How to define a class attribute by other attributes of the same class using "attrs"

Question

I'm new with attrs module & I've encountered something that I didn't understand very well. For simplicity I have this code below:

import attr
from typing import List

@attr.define
class A:
    a: str = attr.ib(init=False, default='hi')
    b: str = attr.ib(init=False, default='bye')
    ab: List[str] = attr.ib(init=False, default=[a, b])


def main() -> None:
    a = A()
    print(a.a)
    print(a.b)
    print(a.ab)


if __name__ == '__main__':
    main()

But while I expected to get the output:

hi
bye
["hi", "bye"]

As a beginner, The output I get is a bit strange to me:

hi
bye
[_CountingAttr(counter=18, _default='hi', repr=True, eq=True, order=True, hash=None, init=False, on_setattr=None, metadata={}), _CountingAttr(counter=19, _default='bye', repr=True, eq=True, order=True, hash=None, init=False, on_setattr=None, metadata={})]

I'll appreciate any explanation to understand the reason & how I can handle it.

Inside the class scope, a and b are literally attr.ib placeholders, not their defaults. So [a, b] is a list of the placeholders; attrs doesn't know that you expect it to peek arbitrarily deep into the default and pick out placeholders there. — MisterMiyagi
– MisterMiyagi, Commented Feb 17, 2022 at 15:57
@MisterMiyagi is there a way to access the default value manually from inside the class? — Benny
– Benny, Commented Feb 17, 2022 at 17:05

Mard · Accepted Answer · 2022-09-03 15:16:41Z

At class definition time. a and b are just the result of attr.ib(...). And if somehow that worked, default=[a, b] means to give this reference to every object that this class creates. Meaning that every A you create would reference the same list.

What you want to do it's to use attr.ib(factory=list) when you want a new list for every object. factory=list it's just an alias to default=attr.Factory(list). attr.Factory has this keyword takes_self that can enhance this functionality further. So you can use attr.ib(default=attr.Factory(lambda self: [self.a, self.b], takes_self=True)) to do what you want.

n [2]: import attr
   ...: from typing import List
   ...: 
   ...: @attr.define
   ...: class A:
   ...:     a: str = attr.ib(init=False, default='hi')
   ...:     b: str = attr.ib(init=False, default='bye')
   ...:     ab: List[str] = attr.ib(default=attr.Factory(lambda self: [self.a, self.b], takes_self=True))
   ...: 
   ...: def main() -> None:
   ...:     a = A()
   ...:     print(a.a)
   ...:     print(a.b)
   ...:     print(a.ab)
   ...: 
   ...: 
   ...: if __name__ == '__main__':
   ...:     main()
   ...: 
hi
bye
['hi', 'bye']

If you need to do something more complex or dislike lambdas, you can also use:

In [3]: import attr
   ...: from typing import List
   ...: 
   ...: @attr.define
   ...: class A:
   ...:     a: str = attr.ib(init=False, default='hi')
   ...:     b: str = attr.ib(init=False, default='bye')
   ...:     ab: List[str] = attr.ib()
   ...: 
   ...:     @ab.default
   ...:     def _(self):
   ...:         return [self.a, self.b]
   ...: 
   ...: def main() -> None:
   ...:     a = A()
   ...:     print(a.a)
   ...:     print(a.b)
   ...:     print(a.ab)
   ...: 
   ...: 
   ...: if __name__ == '__main__':
   ...:     main()
   ...: 
hi
bye
['hi', 'bye']

Benny · Accepted Answer · 2022-02-17 19:57:30Z

0

I have found the solution in attrs.org

attrs module has a post __init__ magic function: __attrs_post_init__ exactly for cases like mine that I described in my question.

That's the solution, post it here for others who will encounter the same question like me:

import attr
from typing import List

@attr.define
class A:
    a: str = attr.ib(init=False, default='hi')
    b: str = attr.ib(init=False, default='bye')
    ab: List[str] = attr.ib(default=[])

    def __attrs_post_init__(self):
        self.ab = [self.a, self.b]


def main() -> None:
    a = A()
    print(a.a)
    print(a.b)
    print(a.ab)


if __name__ == '__main__':
    main()

answered Feb 17, 2022 at 19:57

Benny

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1 Comment

Wör Du Schnaffzig Over a year ago

That works, but it sets the ab value twice and discards the first value, which is unnecessary. Also, it's not recommended to use mutable defaults.

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