'Attempts to get implicit credentials from local credential files.
First checks if the environment variable GOOGLE_APPLICATION_CREDENTIALS
is set with a filename and then falls back to a configuration file (the
"well known" file) associated with the \'gcloud\' command line tool.
Returns:
Credentials object associated with the GOOGLE_APPLICATION_CREDENTIALS
file or the "well known" file if either exist. If neither file is
define, returns None, indicating no credentials from a file can
detected from the current environment.'

@staticmethod
def _implicit_credentials_from_files():

    credentials_filename = _get_environment_variable_file()
    if (not credentials_filename):
        credentials_filename = _get_well_known_file()
        if os.path.isfile(credentials_filename):
            extra_help = '    (produced    automatically    when    running    "gcloud    auth    login"    command)'
        else:
            credentials_filename = None
    else:
        extra_help = (('    (pointed    to    by    ' + GOOGLE_APPLICATION_CREDENTIALS) + '    environment    variable)')
    if (not credentials_filename):
        return
    SETTINGS.env_name = DEFAULT_ENV_NAME
    try:
        return _get_application_default_credential_from_file(credentials_filename)
    except (ApplicationDefaultCredentialsError, ValueError) as error:
        _raise_exception_for_reading_json(credentials_filename, extra_help, error)
