使用标准 json 模块格式化浮点数

2022-01-09 00:00:00 python json floating-point formatting

问题描述

我在 python 2.6 中使用标准 json 模块 来序列化浮点列表.但是，我得到这样的结果:

I am using the standard json module in python 2.6 to serialize a list of floats. However, I'm getting results like this:

>>> import json
>>> json.dumps([23.67, 23.97, 23.87])
'[23.670000000000002, 23.969999999999999, 23.870000000000001]'

我希望浮点数的格式只有两位小数.输出应如下所示:

I want the floats to be formated with only two decimal digits. The output should look like this:

>>> json.dumps([23.67, 23.97, 23.87])
'[23.67, 23.97, 23.87]'

我尝试定义自己的 JSON 编码器类:

I have tried defining my own JSON Encoder class:

class MyEncoder(json.JSONEncoder):
    def encode(self, obj):
        if isinstance(obj, float):
            return format(obj, '.2f')
        return json.JSONEncoder.encode(self, obj)

这适用于唯一的浮动对象:

This works for a sole float object:

>>> json.dumps(23.67, cls=MyEncoder)
'23.67'

但嵌套对象失败:

>>> json.dumps([23.67, 23.97, 23.87])
'[23.670000000000002, 23.969999999999999, 23.870000000000001]'

我不想有外部依赖，所以我更喜欢坚持使用标准的 json 模块.

I don't want to have external dependencies, so I prefer to stick with the standard json module.

我怎样才能做到这一点?

How can I achieve this?

解决方案

注意:这不在任何最新版本的 Python 中都有效.

Note: This does not work in any recent version of Python.

不幸的是，我认为您必须通过猴子修补来做到这一点(在我看来，这表明标准库 json 包中的设计缺陷).例如，这段代码:

Unfortunately, I believe you have to do this by monkey-patching (which, to my opinion, indicates a design defect in the standard library json package). E.g., this code:

import json
from json import encoder
encoder.FLOAT_REPR = lambda o: format(o, '.2f')
    
print(json.dumps(23.67))
print(json.dumps([23.67, 23.97, 23.87]))

发射:

23.67
[23.67, 23.97, 23.87]

如你所愿.显然，应该有一种架构化的方式来覆盖 FLOAT_REPR ，这样如果你愿意，浮点的每一个表示都在你的控制之下；但不幸的是，这不是 json 包的设计方式:-(.

as you desire. Obviously, there should be an architected way to override FLOAT_REPR so that EVERY representation of a float is under your control if you wish it to be; but unfortunately that's not how the json package was designed:-(.

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