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, 3 insertions, 0 deletions

diff --git a/Doc/whatsnew/3.4.rst b/Doc/whatsnew/3.4.rst
index 468ba1f..23ccae2 100644
--- a/Doc/whatsnew/3.4.rst
+++ b/Doc/whatsnew/3.4.rst
@@ -137,6 +137,9 @@ Some smaller changes made to the core Python language are:
 
 * Unicode database updated to UCD version 6.2.
 
+* Import now raises the new exception :exc:`ModuleNotFoundError` (subclass of
+  :exc:`ImportError`) when it cannot find something.
+
 
 
 New Modules