Return a single or double quote, if a string is wrapped in extra quotes.
    Otherwise return an empty string.

def is_quoted(value):
    '''
    Return a single or double quote, if a string is wrapped in extra quotes.
    Otherwise return an empty string.
    '''
    ret = ''
    if isinstance(value, six.string_types) \
            and value[0] == value[-1] \
            and value.startswith(('\'', '"')):
        ret = value[0]
    return ret