Creating a unique id in a python dataclass

Question

I need a unique (unsigned int) id for my python data class. This is very similar to this so post , but without explicit ctors.

import attr
from attrs import field
from itertools import count
@attr.s(auto_attribs=True)
class Person:
    #: each Person has a unique id
    _counter: count[int] = field(init=False, default=count())
    _unique_id: int = field(init=False)

    @_unique_id.default
    def _initialize_unique_id(self) -> int:
        return next(self._counter)

Is there any more-"pythonic" solution?

Answer 1

Use a default factory instead of just a default . This allows to define a call to get the next id on each instantiation.
A simple means to get a callable that counts up is to use count().__next__ , the equivalent of calling next(...) on a count instance. ¹

The common "no explicit ctor" libraries attr and dataclasses both support this:

from itertools import count
from dataclasses import dataclass, field

@dataclass
class C:
    identifier: int = field(default_factory=count().__next__)

import attr

@attr.s
class C:
    identifier: int = attr.field(factory=count().__next__)

To always use the automatically generated value and prevent passing one in as a parameter, use init=False .

@dataclass
class C:
    identifier: int = field(default_factory=count().__next__, init=False)

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¹ If one wants to avoid explicitly addressing magic methods, one can use a closure over a count . For example, factory=lambda counter=count(): next(counter) .