When using tox to run tests the preferred way to run tests creates throw away virtual environments and installs the package to be tested into them. This means that the code to be running is actually inside .tox/venvname/lib/pythonX.Y/site-packages/library.
Normally coverage.py could support this BUT when doing --source library it sees the library/ directory in the current directory (which isn't being used) and doesn't do the package resolution. This leads to 0% coverage reports always when using tox. It would be great if there was a way to say "No really this is a package".