Support positional-only arguments in the strict sense

[maread99]

Summary

Previously valimp ignored any / in a decorated function's signature, allowing arguments intended to be positional-only to be passed as keyword arguments.

valimp now enforces positional-only arguments in the strict sense: a positional-only argument can only be passed positionally. Passing it by keyword raises an InputsError, unless the signature provides for **kwargs, in which case the keyword input is absorbed by **kwargs — consistent with standard Python behaviour.

Implementation

• The new verification is implemented within validate_against_signature, which covers the other signature checks. It receives the list of positional-only parameter names and the list of those invalidly received as keyword arguments.
• inspect.signature is interrogated to identify positional-only parameters, since inspect.getfullargspec does not distinguish them (they are lumped into spec.args).
• In wrapped_f, any keyword input matching a positional-only parameter name is removed from kwargs up front: it never binds to the parameter. If the signature has no **kwargs it is reported via a dedicated "Got positional-only argument(s) passed as keyword argument(s): ..." error; otherwise it is absorbed into **kwargs (and a required positional-only parameter not also passed positionally is correctly reported as missing).
• The coerce/parse logic of the reconstruction loop has been factored into a new apply_metadata helper so that values absorbed by **kwargs under a positional-only name are validated, coerced and parsed consistently with other **kwargs inputs.

Behaviour examples

@parse
def f(a: int, b: str, /, c: int): ...

f(1, "x", 3)        # ok
f(1, "x", c=3)      # ok (c is positional-or-keyword)
f(a=1, b="x", c=3)  # InputsError: positional-only args passed as keyword

@parse
def g(a: int, /, **kwargs: int): ...

g(1, a=2, b=3)      # ok -> a=1, kwargs={'a': 2, 'b': 3}  (matches plain Python)
g(a=2)              # InputsError: missing positional argument 'a'

Documentation

The "does not currently support positional-only arguments" note has been removed from both the module docstring and the README.

Testing

• Added tests covering: passing positional-only args positionally (incl. coerce/parse), the single- and multiple-argument keyword-rejection errors, interaction with other signature errors, **kwargs absorption (with and without a default), the required-but-missing case, validation/coercion of absorbed values, and a decorated method.
• pytest: all 35 tests pass.
• ruff check / ruff format: clean.
• mypy: no new errors introduced (count is one lower than the pre-existing baseline).

https://claude.ai/code/session_015Ci5jP93WGZiN2UE3SXyoh

---

Generated by Claude Code