inspect.getargs fails on some anonymous tuples

[taschini]

BPO	14611
Nosy	@ncoghlan, @bitdancer, @voidspace, @taschini, @ssanderson, @iritkatriel
Files	issue_14611.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 = <Date 2020-08-31.08:47:03.422>
created_at = <Date 2012-04-18.08:35:08.631>
labels = ['type-bug', 'library']
title = 'inspect.getargs fails on some anonymous tuples'
updated_at = <Date 2020-08-31.08:47:03.421>
user = 'https://github.com/taschini'

bugs.python.org fields:

activity = <Date 2020-08-31.08:47:03.421>
actor = 'corona10'
assignee = 'none'
closed = True
closed_date = <Date 2020-08-31.08:47:03.422>
closer = 'corona10'
components = ['Library (Lib)']
creation = <Date 2012-04-18.08:35:08.631>
creator = 'taschini'
dependencies = []
files = ['25269']
hgrepos = []
issue_num = 14611
keywords = ['patch', 'needs review']
message_count = 7.0
messages = ['158599', '158610', '158620', '158708', '252489', '252491', '375822']
nosy_count = 6.0
nosy_names = ['ncoghlan', 'r.david.murray', 'michael.foord', 'taschini', 'Scott Sanderson', 'iritkatriel']
pr_nums = []
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue14611'
versions = ['Python 2.7']

[taschini]

How to reproduce
----------------

Take the following two functions:

    >>> def f(l, (x, y)):
    ...    sup = max(u*x + v*y for u, v in l)
    ...    return ((u, v) for u, v in l if u*x + v*y == sup)

    >>> def g((x, y)):
    ...    def h():
    ...        return x + y
    ...    return h

Inspect.getargs will throw an exception on the former and return a wrong
result on the latter::

    >>> import inspect
    >>> inspect.getargs(f.__code__)
    Traceback (most recent call last):
    ...
    IndexError: list index out of range

    >>> inspect.getargs(g.__code__)
    Arguments(args=['h'], varargs=None, keywords=None)

# h is most definitely not an argument of g!

Analysis
--------

If you disassemble the two functions, you'll see that in both cases
the anonymous tuples are unpacked using STORE_DEREF::

    >>> import dis
    >>> dis.disassemble(f.__code__)
      1           0 LOAD_FAST                1 (.1)
                  3 UNPACK_SEQUENCE          2
                  6 STORE_DEREF              0 (x)
                  9 STORE_DEREF              2 (y)
    <BLANKLINE>
      2          12 LOAD_GLOBAL              0 (max)
    ...

    >>> dis.disassemble(g.__code__)
      1           0 LOAD_FAST                0 (.0)
                  3 UNPACK_SEQUENCE          2
                  6 STORE_DEREF              0 (x)
                  9 STORE_DEREF              1 (y)
    <BLANKLINE>
      2          12 LOAD_CLOSURE             1 (y)
                 15 LOAD_CLOSURE             0 (x)
                 18 BUILD_TUPLE              2
                 21 LOAD_CONST               1 (<code object h ...>)
                 24 MAKE_CLOSURE             0
                 27 STORE_FAST               3 (h)
    <BLANKLINE>
      4          30 LOAD_FAST                3 (h)
                 33 RETURN_VALUE        \

However, the implementation of inspect.getargs only looks for
UNPACK_TUPLE, UNPACK_SEQUENCE, STORE_FAST.

Notes
-----

The version of Python used is::

    >>> import sys
    >>> sys.version_info[:3]
    (2, 7, 3)

[bitdancer]

Formal parameter tuple unpacking was removed in Python3, so this is a Python2-only issue. Would you like to submit a patch for Python2?

[taschini]

I'll give it a try.

[taschini]

I think this should do.

inspect.getargs is now looking for STORE_DEREF besides STORE_FAST, and is making sure that the appropriate namespace (locals vs cell + free vars) is selected depending on the opcode.

The only changes to the test suite are three additional tests, based on the two examples above.

[ssanderson]

This issue is the root cause of at least two open issues in IPython:

ipython/ipython#8293
ipython/ipython#8205

Testing locally, the patch supplied here fixes both of those issues. Is there still work that needs to be done here?

[ssanderson]

Note also that a much simpler repro for this issue is:

inspect.getargs(((x for _ in [0]) for x in [0]).gi_code)

This triggers the same issue because the inner generator expression closes over the loop variable of the outer expression, which causes us to hit the STORE_DEREF case instead of the STORE_FAST case.

[iritkatriel]

I think this was fixed here:

3b23004

and in any case it's a 2.7-only issue, so can this ticket be closed?