tags 52269 patch
thanks

Hi!

The following patch fixes it.  I used site.addsitedir but ensured the
correct ordering of sys.path (we need to make the Guix-installed
packages appear before Python's own site-packages directory otherwise we
wouldn't be able to override its bundled packages such as 'pip').

Here's how I tested:

Copy the sitecustomize.py file to the current directory, then:

--8<---------------cut here---------------start------------->8---
$ pip --version
pip 21.1.3 from $HOME/.guix-profile/lib/python3.9/site-packages/pip (python 3.9)

$ guix show python-pip | recsel -p version
version: 20.2.4
--8<---------------cut here---------------end--------------->8---

Ensure installed pip still overrides Python's own.  PYTHONPATH=. forces
the sitecustomize.py file in the CWD to take precedence over the one
currently installed along Python.

--8<---------------cut here---------------start------------->8---
$ guix shell --pure python python-pip python-pdbpp
[env]$ PYTHONPATH=. python3 -c 'import pip; print(pip.__version__)'
20.2.4
--8<---------------cut here---------------end--------------->8---

Next I created a dummy script to trigger run pdb:

#file: test.py
print('hello')
import pdb; pdb.set_trace()

--8<---------------cut here---------------start------------->8---
$ guix shell --pure python python-pip python-pdbpp
[env]$ python3 test.py
hello
> /tmp/toto/test.py(7)<module>()
-> exit(1)
(Pdb)
--8<---------------cut here---------------end--------------->8---

This is the current bug; this is the regular Pdb, not Pdbpp.  Let's
force our revised sitecustomize.py file:

--8<---------------cut here---------------start------------->8---
$ PYTHONPATH=. python3 test.py
hello
[0] > /tmp/toto/test.py(7)<module>()
-> exit(1)
(Pdb++)
--8<---------------cut here---------------end--------------->8---

Better!