Steps to reproduce:

$ cat a.py
import sys

class MyFinder:
    def __init__(self, path):
        if path != "__myfinder__":
            raise ImportError()

    def find_module(self, fullname, path=None):
        return None

sys.path_hooks.append(MyFinder)
sys.path.insert(0, '__myfinder__')

import time
print('OK')

$ python3.3 -V
Python 3.3.2+

$ python3.3 a.py
OK

$ python3.4 -V
Python 3.4.0rc1

$ python3.4 a.py
Traceback (most recent call last):
  File "a.py", line 14, in <module>
    import time
ImportError: spec missing loader


On Ubuntu 13.10.
python3.3 is ubuntu's package.
python3.4.0rc1 is built by myself.

Vim's custom importer does not work for it.

[vim_dev] Problem with Python 3.4
https://groups.google.com/d/msg/vim_dev/RfuOG7_Jabs/h3YzLTsRgv0J

The problem is that the PEP 451 switch accidentally cut out compatibility code for PathEntryFinder.find_module() since Python 3.3 started the transition to find_loader(). Adding a bit of code to http://hg.python.org/cpython/file/8ac9c3754d33/Lib/importlib/_bootstrap.py#l1865 and a test will fix it.

I should also mention that subclassing importlib.abc.PathEntryFinder solves this coding problem.

I have a fix, I just need to create a test.

Fix and test is attached. Can someone review it so I can check it in and get a cherrypick?

LGTM

It works fine.  Thank you!

I'll try to commit tonight then and request a cherrypick.

New changeset 5dec1604322c by Brett Cannon in branch 'default':
Issue #20763: Fix importlib.machinery.PathFinder to support
http://hg.python.org/cpython/rev/5dec1604322c

Committed for Larry to cherrypick into 3.4.0. No Misc/NEWS so that Larry doesn't have merge issues and since it will be covered by 3.4.0.

Cherrypick bug is http://bugs.python.org/issue20789

New changeset 149be7000c11 by Brett Cannon in branch '3.4':
Issue #20763: Fix importlib.machinery.PathFinder to support
http://hg.python.org/cpython/rev/149be7000c11