Mypy returns Error "Unexpected keyword argument" for subclass of a decorated class with attrs package

Question

I have two decorated classes using attrs package as follows:

@attr.s(kw_only=True)
class Entity:
    """
    base class of all entities
    """
    entity_id = attr.ib(type=str)
    # ...

@attr.s(kw_only=True)
class Customer(Entity):
    customer_name = attr.ib(type=Name)
    # ...

I get Unexpected keyword argument "entity_id" for "Customer" for code like this:

def register_customer(customer_name: str):
    return Customer(
        entity_id=unique_id_generator(),
        customer_name=Name(full_name=customer_name),
    )

So how can I make Mypy aware of the __init__ method of my parent class. I should mention that the code works perfectly and there is (at least it seems) no runtime error.

Answer 1

Your code is correct and should work. If I run the following simplified version:

import attr

@attr.s(kw_only=True)
class Entity:
    """
    base class of all entities
    """
    entity_id = attr.ib(type=str)
    # ...

@attr.s(kw_only=True)
class Customer(Entity):
    customer_name = attr.ib(type=str)

def register_customer(customer_name: str) -> Customer:
    return Customer(
        entity_id="abc",
        customer_name=customer_name,
    )   # ...

through Mypy 0.910 with attrs 21.2.0 on Python 3.9.7 I get:

Success: no issues found in 1 source file

---

My theories:

• Old Mypy (there's a lot of changes all times, sometimes it takes time for the attrs plugin to be updated with new features).
• Old attrs (we try to keep up with the changes in attrs and the features provided by Mypy).
• Python 2 (since you're using the old syntax). kw_only used to be Python 3-only and I wouldn't be surprised if mypy has some resident logic around it?