Here is one way to reproduce this bug:

1. Create a module file (bug.py in this example)

def func():
    pass

2. Run Python

>>> import bug
>>> help(bug)

3. Edit bug.py

def func():
    pass

def newfunc():
    pass

4. Use the same Python interpreter as in step 2

>>> reload(bug)
>>> help(bug)

5. Observe traceback

  [..snip..]
  File "C:\Python27\lib\inspect.py", line 578, in findsource
    if pat.match(lines[lnum]): break
IndexError: list index out of range


Note: A related but different issue is http://bugs.python.org/issue1218234.

Can you explain what makes this one a different problem?  It looks like the same one to me.  Or is your intent in this issue just to avoid the exception?  In that case it seems to me it would better to fix issue 1218234 if we can.

Right, this issue is about the IndexError.

The current patch for issue 1218234 doesn't solve this problem. You will still get IndexErrors, but you will get them when functions are removed rather than when functions are added.

I think we should simply throw an IOError instead of an IndexError. This matches the docstring and prevents help() from blowing up after reloading a module.

This issue appears implicated in https://bitbucket.org/pypa/setuptools/issue/201

I'm unable to reproduce it now. Has it been fixed?

>>> import x
>>> help(x)
Help on module x:

NAME
    x

FUNCTIONS
    func()

FILE
    c:\users\user\src\cpython\x.py


>>> from importlib import reload
>>> reload(x)
<module 'x' from 'C:\\Users\\User\\src\\cpython\\x.py'>
>>> help(x)
Help on module x:

NAME
    x

FUNCTIONS
    func()

    newfunc()

FILE
    c:\users\user\src\cpython\x.py

Sorry, you're right - now the issue is when you remove functions from the module:

>>> reload(x)
<module 'x' from 'C:\\Users\\User\\src\\cpython\\x.py'>
>>> help(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\User\src\cpython\\lib\_sitebuiltins.py", line 103, in __call__
    return pydoc.help(*args, **kwds)
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 2000, in __call__
    self.help(request)
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 2059, in help
    else: doc(request, 'Help on %s:', output=self._output)
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 1779, in doc
    pager(render_doc(thing, title, forceload))
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 1772, in render_doc
    return title % desc + '\n\n' + renderer.document(object, name)
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 472, in document
    if inspect.ismodule(object): return self.docmodule(*args)
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 1273, in docmodule
    contents.append(self.document(value, key, name))
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 474, in document
    if inspect.isroutine(object): return self.docroutine(*args)
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 1510, in docroutine
    doc = getdoc(object) or ''
  File "C:\Users\User\src\cpython\lib\pydoc.py", line 187, in getdoc
    result = _getdoc(object) or inspect.getcomments(object)
  File "C:\Users\User\src\cpython\lib\inspect.py", line 882, in getcomments
    lines, lnum = findsource(object)
  File "C:\Users\User\src\cpython\lib\inspect.py", line 871, in findsource
    if pat.match(lines[lnum]): break
IndexError: list index out of range

New changeset 2e0760bb2edb595050aff82f236cd32b44d3dfb3 by Irit Katriel in branch 'master':
bpo-17735: inspect.findsource now raises OSError when co_lineno is out of range (GH-23633)
https://github.com/python/cpython/commit/2e0760bb2edb595050aff82f236cd32b44d3dfb3

New changeset a4e7d5f750e06e31a80a83c2af02b1a40cecd0ff by Miss Islington (bot) in branch '3.8':
bpo-17735: inspect.findsource now raises OSError when co_lineno is out of range (GH-23633)
https://github.com/python/cpython/commit/a4e7d5f750e06e31a80a83c2af02b1a40cecd0ff

New changeset d1f07419c7560ed3ba52ba4f667f4eec9b5fe95d by Miss Islington (bot) in branch '3.9':
bpo-17735: inspect.findsource now raises OSError when co_lineno is out of range (GH-23633)
https://github.com/python/cpython/commit/d1f07419c7560ed3ba52ba4f667f4eec9b5fe95d

Thanks for reporting this, Kyle!

Thanks for the PR, Irit!