Problem

basic_collation_fn in src/alpamayo_r1/processor/qwen_processor.py uses a mutable list as the default for unstackable_keys:

def basic_collation_fn(batch, unstackable_keys: list[str] = []):

Mutable default arguments are a well-known Python pitfall — the empty list is created once at function-definition time and reused across every call, so any mutation persists across invocations. Even though the current body only iterates over the argument (no mutation today), this pattern is brittle to future edits and is flagged by ruff B006 / pylint W0102.

Fix

Switch to the standard list[str] | None = None pattern and treat None as "no extra unstackable keys":

def basic_collation_fn(batch, unstackable_keys: list[str] | None = None):
    ...
    for k in unstackable_keys or []:
        ...

Two-line change. Behavior is identical for every call site (the default is still effectively an empty iterable). The function is exposed at module level, so callers passing an explicit list continue to work unchanged.

Verification

• unstackable_keys=None → None or [] == [] → loop is skipped (matches old behavior).
• unstackable_keys=["foo"] → loop runs over ["foo"] (matches old behavior).
• unstackable_keys=[] → loop is skipped (matches old behavior).