详解Python对JSON中的特殊类型进行Encoder
Python 处理 JSON 数据时,dumps 函数是经常用到的,当 JSON 数据中有特殊类型时,往往是比较头疼的,因为经常会报这样一个错误。
自定义编码类
#!/usr/bin/env python # -*- coding:utf-8 -*- # Author: wxnacy(wxnacy@gmail.com) import json from datetime import datetime USER_DATA = dict( id = 1, name = 'wxnacy', ts = datetime.now() ) print(json.dumps(USER_DATA))
Traceback (most recent call last): File "/Users/wxnacy/PycharmProjects/study/python/office_module/json_demo/dumps.py", line 74, in <module> dumps_encoder() File "/Users/wxnacy/PycharmProjects/study/python/office_module/json_demo/dumps.py", line 68, in dumps_encoder print(json.dumps(USER_DATA)) File "/Users/wxnacy/.pyenv/versions/3.6.0/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 231, in dumps return _default_encoder.encode(obj) File "/Users/wxnacy/.pyenv/versions/3.6.0/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 199, in encode chunks = self.iterencode(o, _one_shot=True) File "/Users/wxnacy/.pyenv/versions/3.6.0/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 257, in iterencode return _iterencode(o, 0) File "/Users/wxnacy/.pyenv/versions/3.6.0/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 180, in default o.__class__.__name__) TypeError: Object of type 'datetime' is not JSON serializable
原因在于 dumps 函数不知道如何处理 datetime 对象,默认情况下 json 模块使用 json.JSONEncoder 类来进行编码,此时我们需要自定义一下编码类。
#!/usr/bin/env python # -*- coding:utf-8 -*- # Author: wxnacy(wxnacy@gmail.com) class CustomEncoder(json.JSONEncoder): def default(self, x): if isinstance(x, datetime): return int(x.timestamp()) return super().default(self, x)
定义编码类 CustomEncoder 并重写实例的 default 函数,对特殊类型进行处理,其余类型继续使用父类的解析。
#!/usr/bin/env python # -*- coding:utf-8 -*- # Author: wxnacy(wxnacy@gmail.com) import json from datetime import datetime class CustomEncoder(json.JSONEncoder): def default(self, x): if isinstance(x, datetime): return int(x.timestamp()) return super().default(self, x) USER_DATA = dict( id = 1, name = 'wxnacy', ts = datetime.now() ) print(json.dumps(USER_DATA, cls=CustomEncoder)) # {"id": 1, "name": "wxnacy", "ts": 1562938926}
最后整合起来,将类使用 cls 参数传入 dumps 函数即可。
使用 CustomEncoder 实例的 encode 函数可以对对象进行转码
#!/usr/bin/env python # -*- coding:utf-8 -*- # Author: wxnacy(wxnacy@gmail.com) print(CustomEncoder().encode(datetime.now())) # 1562939035
在父类源码中,所有的编码逻辑都在 encode 函数中, default 只负责抛出 TypeError 异常,这就是文章开始报错的出处。
#!/usr/bin/env python # -*- coding:utf-8 -*- # Author: wxnacy(wxnacy@gmail.com) def default(self, o): """Implement this method in a subclass such that it returns a serializable object for ``o``, or calls the base implementation (to raise a ``TypeError``). For example, to support arbitrary iterators, you could implement default like this:: def default(self, o): try: iterable = iter(o) except TypeError: pass else: return list(iterable) # Let the base class default method raise the TypeError return JSONEncoder.default(self, o) """ raise TypeError(f'Object of type {o.__class__.__name__} ' f'is not JSON serializable') def encode(self, o): """Return a JSON string representation of a Python data structure. >>> from json.encoder import JSONEncoder >>> JSONEncoder().encode({"foo": ["bar", "baz"]}) '{"foo": ["bar", "baz"]}' """ # This is for extremely simple cases and benchmarks. if isinstance(o, str): if self.ensure_ascii: return encode_basestring_ascii(o) else: return encode_basestring(o) # This doesn't pass the iterator directly to ''.join() because the # exceptions aren't as detailed. The list call should be roughly # equivalent to the PySequence_Fast that ''.join() would do. chunks = self.iterencode(o, _one_shot=True) if not isinstance(chunks, (list, tuple)): chunks = list(chunks) return ''.join(chunks)
单分派装饰器处理对象
CustomEncoder 如果处理的对象种类很多的话,需要写多个 if elif else 来区分,这样并不是不行,但是不够优雅,不够 pythonic
根据对象的类型不同,而做出不同的处理。刚好有个装饰器可以做到这点,它就是单分派函数 functools.singledispatch
#!/usr/bin/env python # -*- coding:utf-8 -*- # Author: wxnacy(wxnacy@gmail.com) from datetime import datetime from datetime import date from functools import singledispatch class CustomEncoder(json.JSONEncoder): def default(self, x): try: return encode(x) except TypeError: return super().default(self, x) @singledispatch # 1 def encode(x): raise TypeError('Unencode type') @encode.register(datetime) # 2 def _(x): return int(x.timestamp()) @encode.register(date) def _(x): return x.isoformat() print(json.dumps(dict(dt = datetime.now(), d = date.today()), cls=CustomEncoder)) # {"dt": 1562940781, "d": "2019-07-12"}
1 使用 @singledispatch 装饰 encode 函数,是他处理默认类型。同时给他添加一个装饰器构造函数变量。
2 `@encode.register () 是一个装饰器构造函数,接收需要处理的对象类型作为参数。用它装饰的函数不需要名字, _` 代替即可。
最后提一点, json 也可以在命令行中使用
$ echo '{"json": "obj"}' | python -m json.tool { "json": "obj" }
参考链接
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