'Create a Credentials object by reading information from a file.
It returns an object of type GoogleCredentials.
Args:
credential_filename: the path to the file from where the
credentials are to be read
Raises:
ApplicationDefaultCredentialsError: raised when the credentials
fail to be retrieved.'

@staticmethod
def from_stream(credential_filename):

    if (credential_filename and os.path.isfile(credential_filename)):
        try:
            return _get_application_default_credential_from_file(credential_filename)
        except (ApplicationDefaultCredentialsError, ValueError) as error:
            extra_help = '    (provided    as    parameter    to    the    from_stream()    method)'
            _raise_exception_for_reading_json(credential_filename, extra_help, error)
    else:
        raise ApplicationDefaultCredentialsError('The    parameter    passed    to    the    from_stream()    method    should    point    to    a    file.')
