AttributeError raised if --cov enabled twice

If --cov is enabled twice, without specifying paths each time, the following backtrace occurs.

Traceback (most recent call last):
  File "/usr/lib64/python3.5/runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib64/python3.5/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/lib/python3.5/site-packages/pytest.py", line 17, in <module>
    raise SystemExit(pytest.main())
  File "/usr/lib/python3.5/site-packages/_pytest/config.py", line 47, in main
    config = _prepareconfig(args, plugins)
  File "/usr/lib/python3.5/site-packages/_pytest/config.py", line 156, in _prepareconfig
    pluginmanager=pluginmanager, args=args)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 745, in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 339, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 334, in <lambda>
    _MultiCall(methods, kwargs, hook.spec_opts).execute()
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 613, in execute
    return _wrapped_call(hook_impl.function(*args), self.execute)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 250, in _wrapped_call
    wrap_controller.send(call_outcome)
  File "/usr/lib/python3.5/site-packages/_pytest/helpconfig.py", line 32, in pytest_cmdline_parse
    config = outcome.get_result()
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 279, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 265, in __init__
    self.result = func()
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 614, in execute
    res = hook_impl.function(*args)
  File "/usr/lib/python3.5/site-packages/_pytest/config.py", line 923, in pytest_cmdline_parse
    self.parse(args)
  File "/usr/lib/python3.5/site-packages/_pytest/config.py", line 1080, in parse
    self._preparse(args, addopts=addopts)
  File "/usr/lib/python3.5/site-packages/_pytest/config.py", line 1051, in _preparse
    args=args, parser=self._parser)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 745, in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 339, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 334, in <lambda>
    _MultiCall(methods, kwargs, hook.spec_opts).execute()
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 613, in execute
    return _wrapped_call(hook_impl.function(*args), self.execute)
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 254, in _wrapped_call
    return call_outcome.get_result()
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 279, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 265, in __init__
    self.result = func()
  File "/usr/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 614, in execute
    res = hook_impl.function(*args)
  File "/usr/lib/python3.5/site-packages/pytest_cov/plugin.py", line 95, in pytest_load_initial_conftests
    plugin = CovPlugin(ns, early_config.pluginmanager)
  File "/usr/lib/python3.5/site-packages/pytest_cov/plugin.py", line 150, in __init__
    self.start(engine.Central)
  File "/usr/lib/python3.5/site-packages/pytest_cov/plugin.py", line 171, in start
    self.cov_controller.start()
  File "/usr/lib/python3.5/site-packages/pytest_cov/engine.py", line 141, in start
    self.cov.erase()
  File "/usr/lib/python3.5/site-packages/coverage/control.py", line 727, in erase
    self._init()
  File "/usr/lib/python3.5/site-packages/coverage/control.py", line 246, in _init
    self.source.append(files.canonical_filename(src))
  File "/usr/lib/python3.5/site-packages/coverage/files.py", line 61, in canonical_filename
    if not os.path.isabs(filename):
  File "/usr/lib64/python3.5/posixpath.py", line 64, in isabs
    return s.startswith(sep)
AttributeError: 'bool' object has no attribute 'startswith'

I encountered this after adding the following to setup.cfg, which also using --cov on the command line.

[tool:pytest]
addopts =  --cov

The problem is the rather simple handling of multiple --cov at https://github.com/pytest-dev/pytest-cov/blob/master/src/pytest_cov/plugin.py#L87

How can/should I run your coala project locally?

To reproduce the problem:

git clone https://github.com/coala/coala/
cd coala
python3 -m pytest --cov

To see the workaround, remove --cov (it is still enabled in setup.cfg)

python3 -m pytest

Fix double --cov handling. Close #151.

Now fixed, but with a twist: if you use both --cov and --cov=foo then filters will be completely disabled (same as having only --foo). With this behavior you can put in pytest.ini something like addopts=--cov=foo, and add --cov on command line to enable coverage for everything.