= Python DataClasses =

'''`dataclasses`''' is a module that supports the creation of simple classes.

<<TableOfContents>>

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== Decorators ==



=== @DataClass ===

Use the '''`@dataclass`''' decorator on a class to automatically generate [[Python/DunderMethod|dunder methods]].

{{{
import dataclasses

@dataclasses.dataclass
class User:
    name: str       # 'name' has no defualt value
    id: int = 1000  # 'id' defaults to 1000
}}}

A `TypeError` is raised if a field without a default value follows a field with a default value.

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== Classes ==



=== Field ===

A '''`Field`''' object represents a field in a dataclass. For complex logic, a dataclass can be defined in terms of `Field` objects.

A `Field` object is instantiated with the `field()` function. This function takes the following optional named arguments:

 * `default`: default value
 * `default_factory`: callable that generates the default value as needed
 * `init`: boolean indicating that this field should be included in the generated [[Python/DunderMethod#__Init__|__init__]] method
 * `repr`: boolean indicating that this field should be included in the generated [[Python/DunderMethod#__Repr__|__repr__]] method
 * `hash`: boolean indicating that this field should be included in the generated [[Python/DunderMethod#__Hash__|__hash__]] method
 * `compare`: boolean indicating that this field should be included in the generated comparison methods (e.g. [[Python/DunderMethod#__Eq__|__eq__]] and so on)
 * `kw_only`: boolean indicating that this field is keyword-only

A `Field` object has `name` and `type `atributes, as well as attribute matching the names and meanings of the `field()` function's arguments.

To create a dataclass with a field that defaults to an empty list, try:

{{{
import dataclasses

@dataclasses.dataclass
class User:
    groups: list[int] = dataclass.field(default_factory=list)

u = User()
u.groups += [1, 2, 3]
}}}

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== Functions ==



=== AsDict ===

{{{
import dataclasses

@dataclasses.dataclass
class Point:
     x: int
     y: int

p = Point(10, 20)
dataclasses.asdict(p)  # {'x': 10, 'y': 20}
}}}

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=== AsTuple ===

{{{
import dataclasses

@dataclasses.dataclass
class Point:
     x: int
     y: int

p = Point(10, 20)
dataclasses.astuple(p)  # (10, 20)
}}}

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=== Fields ===

Extract the `Field` objects from a dataclass.

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=== Is_DataClass ===

Returns `True` if passed a dataclass.

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=== Make_DataClass ===

An alternative constructor for dataclasses.

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=== Replace ===

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== Constants ==

'''`KW_ONLY`''' is a [[Python/TypeAnnotation|type annotation]] used for dataclasses.

{{{
import dataclasses

@dataclasses.dataclass
class Point:
    x: float
    _: dataclass.KW_ONLY
    y: float
    z: float

p = Point(0, y=1.5, z=2.0)
}}}

'''`MISSING`''' is a sentinel value used internally.

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== See also ==

[[https://docs.python.org/3/library/dataclasses.html|Python dataclasses module documentation]]



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