Fix dataclass inherited field ordering

[jstasiak]

Fixes #5681.

This is how the dataclasses module behaves on Python 3.7.0:

% cat test.py
from dataclasses import dataclass

@dataclass
class Mammal:
    age: int

@dataclass
class Person(Mammal):
    name: str
    age: int

@dataclass
class SpecialPerson(Person):
    special_factor: float

@dataclass
class ExtraSpecialPerson(SpecialPerson):
    age: int
    special_factor: float
    name: str

print(Person(32, 'John'))
print(SpecialPerson(21, 'John', 0.5))
print(ExtraSpecialPerson(21, 'John', 0.5))

% python test.py
Person(age=32, name='John')
SpecialPerson(age=21, name='John', special_factor=0.5)
ExtraSpecialPerson(age=21, name='John', special_factor=0.5)

[ilevkivskyi]

Thanks for PR! This looks good, I have few comments below. Also I have two questions:

• Is this specific to dataclasses, or attrs plugin should be also updated to give correct order?
• Do we have an issue about this? If yes, I would add Fixes #XXXX to the top of PR description.

[ilevkivskyi]

Will this not crash if there are two attrs with the same name? Do we have a test for this?

[jstasiak]

I don't believe it can happen within a single class and with multiple classes the if name not in known_attrs block either adds an attribute that wasn't there or, in case of an attribute being overridden, it reorders attributes without adding duplicates.

The way this assignment is written is to express the fact that we strictly expect single attribute with the name name here, I can't think of a situation in which this condition is not met, any possible name clashes are covered by the tests already.

[jstasiak]

I have no idea about attrs and the attrs plugin, I've never used it. Turns out there was an issue filled about more or less the same thing - I updated the description and the commit message.

[efimk-lu]

Hey @jstasiak , @ilevkivskyi ,

For some reason I keep getting the same issue, I'm using my 0.670.

class AbstractModel(ABC):
   pass
@dataclass(unsafe_hash=True)
class Model(AbstractModel):
    transaction_id: str
    message_id: Optional[str]
    raw: dict = field(hash=False) 
    customer: str
    resource_name: Optional[str] = field(default=None)

@dataclass(unsafe_hash=True)
class TestModel(SpanModel):
    name: str  # <-- Getting here  error: Attributes without a default cannot follow attributes with one

[jstasiak]

Can you provide a complete piece of code that works at runtime yet triggers this? I think there are some pieces missing here (no SpanModel definition).

[efimk-lu]

@jstasiak it looks like it was an internal issue. Solved, Tx.