4.2. Dataclass Define Basic

4.2.1. SetUp

>>> from dataclasses import dataclass, field
>>> from typing import Literal, Final

4.2.2. Required Fields

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str

4.2.3. Default Fields

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

4.2.4. Lists

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     groups: list[str]

4.2.5. Union Fields

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     age: int | float

4.2.6. Optional Fields

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     age: int | None = None

4.2.7. Literal Field

Import:

>>> from typing import Literal

Define class:

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     role: Literal['users', 'staff', 'admins']

4.2.8. ClassVar Fields

  • from typing import ClassVar

  • Defines static field

One of two places where dataclass() actually inspects the type of a field is to determine if a field is a class variable as defined in PEP 526. It does this by checking if the type of the field is typing.ClassVar. If a field is a ClassVar, it is excluded from consideration as a field and is ignored by the dataclass mechanisms. Such ClassVar pseudo-fields are not returned by the module-level fields() function.

Import:

>>> from typing import ClassVar

Define Class:

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     age: int
...     AGE_MIN: ClassVar[int] = 30
...     AGE_MAX: ClassVar[int] = 50

Note, that those fields will not be displayed in repr or while printing.

>>> User('Mark', 'Watney', age=42)
User(firstname='Mark', lastname='Watney', age=42)

4.2.9. Keyword Arguments Only

  • Since Python 3.10

  • from dataclasses import KW_ONLY

Any fields after a pseudo-field with the type of KW_ONLY are marked as keyword-only fields. Note that a pseudo-field of type KW_ONLY is otherwise completely ignored. This includes the name of such a field. By convention, a name of _ is used for a KW_ONLY field.

Import:

>>> from dataclasses import KW_ONLY

Define class:

>>> @dataclass
... class User:
...     firstname: str
...     lastname: str
...     _: KW_ONLY
...     age: int
...     height: float
...     weight: float
>>> User('Mark', 'Watney', age=42, height=178.0, weight=75.5)
User(firstname='Mark', lastname='Watney', age=42, height=178.0, weight=75.5)
>>> mark = User('Mark', 'Watney', 42, height=178.0, weight=75.5)
Traceback (most recent call last):
TypeError: User.__init__() takes 3 positional arguments but 4 positional arguments (and 2 keyword-only arguments) were given
>>> mark = User('Mark', 'Watney', 42, 178.0, weight=75.5)
Traceback (most recent call last):
TypeError: User.__init__() takes 3 positional arguments but 5 positional arguments (and 1 keyword-only argument) were given
>>> mark = User('Mark', 'Watney', 42, 178.0, 75.5)
Traceback (most recent call last):
TypeError: User.__init__() takes 3 positional arguments but 6 were given

4.2.10. Assignments