BSK-E0146 error

Protocol class object violations

Detects two related violations involving Protocol classes and class objects:

1. A Protocol class itself is passed/assigned where typeProto is expected. Only concrete (non-Protocol) subtypes may be used.

2. A class object is assigned to a variable typed as a Protocol instance, but the class does not structurally satisfy the protocol when treated as an object (i.e. class-level access to protocol members gives incompatible types).

class Proto(Protocol):
    def meth(self) -> int: ...

class Concrete:
    def meth(self) -> int: return 42

def fun(cls: type[Proto]) -> int:
    return cls().meth()

fun(Proto)      # E0146 — Protocol class itself passed to type[Proto]
fun(Concrete)   # OK

var: type[Proto]
var = Proto     # E0146 — Protocol class assigned to type[Proto]
var = Concrete  # OK

pa1: ProtoA1 = ConcreteA  # E0146 — class object can't satisfy instance protocol
pa2: ProtoA2 = ConcreteA  # OK    — protocol uses _self/self pattern

How to handle it

Every rule is on by default — strict is the default, not a cage. You can dial BSK-E0146 down per-file or per-path from your editor or pyproject.toml, or fix the code so it type-checks. See the Type System rules and the complete diagnostic reference.

Canonical URL: https://www.basilisk-python.dev/errors/BSK-E0146