对于④,python isdigit()为True,但int()失败

Question

和④有什么区别？

>>> u''.isdigit()
True
>>> int(u'')
4
>>> u'?'.isdigit()
True
>> int(u'?')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '?'

注意:上面是在python3上输出的.在python last上给出了这个异常:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'decimal' codec can't encode character u'\u2463' in position 0: invalid decimal Unicode string