inspect.getargvalues fails if arg name is not bound to a value

[AndrewLutomirski]

BPO	17246
Nosy	@ezio-melotti, @iritkatriel, @macgors
Files	test_cgitb.py: Short script demonstrating the problem issue17246.diff: Proof of concept against 2.7.

^{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 2013-02-19.22:22:36.701>
labels = ['easy', 'type-bug', '3.9', '3.10', '3.11', 'library']
title = 'inspect.getargvalues fails if arg name is not bound to a value'
updated_at = <Date 2022-03-29.18:46:57.608>
user = 'https://bugs.python.org/AndrewLutomirski'

bugs.python.org fields:

activity = <Date 2022-03-29.18:46:57.608>
actor = 'macgors'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2013-02-19.22:22:36.701>
creator = 'Andrew.Lutomirski'
dependencies = []
files = ['29125', '29166']
hgrepos = []
issue_num = 17246
keywords = ['easy']
message_count = 4.0
messages = ['182446', '182705', '396039', '416283']
nosy_count = 4.0
nosy_names = ['ezio.melotti', 'Andrew.Lutomirski', 'iritkatriel', 'macgors']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue17246'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Linked PRs

• gh-61448: Fix inspect.getargvalues fails if arg name is not bound to a value #101318

[AndrewLutomirski]

inspect.formatargvalues assumes (incorrectly) that every argument in args is a key in values. This isn't very hard to break -- see the attachment for a complete example.

[ezio-melotti]

The bug seems to be in inspect indeed:

import inspect
def fun(x):
    del x
    return inspect.currentframe()

inspect.formatargvalues(*inspect.getargvalues(fun(10)))

Attached a proof-of-concept patch that replaces the missing value with <deleted>. If the approach is ok, the same fix might have to be applied to other functions and/or other types of arguments.

[iritkatriel]

Still the same in 3.11:

>>> import inspect
>>> def fun(x):
...     del x
...     return inspect.currentframe()
...
>>> inspect.formatargvalues(*inspect.getargvalues(fun(10)))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\User\src\cpython-dev\lib\inspect.py", line 1444, in formatargvalues
    specs.append(convert(args[i]))
  File "C:\Users\User\src\cpython-dev\lib\inspect.py", line 1441, in convert
    return formatarg(name) + formatvalue(locals[name])
KeyError: 'x'
>>>

[macgors]

In my opinion formatargvalues function is quite useless in current day as ArgInfo provides all the information. I'm however willing to fix this. IMO we should just omit the args/varargs/kwargs that have been deleted, since the values do not exist in the current frame anymore.

I can implement the idea propossed in 2013, but I feel this seems less intuative(?) and might be worse for backwards compatibility (if anyone ever actually expects some values from formatargvalues, but I haven't been able to find an example of that).

Thank you in advance for any input.

[furkanonder]

The PR is ready for review. @iritkatriel @ezio-melotti