Support an allowlist of function calls in argument defaults for B008

[rouge8]

It would be great if B008 supported an allowlist of function calls, similar to extend-immutable-calls for B006. We do a lot of this in our code which as far as I know has no subtle bugs:

def f(x=Decimal("1000")):
    pass

Or maybe it's worth hardcoding Decimal as an exception?

[charliermarsh]

Yeah perhaps we add that to the list of IMMUTABLE_FUNCS in crates/ruff/src/rules/flake8_bugbear/rules/function_call_argument_default.rs.

[JonathanPlasse]

It could be a good first issue.

[rouge8]

Yeah perhaps we add that to the list of IMMUTABLE_FUNCS in crates/ruff/src/rules/flake8_bugbear/rules/function_call_argument_default.rs.

I think it's probably worth doing that and adding an allowlist. Looking at our codebase, we have some custom functions we'd want to allow while still benefiting from flagging this in general.

[rouge8]

Okay, I opened #3764 adding a few more immutable functions. I can work on another PR adding a setting if you're open to that!

[rouge8]

Oh wait, reading the code, it looks like B008 might already support extend-immutable-calls?

[charliermarsh]

Oh wait, you're right...

[rouge8]

Hmm and I don't see it used in https://github.com/charliermarsh/ruff/blob/main/crates/ruff/src/rules/flake8_bugbear/rules/mutable_argument_default.rs ? Is the doc saying it's used for B006 wrong and it's actually used for B008?

[charliermarsh]

Yeah something's a bit off between these.

[aberres]

@rouge8 pathlib.Path is another candidate. See https://peps.python.org/pep-0428/#immutability

[rouge8]

@rouge8 pathlib.Path is another candidate. See https://peps.python.org/pep-0428/#immutability

Good catch! Added it in #3794

[JonathanPlasse]

Why not (Pure)(Posix|Windows)Path too?

[Apakottur]

So at this point should it be possible to use extend-immutable-calls for B008?
I have the following code:

# test.py
from typing import TypeAlias

MyTypeAlias: TypeAlias = str

def func(
    x: str = str("default"),
    y: str = MyTypeAlias("default"),
):
    return x + y

# ruff.toml
[flake8-bugbear]
extend-immutable-calls = ["MyTypeAlias"]

And running ruff check --config ruff.toml --select B008 test.py still gives me:

test.py:9:14: B008 Do not perform function call `MyTypeAlias` in argument defaults
Found 1 error.

[charliermarsh]

It should work for B008 if you use a fully-qualified path to the symbol, rather than the string name. For example:

[flake8-bugbear]
extend-immutable-calls = ["path.to.module.MyTypeAlias"]

Unfortunately, that won't work for symbols defined in the file in which they're used -- they have to be imported from another file. It's just a limitation which we'll resolve eventually (#5486).

[charliermarsh]

Unrelatedly, looking back at this issue, I see a few touch-ups we can do:

• B008 could ignore arguments with immutable annotations.
• B006 could consider extend_immutable_calls when determining whether an annotation is immutable.

[zanieb]

B008 already supports this so I'm going to close this issue.

I've also added the improvements that Charlie suggested in #6781 and #6784