python - json.dump 不调用 default 或 cls

Question

尝试将包含日期时间对象的字典序列化为 json 的键。其他一些答案建议使用 json.dump 参数 default 或 cls 但是，这些方法似乎都没有被调用。请参阅下面的 MWE。我错过了什么？

使用默认

from datetime import datetime
import json

def default(obj):
    print("Default Called")
    if isinstance(obj, (datetime, date)):
        return obj.isoformat()

test = {datetime(1970, 1, 1, 8, 0, 0): 10}

with open("output.json", "w") as fo:
    json.dump(test, fo, default=default)

Traceback (most recent call last):
  File "test.py", line 21, in <module>
    json.dump(test, fo, default=default)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py", line 179, in dump
    for chunk in iterable:
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/encoder.py", line 376, in _iterencode_dict
    raise TypeError(f'keys must be str, int, float, bool or None, '
TypeError: keys must be str, int, float, bool or None, not datetime

使用cls

from datetime import datetime
import json

class DateEncoder(json.JSONEncoder):
    def default(self, obj):
        print("Default called")
        if isinstance(obj, (datetime, date)):
            return obj.isoformat()

        # Let the base class default method raise the TypeError
        return json.JSONEncoder.default(self, obj)

test = {datetime(1970, 1, 1, 8, 0, 0): 10}

with open("output.json", "w") as fo:
    json.dump(test, fo, cls=DateEncoder)

Traceback (most recent call last):
  File "test.py", line 16, in <module>
    json.dump(test, fo, cls=DateEncoder)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py", line 179, in dump
    for chunk in iterable:
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/encoder.py", line 376, in _iterencode_dict
    raise TypeError(f'keys must be str, int, float, bool or None, '
TypeError: keys must be str, int, float, bool or None, not datetime

Answer 1

TLDR:.default 仅针对 dict 值而不是键调用。

详细说明 Abhishek Kumars 的回答；在查看了 json.JSONEncoder 源代码后，.default 仅在不可序列化的 dict 值上调用，从不调用键。键必须是 str、int、float、bool 或 None 之一。

要使用datetime对象作为键，必须覆盖.iterencode以使用递归预处理器方法将datetime转换为海峡:

test = {
        "key_1": "Value_1",
        "key_2": 10,
        "key_3": ["list_" + str(i) for i in range(5)],
        "key_4": {"nestkey_" + str(i) : "nestvalue_" + str(i) for i in range(5) },
        "key_5": datetime.datetime(1970, 1, 1, 8, 0, 0),
        datetime.datetime(1970, 1, 1, 8, 0, 0): "datetime_key"
}

class DateTimeEncoder(json.JSONEncoder):

    def _preprocess_date(self, obj):
        if isinstance(obj, (datetime.date, datetime.datetime, datetime.timedelta)):
            return str(obj)
        elif isinstance(obj, dict):
            return {self._preprocess_date(k): self._preprocess_date(v) for k,v in obj.items()}
        elif isinstance(obj, list):
            return [self._preprocess_date(i) for i in obj]
        return obj

    def default(self, obj):
        if isinstance(obj, (datetime.date, datetime.datetime, datetime.timedelta)):
            return str(obj)
        return super().default(obj)

    def iterencode(self, obj):
        return super().iterencode(self._preprocess_date(obj))

with open('output.json', 'w') as fo:
    json.dump(test, fo, cls=DateTimeEncoder)

对于较大的词典来说，这显然是一项昂贵的操作，因此应谨慎行事。此外，将 json.JSONEncoder 更新为在键和值上使用默认设置会很好 - 创建于:https://bugs.python.org/issue41569 的问题