Regression: "Already imported a file that will be measured", loss of coverage

[mgedmin]

Describe the bug

I'm measuring gtimelog's test coverage with coverage run -m gtimelog.tests. Since coverage 5.0a4 this is missing some coverage (shows 79% instead of 100%) and also produces warnings:

Coverage.py warning: Already imported a file that will be measured: /home/mg/src/gtimelog/src/gtimelog/__init__.py (already-imported)
Coverage.py warning: Already imported a file that will be measured: /home/mg/src/gtimelog/src/gtimelog/timelog.py (already-imported)
Coverage.py warning: Already imported a file that will be measured: /home/mg/src/gtimelog/src/gtimelog/settings.py (already-imported)

To Reproduce
How can we reproduce the problem? Please be specific.

1. What version of Python are you using? 2.7.17 and 3.7.5

2. What version of coverage.py are you using? 5.0.1 (but every version starting with 5.0a4 is affected; 5.0a3 is fine)

3. What versions of what packages do you have installed?

   coverage==5.0.1
   coverage-python-version==0.2.0
   freezegun==0.3.12
   -e git+https://github.com/gtimelog/gtimelog@5f7858cc58925e42b2d7cbc1d872b11745c38872#egg=gtimelog
   mock==3.0.5
   python-dateutil==2.8.1
   six==1.13.0
   </details>
   
   

4. What code are you running? Give us a specific commit of a specific repo that we can check out.
   https://github.com/gtimelog/gtimelog/tree/5f7858cc58925e42b2d7cbc1d872b11745c38872

5. What commands did you run? make coverage for a quick version; .tox/coverage/bin/pip install coverage=$version && .tox/coverage/bin/coverage -m gtimelog.test && .tox/coverage/bin/coverage report to test a specific version of coverage.py

Expected behavior
100% coverage reported, no warnings.

[mgedmin]

git bisect blames commit b7e0eec

[mgedmin]

So it looks like coverage run -m gtimelog.tests imports gtimelog/__init__.py and gtimelog/tests/__init__.py before starting coverage measurement. As a workaround I can apply this patch to my (hacky) test code:

diff --git a/src/gtimelog/tests/__init__.py b/src/gtimelog/tests/__init__.py
index 02f7cc5..31f8aca 100644
--- a/src/gtimelog/tests/__init__.py
+++ b/src/gtimelog/tests/__init__.py
@@ -1,10 +1,9 @@
 """Tests for gtimelog"""
 import unittest
 
-from gtimelog.tests import test_timelog, test_settings, test_main
-
 
 def test_suite():
+    from gtimelog.tests import test_timelog, test_settings, test_main
     return unittest.TestSuite([
         test_timelog.test_suite(),
         test_settings.test_suite(),

and then I have only one wrongly reported missing code line: the __version__ assignment in gtimelog/__init__.py.

I would prefer not having to do that: after all, things worked fine with coverage 4.x (or 5.0a3).

(And anyway the real workaround would be to stop being silly with custom test running modules and start using pytest, which is on my todo list anyway.)

[nedbat]

This is now available in coverage 5.0.2