from ... import`` statements so that
the module usage is consistent for all situations where import
couldn't find what was requested.

This should allow for the common idiom of::

  try:
    import something
  except ImportError:
    pass

to be updated to using ModuleNotFoundError and not accidentally mask
ImportError messages that should propagate (e.g. issues with a
loader).

This work was driven by the fact that the ``from ... import``
statement needed to be able to tell the difference between an
ImportError that simply couldn't find a module (and thus silence the
exception so that ceval can raise it) and an ImportError that
represented an actual problem.

1 files changed, 1 insertions, 0 deletions

diff --git a/Lib/test/exception_hierarchy.txt b/Lib/test/exception_hierarchy.txt
index 1c1f69f..88e1b81 100644
--- a/Lib/test/exception_hierarchy.txt
+++ b/Lib/test/exception_hierarchy.txt
@@ -13,6 +13,7 @@ BaseException
       +-- BufferError
       +-- EOFError
       +-- ImportError
+           +-- ModuleNotFoundError
       +-- LookupError
       |    +-- IndexError
       |    +-- KeyError