Why

In [150]: d = u'\xe7\xac\x91\xe5\x8f\xa3\xe5\xb8\xb8\xe5\xbc\x80'

In [151]: d
Out[151]: u'\xe7\xac\x91\xe5\x8f\xa3\xe5\xb8\xb8\xe5\xbc\x80'

In [152]: d.decode('string_escape')
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-152-0f9f72da24b9> in <module>()
----> 1 d.decode('string_escape')

UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-11: ordinal not in range(128)

In [153]: d.encode("unicode_escape").decode('string_escape')
Out[153]: '\xe7\xac\x91\xe5\x8f\xa3\xe5\xb8\xb8\xe5\xbc\x80'

In [154]: d.encode("unicode_escape").decode('string_escape').decode('utf-8')
     ...:
Out[154]: u'\u7b11\u53e3\u5e38\u5f00'

In [155]: print d.encode("unicode_escape").decode('string_escape').decode('u
     ...: tf-8')
笑口常开