问题:将类实例序列化为JSON

我正在尝试创建类实例的JSON字符串表示形式并且遇到困难。假设该类的构建如下:

class testclass:
    value1 = "a"
    value2 = "b"

像这样对json.dumps进行调用:

t = testclass()
json.dumps(t)

失败了,并告诉我testclass不可JSON序列化。

TypeError: <__main__.testclass object at 0x000000000227A400> is not JSON serializable

我也尝试过使用pickle模块:

t = testclass()
print(pickle.dumps(t, pickle.HIGHEST_PROTOCOL))

它提供类实例信息,但不提供类实例的序列化内容。

b'\x80\x03c__main__\ntestclass\nq\x00)\x81q\x01}q\x02b.'

我究竟做错了什么?

I am trying to create a JSON string representation of a class instance and having difficulty. Let’s say the class is built like this:

class testclass:
    value1 = "a"
    value2 = "b"

A call to the json.dumps is made like this:

t = testclass()
json.dumps(t)

It is failing and telling me that the testclass is not JSON serializable.

TypeError: <__main__.testclass object at 0x000000000227A400> is not JSON serializable

I have also tried using the pickle module :

t = testclass()
print(pickle.dumps(t, pickle.HIGHEST_PROTOCOL))

And it gives class instance information but not a serialized content of the class instance.

b'\x80\x03c__main__\ntestclass\nq\x00)\x81q\x01}q\x02b.'

What am I doing wrong?


回答 0

基本问题是,JSON编码器json.dumps()默认仅知道如何序列化一组有限的对象类型(所有内置类型)。在此处列出:https//docs.python.org/3.3/library/json.html#encoders-and-decoders

一个好的解决方案是使您的类继承自该类JSONEncoder,然后实现该JSONEncoder.default()函数,并使该函数为您的类发出正确的JSON。

一个简单的解决方案是调用该实例json.dumps().__dict__成员。那是一个标准的Python dict,如果您的类很简单,它将是JSON可序列化的。

class Foo(object):
    def __init__(self):
        self.x = 1
        self.y = 2

foo = Foo()
s = json.dumps(foo) # raises TypeError with "is not JSON serializable"

s = json.dumps(foo.__dict__) # s set to: {"x":1, "y":2}

在此博客文章中讨论了上述方法:

使用__dict__将任意Python对象序列化为JSON

The basic problem is that the JSON encoder json.dumps() only knows how to serialize a limited set of object types by default, all built-in types. List here: https://docs.python.org/3.3/library/json.html#encoders-and-decoders

One good solution would be to make your class inherit from JSONEncoder and then implement the JSONEncoder.default() function, and make that function emit the correct JSON for your class.

A simple solution would be to call json.dumps() on the .__dict__ member of that instance. That is a standard Python dict and if your class is simple it will be JSON serializable.

class Foo(object):
    def __init__(self):
        self.x = 1
        self.y = 2

foo = Foo()
s = json.dumps(foo) # raises TypeError with "is not JSON serializable"

s = json.dumps(foo.__dict__) # s set to: {"x":1, "y":2}

The above approach is discussed in this blog posting:

Serializing arbitrary Python objects to JSON using __dict__


回答 1

您可以尝试一种对我有用的方法:

json.dumps()可以采用默认的可选参数,您可以在其中为未知类型指定自定义序列化函数,在我看来,

def serialize(obj):
    """JSON serializer for objects not serializable by default json code"""

    if isinstance(obj, date):
        serial = obj.isoformat()
        return serial

    if isinstance(obj, time):
        serial = obj.isoformat()
        return serial

    return obj.__dict__

前两个if用于日期和时间序列化,然后obj.__dict__返回任何其他对象。

最后的通话如下:

json.dumps(myObj, default=serialize)

当序列化一个集合并且不想__dict__显式地为每个对象调用时,这特别好。在这里,它会自动为您完成。

到目前为止,对我来说非常好,期待您的想法。

There’s one way that works great for me that you can try out:

json.dumps() can take an optional parameter default where you can specify a custom serializer function for unknown types, which in my case looks like

def serialize(obj):
    """JSON serializer for objects not serializable by default json code"""

    if isinstance(obj, date):
        serial = obj.isoformat()
        return serial

    if isinstance(obj, time):
        serial = obj.isoformat()
        return serial

    return obj.__dict__

First two ifs are for date and time serialization and then there is a obj.__dict__ returned for any other object.

the final call looks like:

json.dumps(myObj, default=serialize)

It’s especially good when you are serializing a collection and you don’t want to call __dict__ explicitly for every object. Here it’s done for you automatically.

So far worked so good for me, looking forward for your thoughts.


回答 2

您可以defaultjson.dumps()函数中指定命名参数:

json.dumps(obj, default=lambda x: x.__dict__)

说明:

形成文档(2.73.6):

``default(obj)`` is a function that should return a serializable version
of obj or raise TypeError. The default simply raises TypeError.

(适用于Python 2.7和Python 3.x)

注意:在这种情况下,您需要instance变量而不是class变量,如问题中的示例所示。(我假设询问者是class instance一个类的对象)

我首先从@phihag的答案中学到了这一点。发现它是最简单,最干净的方法。

You can specify the default named parameter in the json.dumps() function:

json.dumps(obj, default=lambda x: x.__dict__)

Explanation:

Form the docs (2.7, 3.6):

``default(obj)`` is a function that should return a serializable version
of obj or raise TypeError. The default simply raises TypeError.

(Works on Python 2.7 and Python 3.x)

Note: In this case you need instance variables and not class variables, as the example in the question tries to do. (I am assuming the asker meant class instance to be an object of a class)

I learned this first from @phihag’s answer here. Found it to be the simplest and cleanest way to do the job.


回答 3

我只是做:

data=json.dumps(myobject.__dict__)

这不是完整的答案,如果您有某种复杂的对象类,那么您肯定不会得到所有。但是,我将其用于一些简单的对象。

在OptionParser模块中获得的“ options”类确实非常有效。这里是JSON请求本身。

  def executeJson(self, url, options):
        data=json.dumps(options.__dict__)
        if options.verbose:
            print data
        headers = {'Content-type': 'application/json', 'Accept': 'text/plain'}
        return requests.post(url, data, headers=headers)

I just do:

data=json.dumps(myobject.__dict__)

This is not the full answer, and if you have some sort of complicated object class you certainly will not get everything. However I use this for some of my simple objects.

One that it works really well on is the “options” class that you get from the OptionParser module. Here it is along with the JSON request itself.

  def executeJson(self, url, options):
        data=json.dumps(options.__dict__)
        if options.verbose:
            print data
        headers = {'Content-type': 'application/json', 'Accept': 'text/plain'}
        return requests.post(url, data, headers=headers)

回答 4

使用jsonpickle

import jsonpickle

object = YourClass()
json_object = jsonpickle.encode(object)

Using jsonpickle

import jsonpickle

object = YourClass()
json_object = jsonpickle.encode(object)

回答 5

JSON并非真正意在序列化任意Python对象。这对于序列化dict对象pickle非常有用,但是实际上通常应该使用该模块。的输出pickle不是真正的人类可读的,但是应该可以使它顺畅运行。如果您坚持使用JSON,则可以签出jsonpickle模块,这是一种有趣的混合方法。

https://github.com/jsonpickle/jsonpickle

JSON is not really meant for serializing arbitrary Python objects. It’s great for serializing dict objects, but the pickle module is really what you should be using in general. Output from pickle is not really human-readable, but it should unpickle just fine. If you insist on using JSON, you could check out the jsonpickle module, which is an interesting hybrid approach.

https://github.com/jsonpickle/jsonpickle


回答 6

这是两个简单的函数,用于对任何不复杂的类进行序列化,没有任何花哨的地方。

我将其用于配置类型的东西,因为我可以在不进行代码调整的情况下将新成员添加到类中。

import json

class SimpleClass:
    def __init__(self, a=None, b=None, c=None):
        self.a = a
        self.b = b
        self.c = c

def serialize_json(instance=None, path=None):
    dt = {}
    dt.update(vars(instance))

    with open(path, "w") as file:
        json.dump(dt, file)

def deserialize_json(cls=None, path=None):
    def read_json(_path):
        with open(_path, "r") as file:
            return json.load(file)

    data = read_json(path)

    instance = object.__new__(cls)

    for key, value in data.items():
        setattr(instance, key, value)

    return instance

# Usage: Create class and serialize under Windows file system.
write_settings = SimpleClass(a=1, b=2, c=3)
serialize_json(write_settings, r"c:\temp\test.json")

# Read back and rehydrate.
read_settings = deserialize_json(SimpleClass, r"c:\temp\test.json")

# results are the same.
print(vars(write_settings))
print(vars(read_settings))

# output:
# {'c': 3, 'b': 2, 'a': 1}
# {'c': 3, 'b': 2, 'a': 1}

Here are two simple functions for serialization of any non-sophisticated classes, nothing fancy as explained before.

I use this for configuration type stuff because I can add new members to the classes with no code adjustments.

import json

class SimpleClass:
    def __init__(self, a=None, b=None, c=None):
        self.a = a
        self.b = b
        self.c = c

def serialize_json(instance=None, path=None):
    dt = {}
    dt.update(vars(instance))

    with open(path, "w") as file:
        json.dump(dt, file)

def deserialize_json(cls=None, path=None):
    def read_json(_path):
        with open(_path, "r") as file:
            return json.load(file)

    data = read_json(path)

    instance = object.__new__(cls)

    for key, value in data.items():
        setattr(instance, key, value)

    return instance

# Usage: Create class and serialize under Windows file system.
write_settings = SimpleClass(a=1, b=2, c=3)
serialize_json(write_settings, r"c:\temp\test.json")

# Read back and rehydrate.
read_settings = deserialize_json(SimpleClass, r"c:\temp\test.json")

# results are the same.
print(vars(write_settings))
print(vars(read_settings))

# output:
# {'c': 3, 'b': 2, 'a': 1}
# {'c': 3, 'b': 2, 'a': 1}

回答 7

关于如何开始执行此操作,有一些很好的答案。但是要记住一些事情:

  • 如果实例嵌套在大型数据结构中怎么办?
  • 如果还想要类的名称怎么办?
  • 如果要反序列化实例怎么办?
  • 如果您正在使用该怎么办 __slots__而不是__dict__呢?
  • 如果您只是不想自己做,该怎么办?

json-tricks是一个库(由我创建,其他人对此做出了贡献)已经有一段时间了。例如:

class MyTestCls:
    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)

cls_instance = MyTestCls(s='ub', dct={'7': 7})

json = dumps(cls_instance, indent=4)
instance = loads(json)

您将恢复实例。这里的json看起来像这样:

{
    "__instance_type__": [
        "json_tricks.test_class",
        "MyTestCls"
    ],
    "attributes": {
        "s": "ub",
        "dct": {
            "7": 7
        }
    }
}

如果您想提出自己的解决方案,请查看以下内容的来源 json-tricks以免忘记某些特殊情况(例如__slots__)。

它还执行其他类型,例如numpy数组,日期时间,复数;它还允许发表评论。

There are some good answers on how to get started on doing this. But there are some things to keep in mind:

  • What if the instance is nested inside a large data structure?
  • What if also want the class name?
  • What if you want to deserialize the instance?
  • What if you’re using __slots__ instead of __dict__?
  • What if you just don’t want to do it yourself?

json-tricks is a library (that I made and others contributed to) which has been able to do this for quite a while. For example:

class MyTestCls:
    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)

cls_instance = MyTestCls(s='ub', dct={'7': 7})

json = dumps(cls_instance, indent=4)
instance = loads(json)

You’ll get your instance back. Here the json looks like this:

{
    "__instance_type__": [
        "json_tricks.test_class",
        "MyTestCls"
    ],
    "attributes": {
        "s": "ub",
        "dct": {
            "7": 7
        }
    }
}

If you like to make your own solution, you might look at the source of json-tricks so as not to forget some special cases (like __slots__).

It also does other types like numpy arrays, datetimes, complex numbers; it also allows for comments.


回答 8

Python3.x

我所能达到的最好的方法就是这个。
注意,此代码也对待set()。
这种方法是通用的,只需要扩展类(在第二个示例中)。
请注意,我只是在处理文件,但是很容易根据自己的喜好修改行为。

但是,这是CoDec。

通过更多的工作,您可以用其他方式构造您的类。我假定使用默认的构造函数来实例化它,然后更新类dict。

import json
import collections


class JsonClassSerializable(json.JSONEncoder):

    REGISTERED_CLASS = {}

    def register(ctype):
        JsonClassSerializable.REGISTERED_CLASS[ctype.__name__] = ctype

    def default(self, obj):
        if isinstance(obj, collections.Set):
            return dict(_set_object=list(obj))
        if isinstance(obj, JsonClassSerializable):
            jclass = {}
            jclass["name"] = type(obj).__name__
            jclass["dict"] = obj.__dict__
            return dict(_class_object=jclass)
        else:
            return json.JSONEncoder.default(self, obj)

    def json_to_class(self, dct):
        if '_set_object' in dct:
            return set(dct['_set_object'])
        elif '_class_object' in dct:
            cclass = dct['_class_object']
            cclass_name = cclass["name"]
            if cclass_name not in self.REGISTERED_CLASS:
                raise RuntimeError(
                    "Class {} not registered in JSON Parser"
                    .format(cclass["name"])
                )
            instance = self.REGISTERED_CLASS[cclass_name]()
            instance.__dict__ = cclass["dict"]
            return instance
        return dct

    def encode_(self, file):
        with open(file, 'w') as outfile:
            json.dump(
                self.__dict__, outfile,
                cls=JsonClassSerializable,
                indent=4,
                sort_keys=True
            )

    def decode_(self, file):
        try:
            with open(file, 'r') as infile:
                self.__dict__ = json.load(
                    infile,
                    object_hook=self.json_to_class
                )
        except FileNotFoundError:
            print("Persistence load failed "
                  "'{}' do not exists".format(file)
                  )


class C(JsonClassSerializable):

    def __init__(self):
        self.mill = "s"


JsonClassSerializable.register(C)


class B(JsonClassSerializable):

    def __init__(self):
        self.a = 1230
        self.c = C()


JsonClassSerializable.register(B)


class A(JsonClassSerializable):

    def __init__(self):
        self.a = 1
        self.b = {1, 2}
        self.c = B()

JsonClassSerializable.register(A)

A().encode_("test")
b = A()
b.decode_("test")
print(b.a)
print(b.b)
print(b.c.a)

编辑

通过更多的研究,我找到了一种使用元类进行泛化而无需SUPERCLASS寄存器方法调用的方法。

import json
import collections

REGISTERED_CLASS = {}

class MetaSerializable(type):

    def __call__(cls, *args, **kwargs):
        if cls.__name__ not in REGISTERED_CLASS:
            REGISTERED_CLASS[cls.__name__] = cls
        return super(MetaSerializable, cls).__call__(*args, **kwargs)


class JsonClassSerializable(json.JSONEncoder, metaclass=MetaSerializable):

    def default(self, obj):
        if isinstance(obj, collections.Set):
            return dict(_set_object=list(obj))
        if isinstance(obj, JsonClassSerializable):
            jclass = {}
            jclass["name"] = type(obj).__name__
            jclass["dict"] = obj.__dict__
            return dict(_class_object=jclass)
        else:
            return json.JSONEncoder.default(self, obj)

    def json_to_class(self, dct):
        if '_set_object' in dct:
            return set(dct['_set_object'])
        elif '_class_object' in dct:
            cclass = dct['_class_object']
            cclass_name = cclass["name"]
            if cclass_name not in REGISTERED_CLASS:
                raise RuntimeError(
                    "Class {} not registered in JSON Parser"
                    .format(cclass["name"])
                )
            instance = REGISTERED_CLASS[cclass_name]()
            instance.__dict__ = cclass["dict"]
            return instance
        return dct

    def encode_(self, file):
        with open(file, 'w') as outfile:
            json.dump(
                self.__dict__, outfile,
                cls=JsonClassSerializable,
                indent=4,
                sort_keys=True
            )

    def decode_(self, file):
        try:
            with open(file, 'r') as infile:
                self.__dict__ = json.load(
                    infile,
                    object_hook=self.json_to_class
                )
        except FileNotFoundError:
            print("Persistence load failed "
                  "'{}' do not exists".format(file)
                  )


class C(JsonClassSerializable):

    def __init__(self):
        self.mill = "s"


class B(JsonClassSerializable):

    def __init__(self):
        self.a = 1230
        self.c = C()


class A(JsonClassSerializable):

    def __init__(self):
        self.a = 1
        self.b = {1, 2}
        self.c = B()


A().encode_("test")
b = A()
b.decode_("test")
print(b.a)
# 1
print(b.b)
# {1, 2}
print(b.c.a)
# 1230
print(b.c.c.mill)
# s

Python3.x

The best aproach I could reach with my knowledge was this.
Note that this code treat set() too.
This approach is generic just needing the extension of class (in the second example).
Note that I’m just doing it to files, but it’s easy to modify the behavior to your taste.

However this is a CoDec.

With a little more work you can construct your class in other ways. I assume a default constructor to instance it, then I update the class dict.

import json
import collections


class JsonClassSerializable(json.JSONEncoder):

    REGISTERED_CLASS = {}

    def register(ctype):
        JsonClassSerializable.REGISTERED_CLASS[ctype.__name__] = ctype

    def default(self, obj):
        if isinstance(obj, collections.Set):
            return dict(_set_object=list(obj))
        if isinstance(obj, JsonClassSerializable):
            jclass = {}
            jclass["name"] = type(obj).__name__
            jclass["dict"] = obj.__dict__
            return dict(_class_object=jclass)
        else:
            return json.JSONEncoder.default(self, obj)

    def json_to_class(self, dct):
        if '_set_object' in dct:
            return set(dct['_set_object'])
        elif '_class_object' in dct:
            cclass = dct['_class_object']
            cclass_name = cclass["name"]
            if cclass_name not in self.REGISTERED_CLASS:
                raise RuntimeError(
                    "Class {} not registered in JSON Parser"
                    .format(cclass["name"])
                )
            instance = self.REGISTERED_CLASS[cclass_name]()
            instance.__dict__ = cclass["dict"]
            return instance
        return dct

    def encode_(self, file):
        with open(file, 'w') as outfile:
            json.dump(
                self.__dict__, outfile,
                cls=JsonClassSerializable,
                indent=4,
                sort_keys=True
            )

    def decode_(self, file):
        try:
            with open(file, 'r') as infile:
                self.__dict__ = json.load(
                    infile,
                    object_hook=self.json_to_class
                )
        except FileNotFoundError:
            print("Persistence load failed "
                  "'{}' do not exists".format(file)
                  )


class C(JsonClassSerializable):

    def __init__(self):
        self.mill = "s"


JsonClassSerializable.register(C)


class B(JsonClassSerializable):

    def __init__(self):
        self.a = 1230
        self.c = C()


JsonClassSerializable.register(B)


class A(JsonClassSerializable):

    def __init__(self):
        self.a = 1
        self.b = {1, 2}
        self.c = B()

JsonClassSerializable.register(A)

A().encode_("test")
b = A()
b.decode_("test")
print(b.a)
print(b.b)
print(b.c.a)

Edit

With some more of research I found a way to generalize without the need of the SUPERCLASS register method call, using a metaclass

import json
import collections

REGISTERED_CLASS = {}

class MetaSerializable(type):

    def __call__(cls, *args, **kwargs):
        if cls.__name__ not in REGISTERED_CLASS:
            REGISTERED_CLASS[cls.__name__] = cls
        return super(MetaSerializable, cls).__call__(*args, **kwargs)


class JsonClassSerializable(json.JSONEncoder, metaclass=MetaSerializable):

    def default(self, obj):
        if isinstance(obj, collections.Set):
            return dict(_set_object=list(obj))
        if isinstance(obj, JsonClassSerializable):
            jclass = {}
            jclass["name"] = type(obj).__name__
            jclass["dict"] = obj.__dict__
            return dict(_class_object=jclass)
        else:
            return json.JSONEncoder.default(self, obj)

    def json_to_class(self, dct):
        if '_set_object' in dct:
            return set(dct['_set_object'])
        elif '_class_object' in dct:
            cclass = dct['_class_object']
            cclass_name = cclass["name"]
            if cclass_name not in REGISTERED_CLASS:
                raise RuntimeError(
                    "Class {} not registered in JSON Parser"
                    .format(cclass["name"])
                )
            instance = REGISTERED_CLASS[cclass_name]()
            instance.__dict__ = cclass["dict"]
            return instance
        return dct

    def encode_(self, file):
        with open(file, 'w') as outfile:
            json.dump(
                self.__dict__, outfile,
                cls=JsonClassSerializable,
                indent=4,
                sort_keys=True
            )

    def decode_(self, file):
        try:
            with open(file, 'r') as infile:
                self.__dict__ = json.load(
                    infile,
                    object_hook=self.json_to_class
                )
        except FileNotFoundError:
            print("Persistence load failed "
                  "'{}' do not exists".format(file)
                  )


class C(JsonClassSerializable):

    def __init__(self):
        self.mill = "s"


class B(JsonClassSerializable):

    def __init__(self):
        self.a = 1230
        self.c = C()


class A(JsonClassSerializable):

    def __init__(self):
        self.a = 1
        self.b = {1, 2}
        self.c = B()


A().encode_("test")
b = A()
b.decode_("test")
print(b.a)
# 1
print(b.b)
# {1, 2}
print(b.c.a)
# 1230
print(b.c.c.mill)
# s

回答 9

我相信与其采用公认的答案所建议的继​​承,不如使用多态性更好。否则,必须有一个很大的if else语句才能自定义每个对象的编码。这意味着为JSON创建通用的默认编码器,如下所示:

def jsonDefEncoder(obj):
   if hasattr(obj, 'jsonEnc'):
      return obj.jsonEnc()
   else: #some default behavior
      return obj.__dict__

然后jsonEnc()在要序列化的每个类中都有一个函数。例如

class A(object):
   def __init__(self,lengthInFeet):
      self.lengthInFeet=lengthInFeet
   def jsonEnc(self):
      return {'lengthInMeters': lengthInFeet * 0.3 } # each foot is 0.3 meter

然后你打电话 json.dumps(classInstance,default=jsonDefEncoder)

I believe instead of inheritance as suggested in accepted answer, it’s better to use polymorphism. Otherwise you have to have a big if else statement to customize encoding of every object. That means create a generic default encoder for JSON as:

def jsonDefEncoder(obj):
   if hasattr(obj, 'jsonEnc'):
      return obj.jsonEnc()
   else: #some default behavior
      return obj.__dict__

and then have a jsonEnc() function in each class you want to serialize. e.g.

class A(object):
   def __init__(self,lengthInFeet):
      self.lengthInFeet=lengthInFeet
   def jsonEnc(self):
      return {'lengthInMeters': lengthInFeet * 0.3 } # each foot is 0.3 meter

Then you call json.dumps(classInstance,default=jsonDefEncoder)


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