usercustomize.py starting with print statement crashes tox env creation with sitepackages = True

[Cielquan]

Hi,
I found a bug but I'm not sure if tox or maybe virtualenv is guilty. I use python 3.8 but I'm sure the bug will also happen in other versions.

I started to use the site module of python because I found the devtools package and in the docs of it the site module was mentioned as a way to globally import the toolset or more specifically the debug() function which then can be used like print(). Instead of a sitecustomize.py file I use a usercustomize.py file. I don't know if this makes a difference but I don't think so.

working usercustomize.py file:

try:
    from devtools import debug
except ImportError:
    pass
else:
    __builtins__['debug'] = debug

I use tox to setup my dev venv and to be able to use this 'global import' of debug() I set sitepackages = True for the dev venv which in itself works but can create another bug #1570.

For information I added print('### Load User Customization') at the top of the usercustomize.py file which triggers the bug. You can see that at the 2nd line starting with ERROR: in the error output below. Somehow the output of the print gets appended to the path for the sys.prefix.

To recreate the bug you only need two files:

An usercustomize.py file at the right place with a print() statement printing something except an empty string. To find the right place call python -m site. The second last line starting with USER_SITE shows the path were to put the file.

Second you need a tox.ini (simplified to the issue):

[tox]
skipsdist = True

[testenv:dev]
basepython = python3.8
sitepackages = True

Then you can create the error message:

$ tox -rvv -e dev

using tox.ini: /home/krys/Python_Projects/.testing/tox.ini (pid 112413)
  removing /home/krys/Python_Projects/.testing/.tox/log
using tox-3.14.6 from /home/krys/.venvs/tox/lib/python3.8/site-packages/tox/__init__.py (pid 112413)
skipping sdist step
dev start: getenv /home/krys/Python_Projects/.testing/.tox/dev
dev cannot reuse: -r flag
dev create: /home/krys/Python_Projects/.testing/.tox/dev
dev uses /home/krys/.venvs/tox/bin/python3.8
  removing /home/krys/Python_Projects/.testing/.tox/dev
setting PATH=/home/krys/Python_Projects/.testing/.tox/dev/bin:/home/krys/bin:/home/krys/.local/bin:/home/krys/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
[112417] /home/krys/Python_Projects/.testing/.tox$ /home/krys/.venvs/tox/bin/python3.8 -m virtualenv --system-site-packages --no-download --python /home/krys/.venvs/tox/bin/python3.8 dev
Already using interpreter /home/krys/.venvs/tox/bin/python3.8
Using base prefix '/usr'
New python executable in /home/krys/Python_Projects/.testing/.tox/dev/bin/python3.8
Also creating executable in /home/krys/Python_Projects/.testing/.tox/dev/bin/python
ERROR: The executable /home/krys/Python_Projects/.testing/.tox/dev/bin/python3.8 is not functioning
ERROR: It thinks sys.prefix is '/home/krys/Python_Projects/.testing/.tox/dev### Load User Customization' (should be '/home/krys/Python_Projects/.testing/.tox/dev')
ERROR: virtualenv is not compatible with this system or executable
ERROR: invocation failed (exit code 100)
ERROR: InvocationError for command /home/krys/.venvs/tox/bin/python3.8 -m virtualenv --system-site-packages --no-download --python /home/krys/.venvs/tox/bin/python3.8 dev (exited with code 100)
dev finish: getenv /home/krys/Python_Projects/.testing/.tox/dev after 0.09 seconds
___________________________________ summary ____________________________________
ERROR:   dev: InvocationError for command /home/krys/.venvs/tox/bin/python3.8 -m virtualenv --system-site-packages --no-download --python /home/krys/.venvs/tox/bin/python3.8 dev (exited with code 100)
*** Exit Code: 1 ***

[gaborbernat]

This should no longer be the case with tox version 4 and virtualenv 20 and later.