python2.7 __str__ cause UnicodeEncodeError when print

Question

I tried to rewrite my class's str method, this is my code:

# encoding: utf8
import json


class Test(object):

    def __str__(self):
        d = {
            'foo': u'中文'
        }

        return json.dumps(d, ensure_ascii=False)


test = Test()
print test.__str__()
print test

what make me confused is that print test.__str__() works fine, but print test cause exception:

Traceback (most recent call last):
  File "test.py", line 17, in <module>
    print(test)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 9-10: ordinal not in range(128)
python test.py  0.05s user 0.03s system 96% cpu 0.078 total

python version 2.7.12

Answer 1

__str__ must return a string value. If you return a unicode object instead, it'll be automatically encoded as ASCII.

Encode explicitly instead:

class Test(object):
    def __str__(self):
        d = {
            'foo': u'中文'
        }
        return json.dumps(d, ensure_ascii=False).encode('utf8')

Personally, I'd not use the __str__ method to provide a JSON encoding. Pick a different method name instead, like tojson() .