从数据类的 __str__ 表示中排除某些属性

Question

我们有这个课程：

from dataclasses import dataclass, field
from datetime import datetime
from typing import List, Dict

@dataclass
class BoardStaff:
    date: str = datetime.now()
    fullname: str
    address: str

    ## attributes to be excluded in __str__:
    degree: str
    rank: int = 10
    badges: bool = False
    cases_dict: Dict[str, str] = field(default_factory=dict)
    cases_list: List[str] = field(default_factory=list)

Emp = BoardStaff('Jack London', address='Unknown', degree='MA')

由于BoardStaff 是一个数据类，因此可以轻松地通过print(Emp) 接收：
BoardStaff(fullname='Jack London', address='Unknown', degree='MA', rank=10, badges=False, cases={}, date=datetime.datetime(2021, 8, 10, 11, 36, 50, 693428))。

但是，我希望从表示中排除一些属性（即最后 5 个），所以我必须定义 __str__ 方法并手动排除一些属性，如下所示：

    def __str__(self):
        str_info = {
            k: v
            for k, v in self.__dict__.items()
            if k not in ['degree', 'rank', 'other'] and v
        }
        return str(str_info)

但是有没有更好的排除方法，比如在定义属性时使用一些参数？

Answer 1

明显的解决方案

只需使用参数repr=False 将您的属性定义为fields：

from dataclasses import dataclass, field
from datetime import datetime
from typing import List, Dict

@dataclass
class BoardStaff:
    date: str = datetime.now()
    fullname: str
    address: str

    ## attributes to be excluded in __str__:
    degree: str = field(repr=False)
    rank: int = field(default=10, repr=False)
    badges: bool = field(default=False, repr=False)
    cases_dict: Dict[str, str] = field(default_factory=dict, repr=False)
    cases_list: List[str] = field(default_factory=list, repr=False)

Emp = BoardStaff('Jack London', address='Unknown', degree='MA')

正如其他人在 cmets 中所建议的那样，这可以很好地与将属性标记为“私有”一起使用，因为它们的名称以前导下划线开头。

更高级的解决方案

如果您正在寻找一种更通用的解决方案，它不涉及使用repr=False 定义这么多fields，您可以这样做。它与您自己想出的解决方案非常相似，但它创建的__repr__ 更类似于通常的dataclass __repr__：

from dataclasses import dataclass, field
from datetime import datetime
from typing import List, Dict
    
@dataclass
class BoardStaff:
    fullname: str
    address: str
    degree: str
    date: str = datetime.now()
    rank: int = 10
    badges: bool = False
    cases_dict: Dict[str, str] = field(default_factory=dict)
    cases_list: List[str] = field(default_factory=list)

    def __repr__(self):
        dict_repr = ', '.join(
            f'{k}={v!r}'
            for k, v in filter(
                lambda item: item[0] in {'fullname', 'address', 'date'},
                self.__dict__.items()
            )
        )

        return f'{self.__class__.__name__}({dict_repr})'

Emp = BoardStaff('Jack London', address='Unknown', degree='MA')
print(Emp)

（请注意，我不得不稍微重新排序您的字段，因为在没有默认值的参数之前有默认参数参数会引发错误。）

如果您不想将您的 __repr__ 字段硬编码到您的 __repr__ 方法中，您可以将您的非 __repr__ 字段标记为私有属性，正如 @DarkKnight 在 cmets 中所建议的那样，并使用它作为__repr__ 方法的信号：

from dataclasses import dataclass, field
from datetime import datetime
from typing import List, Dict
    
@dataclass
class BoardStaff:
    fullname: str
    address: str
    _degree: str
    date: str = datetime.now()
    _rank: int = 10
    _badges: bool = False
    _cases_dict: Dict[str, str] = field(default_factory=dict)
    _cases_list: List[str] = field(default_factory=list)

    def __repr__(self):
        dict_repr = ', '.join(
            f'{k}={v!r}'
            for k, v in filter(
                lambda item: not item[0].startswith('_'),
                self.__dict__.items()
            )
        )

        return f'{self.__class__.__name__}({dict_repr})'

Emp = BoardStaff('Jack London', address='Unknown', _degree='MA')
print(Emp)

您甚至可以编写自己的装饰器，为您逐个类生成自定义 __repr__ 方法。例如，此装饰器将生成 __repr__ 方法，其中仅包含您传递给装饰器的参数：

from dataclasses import dataclass, field
from datetime import datetime
from typing import List, Dict
from functools import partial

def dataclass_with_repr_fields(
    keys, init=True, eq=True, order=False, 
    unsafe_hash=False, frozen=False, cls=None
):
    if cls is None:
        return partial(
            dataclass_with_repr_fields, keys, init=init, 
            eq=eq, order=order,  unsafe_hash=unsafe_hash,
            frozen=frozen)

    cls = dataclass(
        cls, init=init, repr=False, eq=eq, order=order, 
        unsafe_hash=unsafe_hash, frozen=frozen
    )

    def __repr__(self):
        dict_repr = ', '.join(
            f'{k}={v!r}'
            for k, v in filter(
                lambda item: item[0] in keys,
                self.__dict__.items()
            )
        )

        return f'{self.__class__.__name__}({dict_repr})'

    cls.__repr__ = __repr__
    return cls


@dataclass_with_repr_fields({'fullname', 'address', 'date'})
class BoardStaff:
    fullname: str
    address: str
    degree: str
    date: str = datetime.now()
    rank: int = 10
    badges: bool = False
    cases_dict: Dict[str, str] = field(default_factory=dict)
    cases_list: List[str] = field(default_factory=list)
    

@dataclass_with_repr_fields({'name', 'surname'})
class Manager:
    name: str
    surname: str
    salary: int
    private_medical_details: str

Emp = BoardStaff('Jack London', address='Unknown', degree='MA')
print(Emp)
manager = Manager('John', 'Smith', 600000, 'badly asthmatic')
print(manager)