inspect.signature(bytes.hex) raises ValueError "builtin has invalid signature"

[verhovsky]

BPO	43067
Nosy	@verhovsky

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-29.21:00:52.553>
labels = ['type-feature', 'library', '3.10']
title = 'inspect.signature(bytes.hex) raises ValueError "builtin has invalid signature"'
updated_at = <Date 2021-01-29.21:00:52.553>
user = 'https://github.com/verhovsky'

bugs.python.org fields:

activity = <Date 2021-01-29.21:00:52.553>
actor = 'boris'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-01-29.21:00:52.553>
creator = 'boris'
dependencies = []
files = []
hgrepos = []
issue_num = 43067
keywords = []
message_count = 1.0
messages = ['385940']
nosy_count = 1.0
nosy_names = ['boris']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43067'
versions = ['Python 3.10']

[verhovsky]

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

[JelleZijlstra]

I am looking at this because I realized it's preventing us from testing some signatures in typeshed.

I got this to work:

>>> import inspect
>>> inspect.signature(bytes.hex)
<Signature (self, /, sep=Ellipsis, bytes_per_sep=1)>

But it feels wrong to claim Ellipsis as the default, because that's not really true. A more principled solution would be to add a Parameter.unrepresentable marker and use that as the default, but that feels like a change we can make only in 3.13, not in the bugfix branches.

My leaning is to apply the fix suggested above in 3.13, and put some hack in stubtest (or other tools that want to inspect signatures) to make it work on earlier versions.

[JelleZijlstra]

This is basically @sobolevn's idea from https://discuss.python.org/t/ac-null-defaults-prevent-correct-signatures-lets-add-inspect-unrepresentable-to-fix-this/30753/6.

[sobolevn]

@serhiy-storchaka said that future inspect.signatures() is a better solution. But, it is in the works.