Issue 19398: test_trace fails with -S

$ ./python -S -m test test_trace
[1/1] test_trace
test test_trace failed -- Traceback (most recent call last):
  File "/home/antoine/cpython/default/Lib/test/test_trace.py", line 313, in test_coverage
    self.assertTrue("pprint.cover" in files)
AssertionError: False is not true

This is the "emergency" patch to solve this issue.

The main issue is,

[sky@localhost cpython]$ ./python -Sc "import sys; print(sys.path)"
['', '/usr/local/lib/python34.zip', '/home/sky/Code/python/programming_language/cpython/Lib/', '/home/sky/Code/python/programming_language/cpython/Lib/plat-linux', '/home/sky/Code/python/programming_language/cpython/build/lib.linux-x86_64-3.4-pydebug']
[sky@localhost cpython]$ ./python -c "import sys; print(sys.path)"
['', '/usr/local/lib/python34.zip', '/home/sky/Code/python/programming_language/cpython/Lib', '/home/sky/Code/python/programming_language/cpython/Lib/plat-linux', '/home/sky/Code/python/programming_language/cpython/build/lib.linux-x86_64-3.4-pydebug', '/home/sky/.local/lib/python3.4/site-packages']

Let me "zoom" it for you.

[sky@localhost cpython]$ ./python -Sc "import sys; print(sys.path[2])"
/home/sky/Code/python/programming_language/cpython/Lib/
[sky@localhost cpython]$ ./python -c "import sys; print(sys.path[2])"
/home/sky/Code/python/programming_language/cpython/Lib

The "extra" slash is the culprit. I need to investigate whether this is deliberate or not.

I finally found the main culprit which puts extra '/' in my sys.path. The patch removed the extra '/' and also fixed this test automatically.

The test should use the more specific assertXyz method:
self.assertIn("pprint.cover", files)

The error message would then be the more informative
AssertionError: 'pprint.cover' not found in <files listed>

Attached the patch to accommodate Terry J. Reedy's request.

The separator is needed if the relative path is not empty and the prefix doesn't end with the separator.

This patch seems also fixes most problems of issue28655.

New changeset db220f2df5a9 by Serhiy Storchaka in branch '3.5':
Issue #19398: Extra slash no longer added to sys.path components in case of
https://hg.python.org/cpython/rev/db220f2df5a9

New changeset 1a88baaed7a0 by Serhiy Storchaka in branch '3.6':
Issue #19398: Extra slash no longer added to sys.path components in case of
https://hg.python.org/cpython/rev/1a88baaed7a0

New changeset 82607e7c24c7 by Serhiy Storchaka in branch 'default':
Issue #19398: Extra slash no longer added to sys.path components in case of
https://hg.python.org/cpython/rev/82607e7c24c7

New changeset 237ef36fb1bb by Serhiy Storchaka in branch '2.7':
Issue #19398: Extra slash no longer added to sys.path components in case of
https://hg.python.org/cpython/rev/237ef36fb1bb

nosy: + serhiy.storchaka
versions: + Python 3.5, Python 3.6, Python 3.7, - Python 3.4
messages: + msg280500

assignee: serhiy.storchaka

versions: + Python 2.7

messages: + msg201962

messages: + msg201314

nosy: + vajrasky
messages: + msg201313

keywords: + patch