Setting GOOGLE_APPLICATION_CREDENTIALS to empty string throws an error

[tswast]

I would expect an empty environment variable to act identically to an unset environment variable.

Reported by @parthea

If I set my GOOGLE_APPLICATION_CREDENTIALS environment variable to an empty string, I receive an exception instead of the local user web authentication flow. Is it expected?

tony@tonypc:~/pydata-pandas-gbq/pandas_gbq/tests$ export GOOGLE_APPLICATION_CREDENTIALS=''
tony@tonypc:~/pydata-pandas-gbq/pandas_gbq/tests$ pytest test_gbq.py::TestGBQConnectorIntegrationWithLocalUserAccountAuth -v -r s . --maxfail=1
============================= test session starts ==============================
platform linux2 -- Python 2.7.12, pytest-3.0.6, py-1.4.32, pluggy-0.4.0 -- /home/tony/miniconda2/bin/python
cachedir: ../../.cache
rootdir: /home/tony/pydata-pandas-gbq, inifile: 
plugins: cov-2.4.0
collected 100 items 

test_gbq.py::TestGBQConnectorIntegrationWithLocalUserAccountAuth::test_should_be_able_to_make_a_connector ERROR

==================================== ERRORS ====================================
 ERROR at setup of TestGBQConnectorIntegrationWithLocalUserAccountAuth.test_should_be_able_to_make_a_connector 

self = <pandas_gbq.tests.test_gbq.TestGBQConnectorIntegrationWithLocalUserAccountAuth object at 0x7f2bbecf6490>
method = <bound method TestGBQConnectorIntegrationWithLocalUserAccountAuth.test_should_....TestGBQConnectorIntegrationWithLocalUserAccountAuth object at 0x7f2bbecf6490>>

    def setup_method(self, method):
        _setup_common()
        _skip_if_no_project_id()
        _skip_local_auth_if_in_travis_env()
    
>       self.sut = gbq.GbqConnector(_get_project_id(), auth_local_webserver=True)

test_gbq.py:268: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../gbq.py:174: in __init__
    auth_local_webserver=auth_local_webserver)
../gbq.py:182: in get_credentials
    credentials = self.get_application_default_credentials()
../gbq.py:212: in get_application_default_credentials
    credentials, _ = google.auth.default(scopes=[self.scope])
../../../miniconda2/lib/python2.7/site-packages/google/auth/_default.py:277: in default
    credentials, project_id = checker()
../../../miniconda2/lib/python2.7/site-packages/google/auth/_default.py:138: in _get_explicit_environ_credentials
    os.environ[environment_vars.CREDENTIALS])
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

filename = ''

    def _load_credentials_from_file(filename):
        """Loads credentials from a file.
    
        The credentials file must be a service account key or stored authorized
        user credentials.
    
        Args:
            filename (str): The full path to the credentials file.
    
        Returns:
            Tuple[google.auth.credentials.Credentials, Optional[str]]: Loaded
                credentials and the project ID. Authorized user credentials do not
                have the project ID information.
    
        Raises:
            google.auth.exceptions.DefaultCredentialsError: if the file is in the
                wrong format.
        """
>       with io.open(filename, 'r') as file_obj:
E       IOError: [Errno 2] No such file or directory: ''

../../../miniconda2/lib/python2.7/site-packages/google/auth/_default.py:63: IOError
!!!!!!!!!!!!!!!!!!!! Interrupted: stopping after 1 failures !!!!!!!!!!!!!!!!!!!!
=========================== 1 error in 0.61 seconds ============================

[dhermes]

I'm inclined to say this is "working as intended", but maybe I'm missing some subtlety. Why can't they just unset / never set the env. var. in their environment?

[tswast]

Do we want to be throwing an IOError in this case or a DefaultCredentialsError? If an IOError, we should document that in the .default() docstring.

[theacodes]

I think it should probs be a DefaultCredentialsError with File not found.