_posts: add post about python3 external change

Author: classabbyamp

_posts/2023-10-07-python3-external.md

@@ -0,0 +1,66 @@
+---
+title: Changes to Python 3 and pip on Void
+layout: post
+---
+
+Happy Pythonmas! It's October, which means it's Python 3 update season. This
+year, along with the usual large set of updates for Python packages, a safety
+feature for pip, the Python package manager, has been activated. To ensure that
+Python packages installed via XBPS and those installed via pip don't interfere
+with one another, the system-wide Python environment has been marked as
+"externally managed".
+
+If you try to use `pip3` or `pip3 --user` outside of a Python virtual environment,
+you may see this error that provides guidance on how to deploy a virtual
+environment suitable for use with pip:
+
+```
+This system-wide Python installation is managed by the Void Linux package
+manager, XBPS. Installation of Python packages from other sources is not
+normally allowed.
+
+To install a Python package not offered by Void Linux, consider using a virtual
+environment, e.g.:
+
+  python3 -m venv /path/to/venv
+  /path/to/venv/pip install <package>
+
+Appending the flag --system-site-packages to the first command will give the
+virtual environment access to any Python package installed via XBPS.
+
+Invoking python, pip, and executables installed by pip in /path/to/venv/bin
+should automatically use the virtual environment. Alternatively, source its
+activation script to add the environment to the command search path for a shell:
+
+  . /path/to/venv/activate
+
+After activation, running
+
+  deactivate
+
+will remove the environment from the search path without destroying it.
+
+The XBPS package python3-pipx provides pipx, a convenient tool to automatically
+manage virtual environments for individual Python applications.
+```
+
+You can read more about this change on Python's website in [PEP 668](https://peps.python.org/pep-0668/).
+
+To simplify the use of Void-based containers, all [Void container images](https://voidlinux.org/download/#containers)
+tagged `20231003R1` or later will explicitly ignore the "externally managed"
+marker. Containers based on these images will still be able to use pip to
+install Python packages in the container-wide environment.
+
+### Here Be Dragons
+
+If you really want to be able to install packages with pip in the system- or
+user-wide Python environment, there are several options, but beware: this can
+cause hard-to-debug issues with Python applications, or issues when updating with
+XBPS.
+
+1. Use pip's `--break-system-packages` flag. This only applies to the current invocation.
+2. Set pip's configuration: `pip3 config set install.break-system-packages True`.
+   This will apply to all future invocations.
+3. Add a `noextract=/usr/lib/python*/EXTERNALLY-MANAGED` rule to your
+   [XBPS configuration](https://man.voidlinux.org/xbps.d.5) and re-install the
+   `python3` package. This will apply to all future invocations.