'Retrieve Credential from file.
Returns:
oauth2client.client.Credentials'

def locked_get(self):

    credentials = None
    try:
        f = open(self._filename, 'rb')
        content = f.read()
        f.close()
    except IOError:
        return credentials
    try:
        credentials = Credentials.new_from_json(content)
        credentials.set_store(self)
    except ValueError:
        pass
    return credentials
