Issue 17614: IDLE - quickly closing a large file triggers a traceback

Using the bigfile.py generation program from #17613 triggers the following traceback:

Traceback (most recent call last):
  File "/usr/bin/idle", line 5, in <module>
    main()
  File "/usr/lib/python3.3/idlelib/PyShell.py", line 1431, in main
    if flist.open(filename) is None:
  File "/usr/lib/python3.3/idlelib/FileList.py", line 36, in open
    edit = self.EditorWindow(self, filename, key)
  File "/usr/lib/python3.3/idlelib/PyShell.py", line 113, in __init__
    if self.io.filename: self.restore_file_breaks()
  File "/usr/lib/python3.3/idlelib/PyShell.py", line 229, in restore_file_breaks
    filename = self.io.filename
AttributeError: 'NoneType' object has no attribute 'filename'

I misworded the first sentence. It should have read: "Using bigfile.py as generated by the program in #17613 can trigger the following traceback when quickly closing the editor."

Other large files can be used to trigger the bug as well, like Lib/decimal.py.

Like #17613, the root cause is a call to .update() in Lib/idlelib/PyShell.py .restore_file_breaks().

The attached patch fixes the issue.

Is there a better way to reproduce the error in the first place.
On 2.7 tip,
./python -m idelib.idle
Opened and closed bigfile.py and decimal.py - and could not reproduce the error. If I could, I could have tested the patch.

I forgot to specify that the race condition gets triggered by pressing Alt-F4 the moment the IDLE editor window appears. 

I am reliably reproducing the error using the Konsole terminal under Arch Linux.

LGTM

Updated patch based on Ezio's comments. Thank you, Ezio!

Senthil, I wish I had a Mac to test against. I am able to trigger the bug on Win7 with 3.3.

Without objection, I will commit the patch in the upcoming hours.

New changeset 1e5e497ee33b by Roger Serwy in branch '2.7':
#17614: IDLE no longer raises exception when quickly closing a file.
http://hg.python.org/cpython/rev/1e5e497ee33b

New changeset d759b7bc9785 by Roger Serwy in branch '3.3':
#17614: IDLE no longer raises exception when quickly closing a file.
http://hg.python.org/cpython/rev/d759b7bc9785

New changeset 578edbe38f0a by Roger Serwy in branch 'default':
#17614: merge with 3.3.
http://hg.python.org/cpython/rev/578edbe38f0a

stage: patch review