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This morning, I was running an install of a package on Python 3.4.1 when I encountered this error:
C:\Users\jaraco\projects\jaraco.financial [default tip]> ./setup install
running install
running bdist_egg
running egg_info
writing namespace_packages to jaraco.financial.egg-info\namespace_packages.txt
writing jaraco.financial.egg-info\PKG-INFO
writing top-level names to jaraco.financial.egg-info\top_level.txt
writing requirements to jaraco.financial.egg-info\requires.txt
writing dependency_links to jaraco.financial.egg-info\dependency_links.txt
writing entry points to jaraco.financial.egg-info\entry_points.txt
writing manifest file 'jaraco.financial.egg-info\SOURCES.txt'
installing library code to build\bdist.win-amd64\egg
running install_lib
running build_py
copying jaraco\financial\records.py -> build\lib\jaraco\financial
creating build\bdist.win-amd64\egg
creating build\bdist.win-amd64\egg\jaraco
creating build\bdist.win-amd64\egg\jaraco\financial
copying build\lib\jaraco\financial\ledger.py -> build\bdist.win-amd64\egg\jaraco\financial
copying build\lib\jaraco\financial\qif.py -> build\bdist.win-amd64\egg\jaraco\financial
copying build\lib\jaraco\financial\merchant.py -> build\bdist.win-amd64\egg\jaraco\financial
Traceback (most recent call last):
 File "C:\Program Files\Python34\lib\distutils\dir_util.py", line 126, in copy_tree
 names = os.listdir(src)
OSError: [WinError 59] An unexpected network error occurred: 'build\\lib\\jaraco\\financial\\records.py'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "C:\Users\jaraco\projects\jaraco.financial\setup.py", line 72, in <module>
 setuptools.setup(**setup_params)
 File "C:\Program Files\Python34\lib\distutils\core.py", line 148, in setup
 dist.run_commands()
 File "C:\Program Files\Python34\lib\distutils\dist.py", line 955, in run_commands
 self.run_command(cmd)
 File "C:\Program Files\Python34\lib\distutils\dist.py", line 974, in run_command
 cmd_obj.run()
 File "C:\Program Files\Python34\lib\site-packages\setuptools-5.7-py3.4.egg\setuptools\command\install.py", line 67, in run
 File "C:\Program Files\Python34\lib\site-packages\setuptools-5.7-py3.4.egg\setuptools\command\install.py", line 109, in do_egg_install
 File "C:\Program Files\Python34\lib\distutils\cmd.py", line 313, in run_command
 self.distribution.run_command(command)
 File "C:\Program Files\Python34\lib\distutils\dist.py", line 974, in run_command
 cmd_obj.run()
 File "C:\Program Files\Python34\lib\site-packages\setuptools-5.7-py3.4.egg\setuptools\command\bdist_egg.py", line 161, in run
 File "C:\Program Files\Python34\lib\site-packages\setuptools-5.7-py3.4.egg\setuptools\command\bdist_egg.py", line 147, in call_command
 File "C:\Program Files\Python34\lib\distutils\cmd.py", line 313, in run_command
 self.distribution.run_command(command)
 File "C:\Program Files\Python34\lib\distutils\dist.py", line 974, in run_command
 cmd_obj.run()
 File "C:\Program Files\Python34\lib\site-packages\setuptools-5.7-py3.4.egg\setuptools\command\install_lib.py", line 10, in run
 File "C:\Program Files\Python34\lib\distutils\command\install_lib.py", line 111, in install
 outfiles = self.copy_tree(self.build_dir, self.install_dir)
 File "C:\Program Files\Python34\lib\site-packages\setuptools-5.7-py3.4.egg\setuptools\command\install_lib.py", line 38, in copy_tree
 File "C:\Program Files\Python34\lib\distutils\cmd.py", line 357, in copy_tree
 not self.force, dry_run=self.dry_run)
 File "C:\Program Files\Python34\lib\distutils\dir_util.py", line 160, in copy_tree
 verbose=verbose, dry_run=dry_run))
 File "C:\Program Files\Python34\lib\distutils\dir_util.py", line 160, in copy_tree
 verbose=verbose, dry_run=dry_run))
 File "C:\Program Files\Python34\lib\distutils\dir_util.py", line 160, in copy_tree
 verbose=verbose, dry_run=dry_run))
 File "C:\Program Files\Python34\lib\distutils\dir_util.py", line 128, in copy_tree
 (errno, errstr) = e
TypeError: 'OSError' object is not iterable
The original traceback indicates a network error occurred. The secondary error occurs when the error is assumed to be an iterable of two elements. I suspect this issue arises from the conversion of WindowsError to OSError or similar.
I plan to look at this in more depth later.

On a second attempt, the operation completed successfully, so a transient network error was necessary to elicit the bug.

I've started work on a test to capture the failure. Patch 7304b9 is attached. Any comments on the test before I proceed with the fix? I haven't been able to run the test, so if someone could do that, it would be most appreciated.

A similar issue was fixed a couple of days ago in file_util.py, with f01413758114. It would be nice to have similar tests.

I'm referring to the "TypeError: 'OSError' object is not iterable" failure.

Claudiu, thanks for the tip. That was quite helpful. I did not realize that unittest included a mock implementation. I've updated the test to use the same technique. I've furthermore verified that the test captures the failure and adjusted the implementation to match that expectation.

New changeset 7304b9b95438 by Jason R. Coombs in branch '3.4':
#22315: Add test to capture the failure.
http://hg.python.org/cpython/rev/7304b9b95438
New changeset 300cd36eb25c by Jason R. Coombs in branch '3.4':
#22315: Use technique outlined in test_file_util
http://hg.python.org/cpython/rev/300cd36eb25c
New changeset 3402813338db by Jason R. Coombs in branch '3.4':
#22315: Provide an actual directory during test invocation.
http://hg.python.org/cpython/rev/3402813338db
New changeset ddb0f90620b7 by Jason R. Coombs in branch '3.4':
#22315: Use an existent directory for 'src' to trigger appropriate behavior.
http://hg.python.org/cpython/rev/ddb0f90620b7
New changeset 75a5cc4ef31c by Jason R. Coombs in branch '3.4':
#22315: Use advertised API for OSError
http://hg.python.org/cpython/rev/75a5cc4ef31c
New changeset 640c575ab3e1 by Jason R. Coombs in branch 'default':
Merge with 3.4; Closes #22315
http://hg.python.org/cpython/rev/640c575ab3e1