inspect.signature() raises RuntimeError on failed to resolve the default argument value

[tk0miya]

BPO	43118
Nosy	@gvanrossum, @hongweipeng, @miss-islington, @tk0miya
PRs	bpo-43118: in inspect.signature,use 'base' to help parse 'base.__text_signature__' #30285 [3.10] bpo-43118: Fix bug in inspect.signature around 'base.__text_signature__' (GH-30285) #30765 [3.9] bpo-43118: Fix bug in inspect.signature around 'base.__text_signature__' (GH-30285) #30766
Files	inspect.patch

^{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 2021-02-03.15:04:12.206>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'inspect.signature() raises RuntimeError on failed to resolve the default argument value'
updated_at = <Date 2022-01-22.17:28:57.262>
user = 'https://github.com/tk0miya'

bugs.python.org fields:

activity = <Date 2022-01-22.17:28:57.262>
actor = 'gvanrossum'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-02-03.15:04:12.206>
creator = 'tkomiya'
dependencies = []
files = ['49788']
hgrepos = []
issue_num = 43118
keywords = ['patch']
message_count = 5.0
messages = ['386212', '386213', '411188', '411196', '411268']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'hongweipeng', 'miss-islington', 'tkomiya']
pr_nums = ['30285', '30765', '30766']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43118'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[tk0miya]

inspect.signature() raises RuntimeError on failed to resolve the default argument value. For example, it fails to inspect a subclass of io.BufferedReader:

Example:

import inspect
import io


class MyBufferedReader(io.BufferedReader):
 """buffer reader class."""


inspect.signature(MyBufferedReader)

Result:

Traceback (most recent call last):
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2042, in wrap_value
 value = eval(s, module_dict)
 File "<string>", line 1, in <module>
NameError: name 'DEFAULT_BUFFER_SIZE' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2045, in wrap_value
 value = eval(s, sys_module_dict)
 File "<string>", line 1, in <module>
NameError: name 'DEFAULT_BUFFER_SIZE' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "/Users/tkomiya/work/tmp/doc/example.py", line 9, in <module>
 inspect.signature(MyBufferedReader)
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 3130, in signature
 return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2879, in from_callable
 return _signature_from_callable(obj, sigcls=cls,
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2397, in _signature_from_callable
 return _signature_fromstr(sigcls, obj, text_sig)
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2095, in _signature_fromstr
 p(name, default)
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2077, in p
 default_node = RewriteSymbolics().visit(default_node)
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/ast.py", line 407, in visit
 return visitor(node)
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2069, in visit_Name
 return wrap_value(node.id)
 File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2047, in wrap_value
 raise RuntimeError()
RuntimeError

In my investigation, inspect.signature() tries to evaluate the default argument value of the class constructor. But it fails to evaluate because of the 2nd argument of the constructor takes a constant; DEFAULT_BUFFER_SIZE, but it is not found on the current context.

I think it would be better to search the constants for the modules of the base classes. I just made a simple patch to resolve this bug.

[tk0miya]

FWIW, I succeeded to inspect the class with importing the constant from the base module as a workaround:

import inspect
import io

from io import DEFAULT_BUFFER_SIZE


class MyBufferedReader(io.BufferedReader):
 """buffer reader class."""


inspect.signature(MyBufferedReader)