I have a piece of code with an extensive test suite that we run using py.test. I recently ran into an issue where a new module had to import another module in order to work correctly. However, since this module was imported elsewhere in the test suite, py.test did not throw an error. Only many years later did this error appear. I've created a minimal reproducible example.

Minimal reproducible example

Project structure:

set.py
fail/
  __init__.py
  thing.py
  other.py
tests/
  test_thing.py
  test_other.py

      

        
        
        
      

    

If these files contain the following code:

fail/thing.py

:

import fail

def needs_do_it():
    return fail.other.do_it() + 100

      

        
        
        
      

    

fail/other.py

:

def do_it():
    return 100

      

        
        
        
      

    

tests/test_thing.py

:

import fail.thing

def test_needs_do_it():
    assert fail.thing.needs_do_it() == 200

      

        
        
        
      

    

tests/test_other.py

:

import fail.other

def test_do_it():
    assert fail.other.do_it() == 100

      

        
        
        
      

    

Expected behavior

If you try to run the function needs_do_it

, you should get an error as it only imported fail

, not fail.other

:

>>> import fail.thing
>>> fail.thing.needs_do_it()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "fail/thing.py", line 4, in needs_do_it
    return fail.other.do_it() + 100
AttributeError: 'module' object has no attribute 'other'

      

        
        
        
      

    

I would expect a test run under py.test to expose this error on import. However, it completely masks this problem.

Actual behavior

Because it test_other.py

imports test.other

, py.test masks the error.

$ py.test

========== test session starts ==========
platform darwin -- Python 2.7.13, pytest-3.1.2, py-1.4.34, pluggy-0.4.0
rootdir: /Users/eswanson/Sandbox/pytest-test, inifile:
collected 2 items

tests/test_other.py .
tests/test_thing.py .
========== 2 passed in 0.01 seconds ==========

      

        
        
        
      

    

My question

My quesiton has three parts:

• What is the cause of this problem?
• Is this the expected behavior for py.test, or something I should be concerned about?
• Is there anything I can do for the pytest user for me to get more guarantees that I won't import imports in the future.