Import errors when running nosetests that I can't reproduce outside of nose

I'm running into a mysterious import error when using nosetests to run a test suite that I can't reproduce outside of the nose. Furthermore, the import error disappears when I skip a subset of the tests. Executive Summary: I am getting an import error in Nose that a) only appears when tests bearing a certain attribute are excluded and b) cannot be reproduced in an interactive python session, even when I ensure that the sys.path is the same for both. Details: The package structure looks like this: ``` project/ module1/__init__.py module1/foo.py module1/test/__init__.py module1/test/foo_test.py module1/test/test_data/foo_test_data.txt module2/__init__.py module2/bar.py module2/test/__init__.py module2/test/bar_test.py module2/test/test_data/bar_test_data.txt ``` Some of the tests in foo_test.py are slow, so I've created a @slow decorator to allow me to skip them with a nosetests option: ``` def slow(func): """Decorator sets slow attribute on a test method, so nosetests can skip it in quick test mode.""" func.slow = True return func class TestFoo(unittest.TestCase): @slow def test_slow_test(self): load_test_data_from("test_data/") slow_test_operations_here def test_fast_test(self): load_test_data_from("test_data/") ``` When I want to run the fast unit tests only, I use ``` nosetests -vv -a'!slow' ``` from the root directory of the project. When I want to run them all, I remove the final argument. Here comes the detail that I suspect is to blame for this mess. The unit tests need to load test data from files (not best practice, I know.) The files are placed in a directory called "test_data" in each test package, and the unit test code refers to them by a relative path, assuming the unit test is being run from the test/ directory, as shown in the example code above. To get this to work with running nose from the root directory of the project, I added the following code to init.py in each test package: ``` import os import sys orig_wd = os.getcwd() def setUp(): """ test package setup: change working directory to the root of the test package, so that relative path to test data will work. """ os.chdir(os.path.dirname(os.path.abspath(__file__))) def tearDown(): global orig_wd os.chdir(orig_wd) ``` As far as I understand, nose executes the setUp and tearDown package methods before and after running the tests in that package, which ensures that the unit test can find the appropriate test_data directory, and the working directory is reset to the original value when the tests are complete. So much for the setup. The problem is, I get an import error only when I run the full suite of tests. The same modules import just fine when I exclude the slow tests. (To clarify, the tests throwing import errors are not slow, so they execute in either scenario.) ``` $ nosetests ... ERROR: Failure: ImportError (No module named foo_test) ---------------------------------------------------------------------- Traceback (most recent call last): File "/Library/Python/2.7/site-packages/nose/loader.py", line 413, in loadTestsFromName addr.filename, addr.module) File "/Library/Python/2.7/site-packages/nose/importer.py", line 47, in importFromPath return self.importFromDir(dir_path, fqname) File "/Library/Python/2.7/site-packages/nose/importer.py", line 80, in importFromDir fh, filename, desc = find_module(part, path) ImportError: No module named foo_test ``` If I run the test suite without the slow tests, then no error: ``` $ nosetests -a'!slow' ... test_fast_test (module1.test.foo_test.TestFoo) ... ok ``` In a python interactive session, I can import the test module with no trouble: ``` $ python Python 2.7.1 (r271:86832, Aug 5 2011, 03:30:24) [GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> import module1.test >>> module1.test.__path__ ['/Users/USER/project/module1/test'] >>> dir(module1.test) ['__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__', 'orig_wd', 'os', 'setUp', 'sys', 'tearDown'] ``` When I set a breakpoint in nose/importer.py, things look different: ``` > /Library/Python/2.7/site-packages/nose/importer.py(83)importFromDir() -> raise (Pdb) l 78 part, part_fqname, path) 79 try: 80 fh, filename, desc = find_module(part, path) 81 except ImportError, e: 82 import pdb; pdb.set_trace() 83 -> raise 84 old = sys.modules.get(part_fqname) 85 if old is not None: 86 # test modules frequently have name overlap; make sure 87 # we get a fresh copy of anything we are trying to load 88 # from a new path (Pdb) part 'foo_test' (Pdb) path ['/Users/USER/project/module1/test'] (Pdb) import module1.test.foo_test *** ImportError: No module named foo_test #If I import module1.test, it works, but the __init__.py file is not being executed (Pdb) import partition.test (Pdb) del dir (Pdb) dir(partition.test) ['__doc__', '__file__', '__name__', '__package__', '__path__'] #setUp and tearDown missing? (Pdb) module1.test.__path__ ['/Users/USER/project/module1/test'] #Module path is the same as before. (Pdb) os.listdir(partition.test.__path__[0]) #All files are right where they should be... ['.svn', '__init__.py', '__init__.pyc', 'foo_test.py', 'foo_test.pyc','test_data'] ``` I see the same screwy results even if I copy sys.path from my interactive session into the pdb session and repeat the above. Can anyone give me any insight about what might be going on? I realize I'm doing several non-standard things at the same time, which could lead to strange interactions. I'd be as interested in advice on how to simplify my architecture as I would be to get an explanation for this bug.