Issue 32479: inconsistent ImportError message executing same import statement (original) (raw)

While debugging a problem in my job I find an odd case about import, it could be reduced to the following tracebacks:

from keystone.assignment import schema Traceback (most recent call last): File "", line 1, in File "/usr/lib/python2.7/site-packages/keystone/assignment/init.py", line 15, in from keystone.assignment import controllers # noqa File "/usr/lib/python2.7/site-packages/keystone/assignment/controllers.py", line 26, in from keystone.common import controller File "/usr/lib/python2.7/site-packages/keystone/common/controller.py", line 24, in from keystone.common import authorization File "/usr/lib/python2.7/site-packages/keystone/common/authorization.py", line 23, in from keystone.models import token_model File "/usr/lib/python2.7/site-packages/keystone/models/token_model.py", line 15, in from keystoneclient.common import cms ImportError: No module named keystoneclient.common from keystone.assignment import schema Traceback (most recent call last): File "", line 1, in File "/usr/lib/python2.7/site-packages/keystone/assignment/init.py", line 15, in from keystone.assignment import controllers # noqa File "/usr/lib/python2.7/site-packages/keystone/assignment/controllers.py", line 25, in from keystone.assignment import schema ImportError: cannot import name schema

keystoneclient is deliberately not installed. I think it should always report keystoneclient.common could not be found which reveals the root causes. But only the first time it does so. Later tries it always report cannot import schema.

The reason of this behaviour is that first time although keystone.assignment import fails but keystone.assignment.schema is successfully installed in sys.modules. And then, in ensure_fromlist, it calls import_submodule which returns the module in sys.modules, without set schema attribute to keystone.assignment. So it then fails in import_from for not able to get the attribute.

It could be simply reproduced by the following package structure:

test |-- a.py |-- b.py `-- init.py

init.py: from test import a

a.py: from test import b import modulenotexisting

b.py: #emtpy

import test Traceback (most recent call last): File "", line 1, in File "test/init.py", line 1, in from test import a File "test/a.py", line 2, in import dddddddddd ImportError: No module named dddddddddd import test Traceback (most recent call last): File "", line 1, in File "test/init.py", line 1, in from test import a File "test/a.py", line 1, in from test import b ImportError: cannot import name b

Python3 doesn't suffer this problem, at least for 3.7.

I don't know this should be identified as a bug or not. But this does bring me trouble debugging problems and lead to confusions because it doesn't tell users the real module can't be imported.

Since the problem is specific to Python 2.7 and has been resolved in the importlib based import implementation, I'm inclined to close this as "Won't Fix".

The only reason I haven't is that if someone really wanted to dig into the Python 2.7 import implementation and figure out how to improve the situation, I'd actually be willing to review the patch.

Alternatively, it might be worthwhile for OpenStack to investigate migrating over to importlib2 for their Python 2.7 support (or at least supporting it as a debugging option).