The `site` module documentation says that in .pth files, "Blank lines and lines beginning with # are skipped.". However, the implementation does not actually skip blank lines. It then joins the empty string to the site-packages directory, resulting in the site-packages directory being added to sys.path a second time.

Example:

$ python -c 'import sys; print(sys.path)'
['', '/home/smith/.virtualenvs/foo/lib/python36.zip', '/home/smith/.virtualenvs/foo/lib/python3.6', '/home/smith/.virtualenvs/foo/lib/python3.6/lib-dynload', '/usr/lib/python3.6', '/home/smith/.virtualenvs/foo/lib/python3.6/site-packages']
$ echo > /home/smith/.virtualenvs/foo/lib/python3.6/site-packages/test.pth
$ python -c 'import sys; print(sys.path)'
['', '/home/smith/.virtualenvs/foo/lib/python36.zip', '/home/smith/.virtualenvs/foo/lib/python3.6', '/home/smith/.virtualenvs/foo/lib/python3.6/lib-dynload', '/usr/lib/python3.6', '/home/smith/.virtualenvs/foo/lib/python3.6/site-packages', '/home/smith/.virtualenvs/foo/lib/python3.6/site-packages']

A patch fixing this is attached to issue 29326, but was ignored when that issue turned out to be caused by something else.

If this is actually an issue, and not just a documentation lapse I can create a pull request from my original patch.

It's definitely a bug and not a documentation lapse. There's no reason why blank lines should have that effect, and no indication in the code that this was intentional.

I can take this and have a PR

Created a PR: GH-20679 

I did see some of the changes are already in the code for example, test_underpth_nosite_file.test_sity.py already had pth_lines filter out: ''.

I think it's because this issue relevant only for 3.6?
Or maybe this issue was solved already and this bug report is redundant

This is certainly a bug fix, but since it may break backwards-compatibility in delicate ways, I'm not going to backport it to earlier versions. Expect this to land in 3.10.

New changeset 0c71a66b53f6ea262aa5a60784c8c625ae0bb98c by idomic in branch 'master':
bpo-33689: Blank lines in .pth file cause a duplicate sys.path entry (GH-20679)
https://github.com/python/cpython/commit/0c71a66b53f6ea262aa5a60784c8c625ae0bb98c

Thanks for bringing this up again Malcolm, for the original Patch Ammar, and for the work on the PR Ido!