inspect.signature() doesn't parse __text_signature__ containing a newline character

[vstinner]

BPO	41095
Nosy	@vstinner, @serhiy-storchaka, @1st1, @asottile
PRs	bpo-31938: Fix default-value signatures of several functions in the select module #21066

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

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-06-23.22:26:29.252>
labels = ['library', '3.10']
title = "inspect.signature() doesn't parse __text_signature__ containing a newline character"
updated_at = <Date 2020-06-24.01:12:14.804>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2020-06-24.01:12:14.804>
actor = 'Anthony Sottile'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-06-23.22:26:29.252>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 41095
keywords = ['patch']
message_count = 2.0
messages = ['372213', '372222']
nosy_count = 4.0
nosy_names = ['vstinner', 'serhiy.storchaka', 'yselivanov', 'Anthony Sottile']
pr_nums = ['21066']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue41095'
versions = ['Python 3.10']

• PR: gh-85267: Improvements to inspect.signature __text_signature__ handling #98796

• PR: [3.11] gh-85267: Improvements to inspect.signature __text_signature__ handling (GH-98796) #100392

• PR: [3.10] gh-85267: Improvements to inspect.signature __text_signature__ handling (GH-98796) #100393

[vstinner]

$ ./python
Python 3.10.0a0 (heads/unicode_latin1:40855c7064, Jun 24 2020, 00:20:07) 
>>> import select
>>> select.epoll.register.__text_signature__
'($self, /, fd,\n         eventmask=select.EPOLLIN | select.EPOLLPRI | select.EPOLLOUT)'

>>> import inspect
>>> inspect.signature(select.epoll.register)
<Signature (self, /, fd)>

=> eventmask parameter is gone!

Either signature() must raise an exception, or it must handle a __text_signature__ containing a newline character.

Issue spotted on bpo-31938 when fixing "./python -m pydoc select".

By the way, as expected, pydoc shows:

Help on method_descriptor in select.epoll:
---

$ ./python -m pydoc select.epoll.register

select.epoll.register = register(self, /, fd)
    Registers a new fd or raises an OSError if the fd is already registered.
(...)

---

[asottile]

Looking into this, it appears to be due to the default value and not due to the newline

I've stumbled upon two simplifications to the routines in inspect but not a fix for this

1. bpo-20684: Remove unused inspect._signature_get_bound_param #21100
2. gh-99815: remove unused 'invalid' sentinel value and code that checks for it in inspect.signature parsing #21104

the following code is hit because ast.literal_eval(...) fails for the |d expression:

cpython/Lib/inspect.py

Lines 2069 to 2070 in 2f9ada9

	if o is invalid:
	return None

this causes the parameter to be skipped entirely

[vstinner]

Sadly, I don't have the bandwidth to work on this issue, so I just close it.

[JelleZijlstra]

Reopening as there's an active PR fixing this.

[vstinner]

What is the active PR?

[JelleZijlstra]

#98796