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function-call-in-default-argument (B008)#

Derived from the flake8-bugbear linter.

What it does#

Checks for function calls in default function arguments.

Why is this bad?#

Any function call that's used in a default argument will only be performed once, at definition time. The returned value will then be reused by all calls to the function, which can lead to unexpected behaviour.

Calls can be marked as an exception to this rule with the flake8-bugbear.extend-immutable-calls configuration option.

Arguments with immutable type annotations will be ignored by this rule. Types outside of the standard library can be marked as immutable with the flake8-bugbear.extend-immutable-calls configuration option as well.


def create_list() -> list[int]:
    return [1, 2, 3]

def mutable_default(arg: list[int] = create_list()) -> list[int]:
    return arg

Use instead:

def better(arg: list[int] | None = None) -> list[int]:
    if arg is None:
        arg = create_list()

    return arg

If the use of a singleton is intentional, assign the result call to a module-level variable, and use that variable in the default argument:

ERROR = ValueError("Hosts weren't successfully added")

def add_host(error: Exception = ERROR) -> None: