datacls 4.4.2

Slightly improved, backward compatible dataclasses

The Python built-in module dataclasses is almost perfect.

datacls is a thin wrapper around dataclasses, completely backward-compatible, that makes common use cases a little easier.

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@datacls.mutable is exactly like @dataclasses.dataclass() except that the resulting dataclass has four new methods:

• three new instance methods:

  • self.asdict(), like dataclasses.asdict()

  • self.astuple(), like dataclasses.astuple()

  • self.replace(), like dataclasses.replace()

• …and one new class method:

  • cls.fields(), like dataclasses.fields()

The new methods are only added if they do not exist on the target dataclass, so it should be impossible for datacls to override or shadow user methods or members by mistake.

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@datacls.immutable, or just @datacls, is exactly like datacls.mutable except frozen=True by default, so members can’t be changed after construction (without deliberately subverting the immutability).

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datacls.field() is just like dataclasses.field() except the very common default_factory argument is now also the first and only positional parameter.

datacls.make_dataclass() is just like dataclasses.make_dataclass() except that the class created also has the four new methods listed above.

Usage examples

import datacls
from typing import Dict

@datacls
class One:
    one: str = 'one'
    two: int = 2
    three: Dict = datacls.field(dict)

#
# Three new instance methods: asdict(), astuple(), replace()
#
o = One()
assert o.asdict() == {'one': 'one', 'two': 2, 'three': {}}

# Same as:
#
# import dataclasses
#
# assert dataclasses.asdict(o) == {'one': 'one', 'two': 2, 'three': {}}

assert o.astuple() == ('one', 2, {})

o2 = o.replace(one='seven', three={'nine': 9})
assert o2 == One('seven', 2, {'nine': 9})

#
# One new class method: fields()
#
assert [f.name for f in One.fields()] == ['one', 'two', 'three']

#
# @datacls is immutable: use @datacls.mutable for mutable classes
#
try:
    o.one = 'three'
except AttributeError:
    pass
else:
    raise AttributeError('Was mutable!')

@datacls.mutable
class OneMutable:
    one: str = 'one'
    two: int = 2
    three: Dict = datacls.field(dict)

om = OneMutable()
om.one = 'three'
assert str(om) == "OneMutable(one='three', two=2, three={})"

#
# These four new methods won't break your old dataclass by mistake:
#
@datacls
class Overloads:
    one: str = 'one'
    asdict: int = 1
    astuple: int = 1
    fields: int = 1
    replace: int = 1

o = Overloads()

assert datacls.astuple(ov) == ('one', 1, 1, 1, 1)

assert ov.one == 'one'
assert ov.asdict == 1
assert ov.astuple == 1
assert ov.fields == 1
assert ov.replace == 1

# You can still access the methods as functions on `datacls`:
assert (
    datacls.asdict(ov) ==
    {'asdict': 1, 'astuple': 1, 'fields': 1, 'one': 'one', 'replace': 1}
)