Why does import work with Python nose when test directory contains __init__.py?

Question

Consider the following project structure:

a.py
test/
  test_a.py

with test_a.py importing module a:

import a

As expected, running nosetests in the test directory results in import error:

ERROR: Failure: ImportError (No module named a)

However, I noticed that adding an empty __init__.py file to the test directory makes import work with nosetests (but not when you run test_a.py with Python). Could you explain why?

I understand that adding __init__.py makes test a package. But does it mean that import includes the directory containing the package in the lookup?

Answer 1

The presence of an __init__.py file in the directory transforms test from just a plain old directory into a python package. This has an effect on sys.path.

Modify your test_a.py module like this:

import sys

def test_thing():
    for i, p in enumerate(sys.path):
        print i, p

try:
    import a
except ImportError:
    print('caught import error')

Then try running nosetests -s from the test directory, with and without an __init__.py in there.

Note: it is the test runner that munges sys.path. And that is documented in the second "Note" of this section here (thanks @davidism). You won't see any change there just by running python test_a.py with and without the package structure.