Issue #15767: Touch up ModuleNotFoundError usage by import.

Forgot to raise ModuleNotFoundError when None is found in sys.modules.
This led to introducing the C function PyErr_SetImportErrorSubclass()
to make setting ModuleNotFoundError easier.

Also updated the reference docs to mention ModuleNotFoundError
appropriately. Updated the docs for ModuleNotFoundError to mention the
None in sys.modules case.

Lastly, it was noticed that PyErr_SetImportError() was not setting an
exception when returning None in one case. That issue is now fixed.

1 files changed, 7 insertions, 0 deletions

diff --git a/Doc/c-api/exceptions.rst b/Doc/c-api/exceptions.rst
index 1bdcdd3a36..25e2a1c280 100644
--- a/Doc/c-api/exceptions.rst
+++ b/Doc/c-api/exceptions.rst
@@ -292,6 +292,13 @@ in various ways.  There is a separate error indicator for each thread.
 
    .. versionadded:: 3.3
 
+.. c:function:: PyObject* PyErr_SetImportErrorSubclass(PyObject *msg, PyObject *name, PyObject *path)
+
+   Much like :c:func:`PyErr_SetImportError` but this function allows for
+   specifying a subclass of :exc:`ImportError` to raise.
+
+   .. versionadded:: 3.4
+
 
 .. c:function:: void PyErr_SyntaxLocationEx(char *filename, int lineno, int col_offset)