Python:为什么__str__不能调用递归？

Question

将dict转换为JSON对象时遇到一些麻烦.我有这门课:

class ServerResponse(object):

    status = None
    code = None
    message = None
    data = None

    OK_STATUS = True
    ERROR_STATUS = False

    OK_CODE = 200

    def __init__(self, status=OK_STATUS, code=OK_CODE, message=None, data=None, *args, **kwargs):
        self.status = status
        self.code = code
        self.message = message
        self.data = data

    def to_dict(self):
        fields = {
            "status": self.status,
            "code": self.code,
            "message": self.message,
            "data": str(self.data),
        }

        return fields

    def to_json(self):
        return json.dumps(self.to_dict())

    def __str__(self):
        return self.to_json()

我用这个类来生成服务器答案.

from server_response import ServerResponse as Response
...
return_data = {}

for (name, content) in result.items():
    if not previous_hashes or client.is_data_change(previous_hashes[name], data['hash']):
        return_data[name] = Response(data=content)
    else:
        return_data[name] = Response(code=201, message="Data has not changed")

response = Response(data=return_data)
...
self.write(str(response))

从服务器回答我得到下一个JSON

{u'status': True, u'message': None, u'code': 200, u'data': u"{'client': <server_response.ServerResponse object at 0x14e9710>, 'service': <server_response.ServerResponse object at 0x14e90d0>}"}

为什么__str__函数不递归调用？

Answer 1

从这个程序：

class Foo(object):
    def __repr__(self):
        return "REPR"

    def __str__(self):
        return "STR"

x = {}

x['client'] = Foo()

print str(x)
print repr(x)

您可以看到 dict 总是对其成员调用repr，无论 dict 上是否使用了 str 或 repr。