6.3. Dataclass Define Special¶
6.3.1. SetUp¶
>>> from dataclasses import dataclass, field
>>> from typing import Literal, Final
6.3.2. Union Fields¶
>>> @dataclass
... class Astronaut:
... firstname: str
... lastname: str
... age: int | float
6.3.3. Optional Fields¶
>>> @dataclass
... class Astronaut:
... firstname: str
... lastname: str
... agency: str | None = None
6.3.4. Literal Field¶
Import:
>>> from typing import Literal
Define class:
>>> @dataclass
... class Astronaut:
... firstname: str
... lastname: str
... agency: Literal['NASA', 'ESA', 'POLSA']
6.3.5. Final Fields¶
In Python there is no constants
Import:
>>> from typing import Final
Define class:
>>> @dataclass
... class Astronaut:
... firstname: Final[str]
... lastname: Final[str]
... age: int
6.3.6. ClassVar Fields¶
from typing import ClassVarDefines 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 Astronaut:
... fullname: str
... firstname: str
... age: int
... AGE_MIN: ClassVar[int] = 30
... AGE_MAX: ClassVar[int] = 50
6.3.7. 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 Astronaut:
... firstname: str
... lastname: str
... _: KW_ONLY
... age: int
... height: float
... weight: float