The following might be either a bug in the pydoc module or in the inspect module:

$ python
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pydoc
>>> pydoc.help(lambda (a,): lambda x: a)    # this fails
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/pydoc.py", line 1797, in __call__
    self.help(request)
  File "/usr/lib/python2.7/pydoc.py", line 1844, in help
    else: doc(request, 'Help on %s:')
  File "/usr/lib/python2.7/pydoc.py", line 1581, in doc
    pager(render_doc(thing, title, forceload))
  File "/usr/lib/python2.7/pydoc.py", line 1576, in render_doc
    return title % desc + '\n\n' + text.document(object, name)
  File "/usr/lib/python2.7/pydoc.py", line 363, in document
    if inspect.isroutine(object): return self.docroutine(*args)
  File "/usr/lib/python2.7/pydoc.py", line 1321, in docroutine
    args, varargs, varkw, defaults = inspect.getargspec(object)
  File "/usr/lib/python2.7/inspect.py", line 817, in getargspec
    args, varargs, varkw = getargs(func.func_code)
  File "/usr/lib/python2.7/inspect.py", line 791, in getargs
    args[i] = stack[0]
IndexError: list index out of range
>>> pydoc.help(lambda (a,): lambda x: x)    # this succeeds

>>>

Aspiring contributor here!

I believe I have narrowed this error down to the inspect module, specifically in the for loop beginning on line 759. 

There's a bit of "acrobatics" (to quote the code comments) that I'm trying to sift through, I've been using the getargspec method from the inspect module as well as print statements in the inspect module to narrow down the error. It definitely doesn't seem like the previously mentioned for loop is functioning properly, as the first item in the stack is being referenced when it is empty. Additionally, I'm utilizing the dis module to help make sense of the full code object to compare against the behavior in the for loop.

Yury, I've respectfully included you here as you're listed as the expert for the inspect module. I would really like to submit a patch for this, but may need a nudge in the right direction. I will update here with my progress.

Hi Emily!  Python 2.7 is in a maintenance-only mode at this point and mostly receives security-related patches.  The problem with accepting other kind of patches is that we want 2.7 to be super stable.

You can continue working on this bug anyways, once the patch is ready we'll have to ask Benjamin Peterson (Python 2.7 release manager) to review it.

I suspect the bug is that inspect.getargspec doesn't fully support tuple arguments unpacking (it's a rather obscure feature that we removed from Python 3).

Yury, thanks for the encouragement to continue on this patch. I think it will be a good exercise to dive a bit deeper into Python's bytecode and put some knowledge to use.

I believe that tuple argument unpacking is handled appropriately, but there is an even further edge case where a closure is introduced. In the following, inspect.getargspec works for the first bit of code, but the second fails, as 'a' is referenced outside of scope.

>>> dis.dis(lambda (a,): lambda x: x)
  1           0 LOAD_FAST                0 (.0)
              3 UNPACK_SEQUENCE          1
              6 STORE_FAST               1 (a)
              9 LOAD_CONST               1 (<code object <lambda> at 0x10087a130, file "<stdin>", line 1>)
             12 MAKE_FUNCTION            0
             15 RETURN_VALUE
>>> dis.dis(lambda (a,): lambda x: a)
  1           0 LOAD_FAST                0 (.0)
              3 UNPACK_SEQUENCE          1
              6 STORE_DEREF              0 (a)
              9 LOAD_CLOSURE             0 (a)
             12 BUILD_TUPLE              1
             15 LOAD_CONST               1 (<code object <lambda> at 0x10087a930, file "<stdin>", line 1>)
             18 MAKE_CLOSURE             0
             21 RETURN_VALUE

I'll keep poking at this and see where I get.

-- EM

Proposed patch fixes the issue.

New changeset a889c5524520 by Serhiy Storchaka in branch '2.7':
Issue #29354: Fixed inspect.getargs() for parameters which are cell
https://hg.python.org/cpython/rev/a889c5524520

You were on right way Emily. Sorry for crossing your road, but I already had a written patch.

Emily, if you have time and want to help us with the inspect module, we have many other unresolved issues.  I'd be happy to help you with code reviews etc.

Serhiy, thank you for the update. I looked over your patch to ensure that I understood your solution. I also appreciate your tests, as I was able to see other edge cases that I perhaps would not have thought of.

Yury, I'm more than happy to help! I've spent a decent amount of time learning about code objects, byte code, etc. and I'd love to put that knowledge to use. Let me know the best way to find these issues on the tracker or otherwise.

> Yury, I'm more than happy to help! I've spent a decent amount of time learning about code objects, byte code, etc. and I'd love to put that knowledge to use. Let me know the best way to find these issues on the tracker or otherwise.

Certainly!

There seem to be a lot of issues with inspect.findsource and inspect.getsource functions. Quick search results: #28121 and #26890 (and I know there are few more open issues).

For PEP 525 we need to add two new functions to the inspect module (I overlooked them and didn't add them to 3.6): inspect.getasyncgeneratorstate and inspect.getasyncgeneratorlocals (mirroring inspect.getgeneratorstate and inspect.getgeneratorlocals).

There are some issues that nobody even replied to, like #26577.  For those the protocol is usually to reproduce the problem first, and then make a patch/tests.  You can use advanced search on this tracker to find issues like that one.

If you are into async world, you can work on adding contextlib.asyncontextmanager decorator.  Although this one should first be discussed on the python-ideas mailing list.

Also, if you are into C programming, Victor Stinner, INADA Naoki and I are trying to optimize Python ceval loop from time to time (you knowledge of dis/opcodes can be helpful), so you can look for those kind of open issues too.