如何在數據類中使用 field()？

Question

我經常看到這樣的代碼：

@dataclass
class ClassName:
    list_name: list[int] = field(default_factory=list)

但我不明白為什么我需要輸入field(default_factory=list) 。 list_name: list[int]還不夠嗎？

您能否解釋一下何時、為什么以及如何在數據類中使用 field()？

Answer 1

正在做：

list_name: list[int]

不一樣：

list_name: list[int] = field(default_factory=list)

后者使用default_factory ，所以注意：

>>> from dataclasses import dataclass
>>> @dataclass
... class ClassName:
...     list_name: list[int]
...
>>> ClassName()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 1 required positional argument: 'list_name'

但是如果你使用默認工廠：

>>> @dataclass
... class ClassName:
...     list_name: list[int] = field(default_factory=list)
...
>>> ClassName()
ClassName(list_name=[])

注意，如果你做了list_name: list[int] = []那么它會為每個實例重復使用相同的list object ，幾乎肯定不是你想要的。 實際上， dataclass裝飾器明確拒絕了這一點：

>>> @dataclass
... class ClassName:
...     list_name: list[int] = []
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/Users/juanarrivillaga/opt/miniconda3/envs/py39/lib/python3.9/dataclasses.py", line 1021, in dataclass
    return wrap(cls)
  File "/Users/juanarrivillaga/opt/miniconda3/envs/py39/lib/python3.9/dataclasses.py", line 1013, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
  File "/Users/juanarrivillaga/opt/miniconda3/envs/py39/lib/python3.9/dataclasses.py", line 863, in _process_class
    cls_fields = [_get_field(cls, name, type)
  File "/Users/juanarrivillaga/opt/miniconda3/envs/py39/lib/python3.9/dataclasses.py", line 863, in <listcomp>
    cls_fields = [_get_field(cls, name, type)
  File "/Users/juanarrivillaga/opt/miniconda3/envs/py39/lib/python3.9/dataclasses.py", line 747, in _get_field
    raise ValueError(f'mutable default {type(f.default)} for field '
ValueError: mutable default <class 'list'> for field list_name is not allowed: use default_factory

通常，當您需要修改單個字段時，您會顯式使用dataclasses.field 。