Converts a string argument to a unicode string.

def to_unicode(value, encoding=None):
    """Converts a string argument to a unicode string.
    """
    if isinstance(value, text_type):
        return value
    elif isinstance(value, type(None)):
        return text_type(value)

    if not isinstance(value, bytes):
        raise TypeError(
            "Expected bytes, unicode, or None; got %r" % type(value)
        )

    if encoding is not None:
        return value.decode(encoding)
    else:
        try:
            value = value.decode('utf-8')
        except:
            try:
                value = value.decode('gbk')
            except:
                pass

        return value