CoverageException: Can't combine line data with arc data when updating to repository tip

[nedbat]

Originally reported by Florian Bruhin (Bitbucket: The-Compiler, GitHub: The-Compiler)

---

After updating from 3.7.1 to the repository tip, I get this when running my tests:

[...]
  File "/home/florian/proj/qutebrowser/git/.tox/py34/lib/python3.4/site-packages/pytest_cov/plugin.py", line 156, in pytest_sessionfinish
    self.cov_controller.finish()
  File "/home/florian/proj/qutebrowser/git/.tox/py34/lib/python3.4/site-packages/pytest_cov/engine.py", line 123, in finish
    self.cov.combine()
  File "/home/florian/proj/qutebrowser/git/.tox/py34/lib/python3.4/site-packages/coverage/control.py", line 781, in combine
    self.data_files.combine_parallel_data(self.data, aliases=aliases, data_paths=data_paths)
  File "/home/florian/proj/qutebrowser/git/.tox/py34/lib/python3.4/site-packages/coverage/data.py", line 649, in combine_parallel_data
    data.update(new_data, aliases=aliases)
  File "/home/florian/proj/qutebrowser/git/.tox/py34/lib/python3.4/site-packages/coverage/data.py", line 434, in update
    raise CoverageException("Can't combine line data with arc data")
coverage.misc.CoverageException: Can't combine line data with arc data

It goes away when deleting .coverage.

I can't reproduce it with a minimal example, but here is how you should be able to reproduce it:

• Install tox and PyQt, see this document
• Clone https://github.com/The-Compiler/qutebrowser
• Run tox -e py34 (note: some windows will pop-up and close during the test, so it's best to not do anything in the meantime - the testsuite should take about 30-60 seconds)
• Change tox.ini to use the hg version:

diff --git a/tox.ini b/tox.ini
index [d78d979 (bb)](https://bitbucket.org/ned/coveragepy/commits/d78d979)..e4777ea [100644 (bb)](https://bitbucket.org/ned/coveragepy/commits/100644)
--- a/tox.ini
+++ b/tox.ini
@@ -22,7 +22,7 @@ deps =
     pytest-html==1.5.1
     hypothesis==1.10.1
     hypothesis-pytest==0.17.0
-    coverage==3.7.1
+    hg+https://bitbucket.org/ned/coveragepy
     pytest-cov==2.0.0
     cov-core==1.15.0
     beautifulsoup4==4.4.0

• If it's present (it shouldn't), remove .coverage.xml as this will cause other issues
• Re-run the command above.

I attached .coverage before and after running 4.0 (the latter was named .coverage.ws042.24666.991896 - I guess it gets renamed later and it crashed before that?)

---

• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/399
• This issue had attachments: coverage-4.0, coverage-3.7.1. See the original issue for details.

[nedbat]

I've reproduced this locally, working on understanding what's going on...

[nedbat]

Original comment by Florian Bruhin (Bitbucket: The-Compiler, GitHub: The-Compiler)

---

I can reproduce this with this minimal example, without doing the 3.7.1 -> 4.0 switch:

import sys
import subprocess

def test_foo():
    subprocess.check_call([sys.executable, '-c', 'print("Hello World")'])

I guess it has something to do with pytest-cov turning subprocess coverage on automatically.

[nedbat]

Original comment by Florian Bruhin (Bitbucket: The-Compiler, GitHub: The-Compiler)

---

The above sample is actually flawed - it only worked because my conftest.py (pytest configuration file) imported something from qutebrowser, otherwise no data would be collected.

Here is a truly standalone example:

$ virtualenv .venv --python=python3

$ ./.venv/bin/pip install hg+https://bitbucket.org/ned/coveragepy pytest pytest-cov
Collecting hg+https://bitbucket.org/ned/coveragepy                                   
  Cloning hg https://bitbucket.org/ned/coveragepy to /tmp/pip-q_79karm-build
Collecting pytest
  Using cached pytest-2.7.2-py2.py3-none-any.whl
Collecting pytest-cov
  Using cached pytest_cov-2.0.0-py2.py3-none-any.whl
Collecting py>=1.4.29 (from pytest)
  Using cached py-1.4.30-py2.py3-none-any.whl
[...]

$ mkdir pkg
$ echo pass > pkg/mod.py

$ cat > .coveragerc
[run]
branch = true

$ cat > test_foo.py
import sys
import subprocess

from pkg import mod

def test_foo():
        subprocess.check_call([sys.executable, '-c', 'print("Hello World")'])

$ ./.venv/bin/python -m py.test test_foo.py --cov pkg
[...]

test_foo.py .Traceback (most recent call last):
[...]
  File "/home/florian/tmp/coverage_issue/.venv/lib/python3.4/site-packages/pytest_cov/plugin.py", line 156, in pytest_sessionfinish
    self.cov_controller.finish()
  File "/home/florian/tmp/coverage_issue/.venv/lib/python3.4/site-packages/pytest_cov/engine.py", line 123, in finish
    self.cov.combine()
  File "/home/florian/tmp/coverage_issue/.venv/lib/python3.4/site-packages/coverage/control.py", line 781, in combine
    self.data_files.combine_parallel_data(self.data, aliases=aliases, data_paths=data_paths)
  File "/home/florian/tmp/coverage_issue/.venv/lib/python3.4/site-packages/coverage/data.py", line 649, in combine_parallel_data
    data.update(new_data, aliases=aliases)
  File "/home/florian/tmp/coverage_issue/.venv/lib/python3.4/site-packages/coverage/data.py", line 434, in update
    raise CoverageException("Can't combine line data with arc data")
coverage.misc.CoverageException: Can't combine line data with arc data

[nedbat]

Fixed in 74e2f7c366a2 (bb)