Running the test test_cmd_line with my current PYTHONSTARTUP set to the attached file breaks the test.

$ ./python.exe -m test test_cmd_line

Changing line 280 on Lib/test/test_cmd_line.py to include either -I or -E makes the error be different but still breaks the test.

adding a line with `del env['PYTHONSTARTUP']` before the subprocess works around the issue.

This leads me to believe that -E and -I aren't fully ignoring the environment variable.

Running with:
$ PYTHONSTARTUP= ./python.exe -m test test_cmd_line

Also works around the issue.

Ran the following to build python and run tests

$ hg clone https://hg.python.org/cpython
$ cd cpython
$ ./configure --with-pydebug && make -j4
$ ./python -m test -j4

Currently on revision: 2a18f6b85da2 2015-04-12 | [rdmurray] #23464: remove JoinableQueue that was deprecated in 3.4.4.

As pointed by ncoghlan, the test needs to (by definition) not use -E or -I, but PYTHONSTARTUP env var breaks the test, so the patch makes the env not have any pre-existing PYTHON* env vars.

Ideally this should be another type of helper in script_helper, no?

New changeset d0819a8fda1b by R David Murray in branch '3.4':
#23925: stop (eg) PYTHONSTARTUP from causing test_cmd_line failure.
https://hg.python.org/cpython/rev/d0819a8fda1b

New changeset 3a3dc86a5b6e by R David Murray in branch 'default':
Merge: #23925: stop (eg) PYTHONSTARTUP from causing test_cmd_line failure.
https://hg.python.org/cpython/rev/3a3dc86a5b6e

Thanks, Jamiel.  I tweaked the comment.  I don't think it is worth making this a helper, since the only time it is likely to be needed is in test_cmd_line.  If there are other tests in test_cmd_line that test PYTHON environment variables then it might be worth adding it as a helper in test_cmd_line.