webbrowser._invoke opens /dev/null, never closes it and a warning is
printed.

I'm attaching a patch.
The diff looks messy, but I'm just wrapping the code in a try-finally
block, the rest is just indented.

Thanks.  Is this warning printed by the webbrowser unit tests?  If not can you see a way to add one that does?

The warning is printed by the file object when it closes itself in __del__:

  ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='r+' encoding='UTF-8'>

There isn't much to test, or is there?

To clarify, I discovered this when I was simply running webbrowser.open
in REPL.

Are there any webbrowser unit tests?

(this could probably use the new subprocess.DEVNULL constant in 3.3)

@Anton: That's what I was guessing.  If we had a unit test in test_webbrowser that did the same thing, we'd have seen the resource warning when running the tests and fixed it.  However, it looks like there aren't *any* tests for webbrowser, not even in test_sundry (which just makes sure modules without tests are importable).

So adding a test that will trigger this resource warning requires setting up a test_webbrowser file first, even before we get to the problem of how to test something that wants to start up a web browser...(but that should be solvable with unittest.mock, I think).

Adding a patch that uses subprocess.DEVNULL instead.

Writing tests for webbrowser should be a separate issue, right?

You could do it either way.  Normally we prefer to have a test along with any fix; in this case adding a test involves adding the test module as well, but it is not different in principle.  If you want to work on it and prefer to have it as a separate issue that's fine, we'll just make the test issue dependent on this one.

An updated patch with the same issue fixed in Konqueror class.

Added tests in #15557.

New changeset 901c790e4417 by R David Murray in branch 'default':
#15447: Use subprocess.DEVNULL in webbrowser, instead of opening
http://hg.python.org/cpython/rev/901c790e4417

New changeset 5da3b2df38b3 by R David Murray in branch 'default':
#15557,#15447,#15509: webbrowser test suite added.
http://hg.python.org/cpython/rev/5da3b2df38b3

Thanks, Anton.