test-priority.py

import os

from traitlets.config import Config, Configurable, Application
from traitlets import Unicode

os.environ['FOO_VALUE'] = 'foo-from-env'
os.environ['APP_VALUE'] = 'app-from-env'

class Foo(Configurable):
    value = Unicode('default', envvar='FOO_VALUE', config=True)

class MyApp(Application):
    value = Unicode('app-default', envvar='APP_VALUE', config=True)
    skip_env = False

    def initialize(self, argv=None):
        super().initialize(argv)
        self.load_config_file(__file__, skip_env=self.skip_env)
        self.foo = Foo(parent=self)

    def start(self):
        print('app value', self.value)
        print('foo value', self.foo.value)


if __name__ == '__main__':
    argv = ['--MyApp.value=app-cli', '--Foo.value=foo-cli']
    for skip_env in (True, False):
        MyApp.skip_env = skip_env
        print("no cli (skip_env=%s)" % skip_env)
        MyApp.launch_instance([])
        print('cli args and env, cli should win (skip_env=%s)' % skip_env)
        MyApp.launch_instance(argv)
else:
    # I'm my own config file!
    try:
        c = get_config()
    except NameError:
        pass
    else:
        # I'm a config file!
        c.MyApp.value = 'app_cfg_file'
        c.Foo.value = 'app_foo_file'

Results with ipython/traitlets@6f05aad

no cli (skip_env=True):
app value app_cfg_file # correct
foo value foo-from-env # incorrect
cli args and env, cli should win (skip_env=True):
app value app-cli # correct
foo value foo-from-env # incorrect
no cli (skip_env=False):
app value app-from-env # correct
foo value foo-from-env # correct
cli args and env, cli should win (skip_env=False):
app value app-from-env # incorrect
foo value foo-from-env # incorrect