4.9. Dataclass Parameters

• init - Generate __init__() method

• repr - Generate __repr__() method

• eq - Generate __eq__() and __ne__() methods

• order - Generate __lt__(), __le__(), __gt__(), and __ge__() methods

• unsafe_hash - If False: the __hash__() method is generated according to how eq and frozen are set

• frozen - If True: assigning to fields will generate an exception

• match_args - Generate __match_args__() method

• kw_only - Mark all fields as keyword-only

• slots - Create class with __slots__

Table 4.2. Dataclass options

Option	Default	Description (if True)
init	True	Generate __init__() method
repr	True	Generate __repr__() method
eq	True	Generate __eq__() and __ne__() methods
order	False	Generate __lt__(), __le__(), __gt__(), and __ge__() methods
unsafe_hash	False	If False: the __hash__() method is generated according to how eq and frozen are set
frozen	False	If True: assigning to fields will generate an exception
match_args	True	Generate __match_args__() method
kw_only	False	Mark all fields as keyword-only
slots	False	Create class with __slots__

4.9.1. SetUp

>>> from dataclasses import dataclass

4.9.2. Example

>>> @dataclass(init=True, repr=True, eq=True, order=False, unsafe_hash=False,
...            frozen=False, match_args=True, kw_only=False, slots=False)
... class User:
...     firstname: str
...     lastname: str
>>>
>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')
>>> c = User('Melissa', 'Lewis')

4.9.3. Init

• init=True by default

• Generate __init__() method

>>> @dataclass(init=True)
... class Point:
...     x: int
...     y: int
>>>
>>>
>>> p = Point(10, 20)
>>>
>>> print(p)
Point(x=10, y=20)

>>> @dataclass(init=False)
... class Point:
...     x: int
...     y: int
>>>
>>>
>>> p = Point(10, 20)
Traceback (most recent call last):
TypeError: Point() takes no arguments

4.9.4. Repr

• repr=True by default

• Generate __repr__() for pretty printing

>>> @dataclass(repr=True)
... class Point:
...     x: int
...     y: int
>>>
>>>
>>> p = Point(10, 20)
>>>
>>> print(p)
Point(x=10, y=20)

>>> @dataclass(repr=False)
... class Point:
...     x: int
...     y: int
>>>
>>>
>>> p = Point(10, 20)
>>>
>>> print(p)  
<__main__.Point object at 0x...>

4.9.5. Frozen

• frozen=False by default

• Prevents object from modifications

• Assigning to fields will generate an exception

>>> @dataclass(frozen=False)
... class Point:
...     x: int
...     y: int
>>>
>>>
>>> p = Point(10, 20)
>>> p.x = 30
>>>
>>> print(p)
Point(x=30, y=20)

>>> @dataclass(frozen=True)
... class Point:
...     x: int
...     y: int
>>>
>>>
>>> p = Point(10, 20)
>>> p.x = 30
Traceback (most recent call last):
dataclasses.FrozenInstanceError: cannot assign to field 'x'

4.9.6. Eq

• eq=True by default

• when eq=False compare objects by id() not values

• when eq=True compare objects by value not id()

>>> @dataclass(eq=True)
... class User:
...     firstname: str
...     lastname: str
>>>
>>>
>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')
>>> c = User('Melissa', 'Lewis')
>>>
>>> a == a
True
>>> a == b
True
>>> a == c
False

>>> @dataclass(eq=False)
... class User:
...     firstname: str
...     lastname: str
>>>
>>>
>>> a = User('Mark', 'Watney')
>>> b = User('Mark', 'Watney')
>>> c = User('Melissa', 'Lewis')
>>>
>>> a == a
True
>>> a == b
False
>>> a == c
False

4.9.7. Hash

• hash=False by default

• the __hash__() method is generated according to how eq and frozen are set

4.9.8. Order

• order=False by default

• Generate __lt__(), __le__(), __gt__(), and __ge__() methods

4.9.9. Match_args

• match_args=True by default

• Since Python 3.10

If true, the __match_args__ tuple will be created from the list of parameters to the generated __init__() method (even if __init__() is not generated, see above). If false, or if __match_args__ is already defined in the class, then __match_args__ will not be generated. New in version 3.10.

4.9.10. Kw_only

• kw_only=False by default

• Mark all fields as keyword-only

• Since Python 3.10

If true, then all fields will be marked as keyword-only. If a field is marked as keyword-only, then the only affect is that the __init__() parameter generated from a keyword-only field must be specified with a keyword when __init__() is called. There is no effect on any other aspect of dataclasses.

4.9.11. Slots

• slots=False by default

• Create class with __slots__

• Since Python 3.10

If true, __slots__ attribute will be generated and new class will be returned instead of the original one. If __slots__ is already defined in the class, then TypeError is raised.

>>> @dataclass(slots=True)
... class User:
...     firstname: str
...     lastname: str
...     __slots__ = ('firstname', 'lastname')
...
...     def say_hello(self):
...         return f'Hello {self.firstname} {self.lastname}'
...
Traceback (most recent call last):
TypeError: User already specifies __slots__

>>> @dataclass(slots=True)
... class User:
...     firstname: str
...     lastname: str
...
...     def say_hello(self):
...         return f'Hello {self.firstname} {self.lastname}'
>>>
>>> vars(User)  
mappingproxy({'__module__': '__main__',
              '__annotations__': {'firstname': <class 'str'>, 'lastname': <class 'str'>},
              'say_hello': <function User.say_hello at 0x...>,
              '__doc__': 'User(firstname: str, lastname: str)',
              '__dataclass_params__': _DataclassParams(init=True,repr=True,eq=True,order=False,unsafe_hash=False,frozen=False),
              '__dataclass_fields__': {'firstname': Field(name='firstname',type=<class 'str'>,default=<dataclasses._MISSING_TYPE object at 0x...>,default_factory=<dataclasses._MISSING_TYPE object at 0x...>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),kw_only=False,_field_type=_FIELD),
               'lastname': Field(name='lastname',type=<class 'str'>,default=<dataclasses._MISSING_TYPE object at 0x...>,default_factory=<dataclasses._MISSING_TYPE object at 0x...>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),kw_only=False,_field_type=_FIELD)},
              '__init__': <function User.__init__ at 0x...>,
              '__repr__': <function User.__repr__ at 0x...>,
              '__eq__': <function User.__eq__ at 0x...>,
              '__hash__': None,
              '__match_args__': ('firstname', 'lastname'),
              '__slots__': ('firstname', 'lastname'),
              'firstname': <member 'firstname' of 'User' objects>,
              'lastname': <member 'lastname' of 'User' objects>})
>>>
>>> a = User('Mark', 'Watney')
>>>
>>> a
User(firstname='Mark', lastname='Watney')
>>>
>>> vars(a)
Traceback (most recent call last):
TypeError: vars() argument must have __dict__ attribute
>>>
>>> a.__slots__
('firstname', 'lastname')
>>>
>>> {attrname: getattr(a, attrname) for attrname in a.__slots__}
{'firstname': 'Mark', 'lastname': 'Watney'}