sig.py

def sig(fn):
    code = fn.__code__
    defaults = fn.__defaults__
    kwdefaults = fn.__kwdefaults__
    if defaults is None:
        defaults = ()
    if kwdefaults is None:
        kwdefaults = {}

    pos, varnames = code.co_varnames[:code.co_argcount], code.co_varnames[code.co_argcount:]
    pos_only, pos_or_kw = pos[:code.co_posonlyargcount], pos[code.co_posonlyargcount:]
    kw_only, varnames = varnames[:code.co_kwonlyargcount], varnames[code.co_kwonlyargcount:]

    if code.co_flags & 4:
        var_pos, varnames = varnames[0], varnames[1:]
    else:
        var_pos = None

    if code.co_flags & 8:
        var_kw = varnames[0]
    else:
        var_kw = None

    pieces = []
    idx = len(defaults) - code.co_argcount
    for arg in pos_only:
        if idx >= 0:
            pieces.append(f"{arg}={defaults[idx]!r}")
        else:
            pieces.append(arg)
        idx += 1
    if pos_only:
        pieces.append("/")
    for arg in pos_or_kw:
        if idx >= 0:
            pieces.append(f"{arg}={defaults[idx]!r}")
        else:
            pieces.append(arg)
        idx += 1
    if var_pos is not None:
        pieces.append(f"*{var_pos}")
    elif kw_only:
        pieces.append("*")
    for kwarg in kw_only:
        if kwarg in kwdefaults:
            pieces.append(f"{kwarg}={kwdefaults[kwarg]!r}")
        else:
            pieces.append(kwarg)
    if var_kw is not None:
        pieces.append(f"**{var_kw}")

    return ", ".join(pieces)

if __name__ == "__main__":
    import inspect

    tests = [
        ", ".join([*pos_only, *pos_or_kw, *var_pos, *kw_only, *var_kw])
        for var_kw in [[], ["**h"]]
        for kw_only in [[], ["f", "g"], ["f", "g='g'"], ["f='f'", "g"], ["f='f'", "g='g'"]]
        for var_pos in [["*"] if kw_only else [], ["*e"]]
        for pos_or_kw in [[], ["c", "d"], ["c", "d='d'"], ["c='c'", "d='d'"]]
        for pos_only in [[], ["a", "b", "/"], *([["a", "b='b'", "/"], ["a='a'", "b='b'", "/"]] if "c='c'" in pos_or_kw else [])]
    ]

    for test in tests:
        fn = eval(f"lambda {test}: ...")
        assert inspect.signature(fn).format() == f"({test})"
        assert sig(fn) == test