Unexpected effect of sys.pycache_prefix = ""

[akulakov]

BPO	45977
Nosy	@tiran, @akulakov

^{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 2021-12-12.01:43:28.393>
created_at = <Date 2021-12-04.03:58:31.172>
labels = ['interpreter-core', 'type-bug', 'invalid', '3.11']
title = 'Unexpected effect of sys.pycache_prefix = ""'
updated_at = <Date 2021-12-12.01:43:28.392>
user = 'https://github.com/akulakov'

bugs.python.org fields:

activity = <Date 2021-12-12.01:43:28.392>
actor = 'andrei.avk'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-12.01:43:28.393>
closer = 'andrei.avk'
components = ['Interpreter Core']
creation = <Date 2021-12-04.03:58:31.172>
creator = 'andrei.avk'
dependencies = []
files = []
hgrepos = []
issue_num = 45977
keywords = []
message_count = 7.0
messages = ['407632', '407633', '407729', '407763', '407764', '407775', '408358']
nosy_count = 2.0
nosy_names = ['christian.heimes', 'andrei.avk']
pr_nums = []
priority = 'low'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45977'
versions = ['Python 3.11']

[akulakov]

Setting sys.pycache_prefix = "", re-creates CWD structure under current directory, e.g. if run from /Users/foo/test, will create /Users/foo/test/Users/foo/test/myfile.pyc . Is that intentional? It seems a little weird. At least it might be good to document it.

[akulakov]

(I forgot to mention this happens on MacOS).

[tiran]

It works as designed and documented. An empty string is considered a relative directory.

https://docs.python.org/3/library/sys.html#sys.pycache_prefix

If this is set (not None), Python will write bytecode-cache .pyc files to (and read them from) a parallel directory tree rooted at this directory...

A relative path is interpreted relative to the current working directory.

[akulakov]

If CWD=/a/b and pycache_prefix=c, the resulting dir is /a/b/c as I would expect.

If CWD=/a/b and pycache_prefix='', I would expect resulting dir to be /a/b
instead of /a/b/a/b .

For example as the shell cd command accepts relative path as arg, I would expect that cd c changes to /a/b/c ; but it would be very strange to expect cd with empty arg to change to /a/b/a/b .

[akulakov]

Can this also create the risk of 'path too long' issues?

[tiran]

The cache prefix mirrors the directory structure. Python takes the absolute path to the .py file, converts the path to a relative path by stripping off the leading slash, and finally joins the path with the prefix.

Take this example:

$ PYTHONPYCACHEPREFIX=/tmp/pycache python3.10 -c pass

This will create the cache file "/tmp/pycache/usr/lib64/python3.10/os.cpython-310.pyc" for the Python file "/usr/lib64/python3.10/os.py.

[akulakov]

Thanks, I've misunderstood how it works. It makes sense now. Closing as not a bug.