Blank lines in .pth file cause a duplicate sys.path entry

[mhsmith]

BPO	33689
Nosy	@taleinat, @mhsmith, @ammaraskar, @idomic
PRs	bpo-33689: Blank lines in .pth file cause a duplicate sys.path entry #20679

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-09-19.19:14:49.996>
created_at = <Date 2018-05-29.23:03:14.459>
labels = ['type-bug', 'library', '3.10']
title = 'Blank lines in .pth file cause a duplicate sys.path entry'
updated_at = <Date 2020-09-19.19:14:49.993>
user = 'https://github.com/mhsmith'

bugs.python.org fields:

activity = <Date 2020-09-19.19:14:49.993>
actor = 'taleinat'
assignee = 'none'
closed = True
closed_date = <Date 2020-09-19.19:14:49.996>
closer = 'taleinat'
components = ['Library (Lib)']
creation = <Date 2018-05-29.23:03:14.459>
creator = 'Malcolm Smith'
dependencies = []
files = []
hgrepos = []
issue_num = 33689
keywords = ['patch']
message_count = 8.0
messages = ['318137', '318201', '364185', '364235', '370824', '377181', '377183', '377184']
nosy_count = 4.0
nosy_names = ['taleinat', 'Malcolm Smith', 'ammar2', 'Ido Michael']
pr_nums = ['20679']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33689'
versions = ['Python 3.10']

[mhsmith]

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 bpo-29326, but was ignored when that issue turned out to be caused by something else.

[ammaraskar]

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

[mhsmith]

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.

[idomic]

I can take this and have a PR

[idomic]

Created a PR: #64878

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

[taleinat]

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.

[taleinat]

New changeset 0c71a66 by idomic in branch 'master':
bpo-33689: Blank lines in .pth file cause a duplicate sys.path entry (GH-20679)
0c71a66

[taleinat]

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