astral-sh/ruff · error

Missing argument `member`

Error message

Missing argument `member`

What it means

`generate_assert` handles `assertIn`/`assertNotIn` by emitting `assert member in container`. When the rewritten call's argument map lacks a `"member"` entry (the first required argument), the rewrite fails with this error and no fix is produced.

Source

Thrown at crates/ruff_linter/src/rules/flake8_pytest_style/rules/unittest_assert.rs:347

                    | UnittestAssert::FailUnlessEqual => CmpOp::Eq,
                    UnittestAssert::NotEqual
                    | UnittestAssert::NotEquals
                    | UnittestAssert::FailIfEqual => CmpOp::NotEq,
                    UnittestAssert::Greater => CmpOp::Gt,
                    UnittestAssert::GreaterEqual => CmpOp::GtE,
                    UnittestAssert::Less => CmpOp::Lt,
                    UnittestAssert::LessEqual => CmpOp::LtE,
                    UnittestAssert::Is => CmpOp::Is,
                    UnittestAssert::IsNot => CmpOp::IsNot,
                    _ => unreachable!(),
                };
                let expr = compare(first, cmp_op, second);
                Ok(assert(&expr, msg))
            }
            UnittestAssert::In | UnittestAssert::NotIn => {
                let member = args
                    .get("member")
                    .ok_or_else(|| anyhow!("Missing argument `member`"))?;
                let container = args
                    .get("container")
                    .ok_or_else(|| anyhow!("Missing argument `container`"))?;
                let msg = args.get("msg").copied();
                let cmp_op = if matches!(self, UnittestAssert::In) {
                    CmpOp::In
                } else {
                    CmpOp::NotIn
                };
                let expr = compare(member, cmp_op, container);
                Ok(assert(&expr, msg))
            }
            UnittestAssert::IsNone | UnittestAssert::IsNotNone => {
                let expr = args
                    .get("expr")
                    .ok_or_else(|| anyhow!("Missing argument `expr`"))?;
                let msg = args.get("msg").copied();
                let cmp_op = if matches!(self, UnittestAssert::IsNone) {

View on GitHub (pinned to 15f3fe6b15)

Solutions

1. Fix the call to pass the value first: `self.assertIn(item, collection)`.
2. Disable rule PT009 (`lint.ignore = ["PT009"]`) if unittest asserts should not be rewritten.
3. If editing Ruff, verify `arg_spec = ['member', 'container', 'msg']` and the positional/keyword collection in `args_map`.
4. Run `ruff check --no-fix` to pinpoint the exact call being rewritten.

Example fix

// before
self.assertIn(key='k', container=d)
// after
self.assertIn('k', d)

def valid_assert_in(call_args, call_kwargs):
    return len(call_args) >= 1 or 'member' in call_kwargs

def is_membership_assert(args: tuple, kwargs: dict) -> bool:
    return (len(args) >= 1 or 'member' in kwargs) and (len(args) >= 2 or 'container' in kwargs)

When it happens

Trigger: `UnittestAssert::In | NotIn.generate_assert` where the source `self.assertIn(...)` call supplied zero positional args or skipped `member` without a `member=` keyword (e.g. only `container`/`msg` given).

Common situations: Incomplete `assertIn()` calls in suites being bulk-fixed with PT009; dynamically defined methods named `assertIn` on non-unittest classes with different signatures.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Missing argument `first`
• Missing argument `second`
• Missing argument `container`
• Missing argument `obj`
• Missing argument `cls`

AI-assisted analysis of astral-sh/ruff@15f3fe6b15 (2026-09-05). Data as JSON: /api/errors/20ac082c8fb266cb. Report an issue: GitHub.