How to create dict from class without None fields?

问题

I have the following dataclass:

@dataclass
class Image:
    content_type: str
    data: bytes = b''
    id: str = ""
    upload_date: datetime = None
    size: int = 0

    def to_dict(self) -> Dict[str, Any]:
        result = {}
        if self.id:
            result['id'] = self.id
        if self.content_type:
            result['content_type'] = self.content_type
        if self.size:
            result['size'] = self.size
        if self.upload_date:
            result['upload_date'] = self.upload_date.isoformat()
        return result

Is there any way to simplify to_dict method? I don't want to list all of the fields using if.

回答1:

As suggested by meowgoesthedog, you can use asdict and filter the result to skip falsy values:

from dataclasses import dataclass, asdict
from datetime import datetime
from typing import Dict, Any

@dataclass
class Image:
    content_type: str
    data: bytes = b''
    id: str = ""
    upload_date: datetime = None
    size: int = 0

    def to_dict(self) -> Dict[str, Any]:
        return {k: v for k, v in asdict(self).items() if v}

print(Image('a', b'b', 'c', None, 0).to_dict())
# {'content_type': 'a', 'data': b'b', 'id': 'c'}

回答2:

You can get the dict representation of a class object in python by using the vars method.

First, you need to implement __setattr__ inside the class. Add this method inside your class.

def __setattr__(self, name, value):
    if value is not None:
        self.__dict__[name] = value
    if value is not None and name == 'upload_date':
        self.__dict__[name] = value.isoformat()

This will prevent the None fields from being added into the class dict.

>>> img = Image(content_type='something')
>>> vars(i)
{'content_type': 'something', 'data': b'', 'id': '', 'size': 0}

However, using dataclasses.asdict will still keep the None fields.

>>> import dataclasses
>>> img = Image(content_type='something')
>>> dataclasses.asdict(img)
{'content_type': 'something', 'data': b'', 'id': '', 'upload_date': None, 'size': 0}

But you can use vars since that works. object.__dict__ returns the same thing as vars(object).

来源：https://stackoverflow.com/questions/57305749/how-to-create-dict-from-class-without-none-fields