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

Here's what I did to define the field as a property in __post_init__. This is a total hack, but it works with dataclasses dict-based initialization and even with marshmallow_dataclasses.

from dataclasses import dataclass, field, asdict


@dataclass
class Test:
    name: str = "schbell"
    _name: str = field(init=False, repr=False)

    def __post_init__(self):
        # Just so that we don't create the property a second time.
        if not isinstance(getattr(Test, "name", False), property):
            self._name = self.name
            Test.name = property(Test._get_name, Test._set_name)

    def _get_name(self):
        return self._name

    def _set_name(self, val):
        self._name = val


if __name__ == "__main__":
    t1 = Test()
    print(t1)
    print(t1.name)
    t1.name = "not-schbell"
    print(asdict(t1))

    t2 = Test("llebhcs")
    print(t2)
    print(t2.name)
    print(asdict(t2))

This would print:

Test(name='schbell')
schbell
{'name': 'not-schbell', '_name': 'not-schbell'}
Test(name='llebhcs')
llebhcs
{'name': 'llebhcs', '_name': 'llebhcs'}

I actually started off from this blog post mentioned somewhere in this SO, but ran into the issue that the dataclass field was being set to type property because the decorator is applied to the class. That is,

@dataclass
class Test:
    name: str = field(default='something')
    _name: str = field(init=False, repr=False)

    @property
    def name():
        return self._name

    @name.setter
    def name(self, val):
        self._name = val

would make name to be of type property and not str. So, the setter will actually receive property object as the argument instead of the field default.