4.7. Dataclass Field

default - Default value for the field
default_factory - Field factory
init - Use this field in __init__()
repr - Use this field in __repr__()
hash - Use this field in __hash__()
compare - Use this field in comparison functions (le, lt, gt, ge, eq, ne)
metadata - For storing extra information about field
kw_only - field will become a keyword-only parameter to __init__()

def field(*,
          default: Any,
          default_factory: Any,
          init: bool = True,
          repr: bool = True,
          hash: bool = None,
          compare: bool = True,
          metadata: dict[str,Any] = None,
          kw_only: bool) -> None

4.7.1. SetUp

>>> from dataclasses import dataclass, field
>>> from typing import ClassVar

4.7.2. Default

default - Default value for the field

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     role: str = field(default='user')

>>> User('Mark', 'Watney')
User(firstname='Mark', lastname='Watney', role='user')

>>> User('Mark', 'Watney', role='admin')
User(firstname='Mark', lastname='Watney', role='admin')

Note, that this is equal to:

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     role: str = 'user'

4.7.3. Default Factory

default_factory - Field factory

The following code will not work:

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     groups: list[str] = ['users', 'staff', 'admins']
Traceback (most recent call last):
ValueError: mutable default <class 'list'> for field groups is not allowed: use default_factory

If you want to create a list with default values, you have to create a field with default_factory=lambda: ['users', 'staff', 'admins']. Lambda expression will be evaluated on field initialization.

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     groups: list[str] = field(default_factory=lambda: ['users', 'staff', 'admins'])

>>> User('Mark', 'Watney')
User(firstname='Mark', lastname='Watney', groups=['users', 'staff', 'admins'])

4.7.4. Init

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     role: str = field(default='user', init=False)

>>> User('Mark', 'Watney')
User(firstname='Mark', lastname='Watney', role='user')

>>> User('Mark', 'Watney', role='admin')
Traceback (most recent call last):
TypeError: User.__init__() got an unexpected keyword argument 'role'

4.7.5. Repr

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     role: str = field(repr=False)

>>> User('Mark', 'Watney', role='admin')
User(firstname='Mark', lastname='Watney')

4.7.6. kw_only

Since Python 3.10
keyword-only

If true, this field will be marked as keyword-only. This is used when the generated __init__() method's parameters are computed.

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     role: str = field(kw_only=True)

>>> User('Mark', 'Watney', role='admin')
User(firstname='Mark', lastname='Watney', role='admin')

>>> User('Mark', 'Watney', 'admin')
Traceback (most recent call last):
TypeError: User.__init__() takes 3 positional arguments but 4 were given

4.7.7. Use Case - 1

Validation

>>> from typing import ClassVar, Self
>>> from dataclasses import dataclass, field
>>> from datetime import time, datetime, timezone
>>>
>>>
>>> @dataclass
... class Group:
...     gid: int
...     name: str
>>>
>>> @dataclass(frozen=True)
... class User:
...     firstname: str
...     lastname: str
...     roles: list[str] = field(default_factory=lambda: ['users', 'staff', 'admins'])
...     friends: list[Self] = field(default_factory=list, kw_only=True)
...     groups: list[Group] = field(default_factory=list, kw_only=True)
...     account_created: datetime = field(default_factory=lambda: datetime.now(tz=timezone.utc), kw_only=True)
...     permissions: list[dict] = field(default_factory=dict, kw_only=True)