Unmutable field on python's dataclass

Question

I have this dataclass:

@dataclass
class Couso:
    nome: str
    date: str = field(default=datetime.now(), init = False)
    id_: str = field(default=key())

Being key() a simple function that returns a str on len 32.

And when i create multiple classes of it (without specifing the id_ obviously) they all share the same id_

But why does it work this way? I cant understand.

Also, would this happen again with the attribute date?

Answer 1

key is called before field is called to create the field, so that every instance will have the same default id_ attribute. It's the same as if you had written

x = key()


@dataclass
class Couso:
    ...
    id_ : str = field(default=x)

If you want to call key each time you create a new instance, use default_factory instead.

id_: str = field(default_factory=key)  # key is not called; it's passed as an object.

The same goes for datetime.now :

date: str = field(default_factory=datetime.now, init = False)