Dataclasses and property decorator

Question

I\'ve been reading up on Python 3.7\'s dataclass as an alternative to namedtuples (what I typically use when having to group data in a structure). I was wondering if datacla

Answer 1

Currently, the best way I found was to overwrite the dataclass fields by property in a separate child class.

from dataclasses import dataclass, field

@dataclass
class _A:
    x: int = 0

class A(_A):
    @property
    def x(self) -> int:
        return self._x

    @x.setter
    def x(self, value: int):
        self._x = value

The class behave like a regular dataclass. And will correctly define the __repr__ and __init__ field (A(x=4) instead of A(_x=4). The drawback is that the properties cannot be read-only.

This blog post, tries to overwrite the wheels dataclass attribute by the property of the same name. However, the @property overwrite the default field, which leads to unexpected behavior.

from dataclasses import dataclass, field

@dataclass
class A:

    x: int

    # same as: `x = property(x)  # Overwrite any field() info`
    @property
    def x(self) -> int:
        return self._x

    @x.setter
    def x(self, value: int):
        self._x = value

A()  # `A(x=)`   Oups

print(A.__dataclass_fields__)  # {'x': Field(name='x',type=,default=,init=True,repr=True}

One way solve this, while avoiding inheritance would be to overwrite the field outside the class definition, after the dataclass metaclass has been called.

@dataclass
class A:
  x: int

def x_getter(self):
  return self._x

def x_setter(self, value):
  self._x = value

A.x = property(x_getter)
A.x = A.x.setter(x_setter)

print(A(x=1))
print(A())  # missing 1 required positional argument: 'x'

It should probably possible to overwrite this automatically by creating some custom metaclass and setting some field(metadata={'setter': _x_setter, 'getter': _x_getter}).