BSK-E0129 error

Literal value assignment incompatibility

Detects two classes of Literal-related assignment errors inside function bodies:

1. **Literal0 vs LiteralFalse non-equivalence (PEP 586)**: Literal0 and LiteralFalse are distinct types despite 0 == False in Python. Assigning a Literal0-typed parameter to a LiteralFalse local (or vice versa) is a type error.

2. **Augmented assignment widens a Literal type**: a += 3 where a is typed Literal3, 4, 5 produces an int result, which is not assignable back to Literal3, 4, 5.

def func(a: Literal[0], b: Literal[False]):
    x1: Literal[False] = a  # E — int 0 ≠ bool False in Literal
    x2: Literal[0] = b      # E — bool False ≠ int 0 in Literal

def func2(a: Literal[3, 4, 5]):
    a += 3  # E — result type is `int`, not `Literal[3, 4, 5]`

How to handle it

Every rule is on by default — strict is the default, not a cage. You can dial BSK-E0129 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-E0129