On Python 3.9

>>> import inspect
>>> inspect.signature(bytes.hex)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.9/inspect.py", line 3118, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
  File "/usr/lib/python3.9/inspect.py", line 2867, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File "/usr/lib/python3.9/inspect.py", line 2322, in _signature_from_callable
    return _signature_from_builtin(sigcls, obj,
  File "/usr/lib/python3.9/inspect.py", line 2135, in _signature_from_builtin
    return _signature_fromstr(cls, func, s, skip_bound_arg)
  File "/usr/lib/python3.9/inspect.py", line 2005, in _signature_fromstr
    raise ValueError("{!r} builtin has invalid signature".format(obj))
ValueError: <method 'hex' of 'bytes' objects> builtin has invalid signature


This is also true for memoryview.hex and bytearray.hex . This happens because

>>> bytes.hex.__text_signature__
'($self, /, sep=<unrepresentable>, bytes_per_sep=1)'

when it would make more sense for it to return

'($self, /, sep='', bytes_per_sep=1)'

or at least sep=None