Python将对象列表序列化为JSON

问题描述

我正在尝试将对象的__dict__序列化为JSON，该方法工作正常，直到将对象追加到第一个对象的instance属性之一为止.

I am trying to serialize to JSON the __dict__ of an object, which is working fine, until I append objects to one of the instance attribute of my first object:

from json import dumps

class A(object):
    def __init__(self):
        self.b_list = []

class B(object):
    def __init__(self):
        self.x = 'X'
        self.y = 'Y'
    def __repr__(self):
        return dumps(self.__dict__)

a = A()

print dumps(a.__dict__)  # works fine

a.b_list.append(B())

print dumps(a.__dict__)

第二次打电话给dumps时，我得到了以下TypeError:

When calling for the second time dumps, I got the following TypeError:

TypeError: {"y": "Y", "x": "X"} is not JSON serializable

我不明白为什么为什么一直收到此错误，却看不到为什么无法序列化为JSON.

I don't understand why I keep getting this error while I can't see why this is not serializable to JSON.

推荐答案

这是因为B的实例不是简单类型.因为您为B提供了__repr__方法，所以实例被打印为 printed (因为它是JSON表示形式)，但它本身并不是受支持的JSON类型.

That's because instances of B are not a simple type. Because you gave B a __repr__ method, the instance is printed as it's JSON representation, but it is not itself a supported JSON type.

删除__repr__方法，回溯就不那么混乱了:

Remove the __repr__ method and the traceback is much less confusing:

>>> class A(object):
...     def __init__(self):
...         self.b_list = []
...
>>> class B(object):
...     def __init__(self):
...         self.x = 'X'
...         self.y = 'Y'
...
>>> a = A()
>>> a.b_list.append(B())
>>>
>>> print dumps(a.__dict__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/__init__.py", line 243, in dumps
    return _default_encoder.encode(obj)
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: <__main__.B object at 0x10a753e10> is not JSON serializable

使用default关键字参数对自定义对象进行编码:

Use the default keyword argument to encode custom objects:

def encode_b(obj):
    if isinstance(obj, B):
        return obj.__dict__
    return obj

json.dumps(a, default=encode_b)

演示:

>>> def encode_b(obj):
...     if isinstance(obj, B):
...         return obj.__dict__
...     return obj
...
>>> dumps(a.__dict__, default=encode_b)
'{"b_list": [{"y": "Y", "x": "X"}]}'

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