TypeError when no arguments supplied

[jaraco]

Consider this watcher_getter:

 proc = watcher_getter(
        name='/home/vagrant/.virtualenvs/gryphon/services/dragoman/bin/dragoman',
        kwargs=dict(
            env=dict(
                PORT=str(port),
                STORAGE_URI=dragoman_db_uri,
            ),
        ),
        checker=functools.partial(airportlocker_listening, port),
    )

Causes this TypeError:

name = '/home/vagrant/.virtualenvs/gryphon/services/dragoman/bin/dragoman', arguments = None
kwargs = {'env': {'PORT': '9000', 'STORAGE_URI': "mongodb://['localhost:47667']/translations"}}
timeout = 20, checker = <functools.partial object at 0x7fef84e1e730>

    def watcher_getter_function(name, arguments=None, kwargs=None, timeout=20, checker=None):
        executable = find_executable(name)
        assert executable, 'You have to install {0} executable.'.format(name)
    
>       cmd = [name] + arguments or []
E       TypeError: can only concatenate list (not "NoneType") to list

/home/vagrant/.virtualenvs/gryphon/local/lib/python2.7/site-packages/pytest_services/service.py:63: TypeError

The issue is with the bind order of that expression. A distilled example is this:

>>> [] + None or []
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "NoneType") to list

Probably what was intended was this:

>>> [] + (None or [])
[]

One can workaround the problem by supplying arguments=[], but it would be better if services handled its own defaults.

[bubenkoff]

service_path is a string fixture, in your example instead you use it as a function, any reason why?
just concatenate with os.path?

[jaraco]

Oh, I wasn't aware of service_path. In this case, it's a global privately defined. It returns a string. I'll update the example to eliminate that point of confusion.