为什么JSONEncoder无法用于namedtuple? [英] Why doesn't JSONEncoder work for namedtuples?

问题描述

我无法转储collections.namedtuple作为正确的JSON.

I am unable to to dump collections.namedtuple as correct JSON.

第一 ，请考虑使用自定义JSON序列化程序的官方示例:

import json

class ComplexEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, complex):
            return [obj.real, obj.imag]
        # Let the base class default method raise the TypeError
        return json.JSONEncoder.default(self, obj)

json.dumps(2 + 1j, cls=ComplexEncoder)   # works great, without a doubt

第二 ，现在考虑以下示例，该示例告诉Python如何对Friend对象进行JSON化:

Second, now consider the following example which tells Python how to JSONize a Friend object:

import json

class Friend():
    """ struct-like, for storing state details of a friend """
    def __init__(self, _id, f_name, l_name):
        self._id = _id
        self.f_name = f_name
        self.l_name = l_name

t = Friend(21, 'Steve', 'Rogerson')

class FriendEncoder(json.JSONEncoder):
    """ take a Friend object and make it truly json """
    def default(self, aFriend):
        if isinstance(aFriend, Friend):
            return {
                "id": aFriend._id,
                "f_name": aFriend.f_name,
                "l_name": aFriend.l_name,
            }
        return super(FriendEncoder, self).default(aFriend)

json.dumps(t, cls=FriendEncoder) # returns correctly JSONized string

最后 ，当我们尝试使用namedtuples实现同一事物时，json.dumps(t, cls=FriendEncoder)不会给出任何错误，但是给出错误的输出.看看:

Finally when we try to implement the same thing using namedtuples, json.dumps(t, cls=FriendEncoder) doesn't give any errors but gives the wrong output. Take a look:

import pdb
import json
from collections import namedtuple

Friend = namedtuple("Friend", ["id", 'f_name', 'l_name'])

t = Friend(21, 'Steve', 'Rogerson')

print(t)

class FriendEncoder(json.JSONEncoder):
    """ take a Friend collections.namedtuple object and make it truly json """
    def default(self, obj):
        if True:    # if isinstance(obj, Friend):
            ans = dict(obj._asdict())
            pdb.set_trace()     # WOW!! even after commenting out the if and hardcoding True, debugger doesn't get called
            return ans
        return json.JSONEncoder.default(self, obj)

json.dumps(t, cls=FriendEncoder)

我得到的输出不是类似dict的，而是一个值列表，即[21, 'Steve', 'Rogerson']

The output I get is not a dict-like but rather just a list of values i.e. [21, 'Steve', 'Rogerson']

默认行为是信息丢失吗?
json.dumps是否忽略显式传递的编码器?

Is the default behavior such that information is lost?
Does json.dumps ignore the explicitly passed encoder?

通过正确的json化namedtuple，我的意思是json.dumps应该返回 exactly dict(nt._asdict())之类的数据，其中nt是预定义的namedtuple

by correctly jsonized namedtuple I mean that json.dumps should return data like exactly dict(nt._asdict()), where nt is a pre defined namedtuple

推荐答案

正如我在评论中所说，json.JSONEncoder仅在遇到对象类型时才调用default，它不知道如何序列化自身.在表中， c8>文档.这是它的屏幕截图，以方便参考:

As I said in a comment, the json.JSONEncoder only calls default when it encounters an object type it doesn't already know how to serialize itself. There's a table of them in the json documentation. Here's a screenshot of it for easy reference:

请注意，tuple在列表中，并且由于namedtuple是tuple的子类，因此它也适用于它们. (即因为isinstance(friend_instance, tuple)→True).

Note that tuple is on the list, and since namedtuple is a subclasses of tuple, so it applies to them, too. (i.e. because isinstance(friend_instance, tuple) → True).

这就是为什么从未调用用于处理Friend类实例的代码的原因.

This is why your code for handling instances of the Friend class never gets called.

以下是一种解决方法-即通过创建一个简单的Wrapper类，该类的实例不会是json.JSONEncoder认为已经知道如何处理的类型，然后指定一个default=关键字自变量函数，遇到某个对象尚不知道如何操作时将调用该函数.

Below is one workaround — namely by creating a simple Wrapper class whose instances won't be a type that the json.JSONEncoder thinks it already knows how to handle, and then specifying a default= keyword argument function that to be called whenever an object is encountered that it doesn't already know how to do.

这是我的意思:

import json
from collections import namedtuple

class Wrapper(object):
    """ Container class for objects with an _asdict() method. """
    def __init__(self, obj):
        assert hasattr(obj, '_asdict'), 'Cannot wrap object with no _asdict method'
        self.obj = obj


if __name__ == '__main__':

    Friend = namedtuple("Friend", ["id", 'f_name', 'l_name'])
    t = Friend(21, 'Steve', 'Rogerson')
    print(t)
    print(json.dumps(t))
    print(json.dumps(Wrapper(t), default=lambda wrapper: wrapper.obj._asdict()))

输出:

Friend(id=21, f_name='Steve', l_name='Rogerson')
[21, "Steve", "Rogerson"]
{"id": 21, "f_name": "Steve", "l_name": "Rogerson"}

有关其他信息和见解，请查看我对有关问题

For some additional information and insights, also check out my answer to the related question Making object JSON serializable with regular encoder.

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