B006 (MutableArgumentDefault) should exempt parameters with immutable type annotations #572
Description
B006 aims to prevent bugs that arise from mutating the default value of a function’s parameter, with unintended effects on all future calls to that function. But for a parameter that’s annotated with an immutable abstract type such as Sequence[int], mypy already prevents the value from being mutated. So such parameters should be exempt from B006. For example:
from typing import Sequence
def f(a: Sequence[int] = [1, 2, 3]): # should not raise B006
for x in a:
print(x)This was suggested at PyCQA/flake8-bugbear#137 (comment), but wasn’t implemented due to flake8-bugbear lacking the machinery to accurately track typing imports. Since ruff has that now (#533), perhaps this is easy to implement in ruff.
Types that should be considered immutable:
object
collections.abc.Container[T]
collections.abc.Iterable[T]
collections.abc.Reversible[T]
collections.abc.Sized
collections.abc.Collection[T]
collections.abc.Sequence[T]
collections.abc.Set[T]
collections.abc.Mapping[K, V]
typing.Container[T]
typing.Iterable[T]
typing.Reversible[T]
typing.Sized
typing.Collection[T]
typing.Sequence[T]
typing.AbstractSet[T] # not typing.Set[T]
typing.Mapping[K, V]
typing.Optional[I] # where I is any of the above
I | None # where I is any of the above
typing.Annotated[I, …] # where I is any of the above