Explain how to switch to add native namespace

docs/developer-guide/index.md

@@ -24,4 +24,5 @@ You can help consolidate all of development documentation here, even if it is to
 develop-volto-add-ons-index
 create-a-distribution
 standardize-python-project-configuration
+native-namespace
 ```

docs/developer-guide/native-namespace.md

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+---
+myst:
+  html_meta:
+    "description": "How to convert a Python distribution from a pkg_resources namespace to a native namespace"
+    "property=og:description": "How to convert a Python distribution from a pkg_resources namespace to a native namespace"
+    "property=og:title": "Native namespace"
+    "keywords": "Plone 6, developer guide, native namespaces, pkg_resources, Python"
+---
+
+# Native namespace
+
+This chapter is guide for how to convert a Python distribution from a `pkg_resources` namespace to a native namespace.
+
+Python 3.3 added support for native namespaces.
+See [PEP 420](https://peps.python.org/pep-0420/) for more details.
+
+Plone has been using `pkg_resources`-style namespaces, but they are deprecated in `setuptools`.
+`setuptools` is planning to remove `pkg_resources`'s namespace support by the end of 2025.
+[PLIP 3928](https://github.com/plone/Products.CMFPlone/issues/3928) tracks the changes needed for Plone to adapt to the _new_ native namespaces.
+
+To convert a given Python distribution to use a native namespace, follow these steps.
+
+
+## Create maintenance branch
+
+```{note}
+This step is relevant only for Plone core packages.
+```
+
+```{tip}
+This part is only needed when the `main` or `master` branch is used on multiple versions of the Plone core development buildout.
+```
+
+If you haven't cloned the package's repository, then do so.
+
+```shell
+git clone git@github.com:plone/$package
+cd $package
+```
+
+Otherwise, pull the latest changes into your local repository.
+
+```shell
+git fetch -p
+git checkout main # or master
+git rebase
+```
+
+Find the last release tag, and create a branch for it.
+
+List all tags.
+
+```shell
+git for-each-ref --sort=taggerdate --format '%(tag)' refs/tags
+```
+
+Get the last tag's major number.
+
+```shell
+MAJOR=`git for-each-ref --sort=taggerdate --format '%(tag)' refs/tags | tail -n1 | cut -d"." -f1`
+```
+
+Create a branch for it.
+
+```shell
+git checkout -b $MAJOR.x
+```
+
+Push the newly created branch.
+
+```shell
+git push
+```
+
+
+## Update `buildout.coredev`
+
+```{note}
+This step is relevant only for Plone core packages.
+```
+
+Update `buildout.coredev`'s branch `6.1` to use the newly created branch.
+
+```shell
+cd buildout.coredev
+```
+
+Ensure you have the latest changes.
+
+```shell
+git fetch -p
+git checkout 6.1
+git rebase
+```
+
+Update the branch being used by the Python distribution.
+
+```shell
+sed -i "s/$package.git branch=master/$package.git branch=$MAJOR.x/" sources.cfg
+```
+
+Add the changes, commit, and push.
+
+```shell
+git add sources.cfg
+git commit -m"chore: use branch $MAJOR.x for $package"
+git push
+```
+
+Now check if you need to do the same on the `6.0` branch of `buildout.coredev`.
+
+```{tip}
+You can use this [handy table](https://jenkins.plone.org/roboto/branches) to know which branch is used by a given package for each Plone version.
+```
+
+```{tip}
+To lower the amount of builds in Jenkins, either do a few at a time or add a `[ci-skip]` on the commit message.
+```
+
+## Counts before change
+
+There is a risk when changing to the native namespaces that some files, or tests, might be left behind.
+To ensure all tests and files are kept after the switch, gather the counts before the change.
+
+```shell
+tox run -e test -- --list-tests | wc -l
+```
+
+```{note}
+Adapt to whichever way you are using to run the tests.
+The above is meant for repositories that follow `plone.meta` conventions.
+The `--list-tests` comes from `zope.testrunner`, if you are using that, but not `plone.meta`, although you can use that as well.
+```
+
+Create a distribution to get a listing of how many files are currently packaged.
+
+```shell
+rm -rf dist/
+uvx --from build pyproject-build
+```
+
+A `dist` folder is created with two archives in it, one each with the file suffix of `.tar.gz` and `.whl`.
+To get the count of files in them, run the following commands.
+
+```shell
+python -c "import glob; import tarfile; print(len(tarfile.open(glob.glob('dist/*.tar.gz')[0], 'r:gz').getnames()))"
+python -c "import glob; from zipfile import ZipFile; print(len(ZipFile(glob.glob('dist/*.whl')[0]).namelist()))"
+```
+
+Note these counts for later in the step {ref}`compare-counts-after-change`.
+
+
+## Build backend
+
+To ensure the package continues to build, verify that `setuptools` is defined as its build backend.
+For that, inspect the {file}`pyproject.toml`.
+It should have the following lines.
+
+```toml
+[build-system]
+requires = ["setuptools>=68.2,<80", "wheel"]
+```
+
+If they are not there, then add them, commit, and push the changes.
+
+
+## Convert to native namespace
+
+Use `plone.meta`'s `switch-to-pep420` script.
+
+```shell
+cd $package
+uvx --from plone.meta switch-to-pep420 --no-tests .
+```
+
+```{tip}
+This will also bump the version to a new major release.
+
+If the `main` or `master` branch is already an alpha version that is only used in Plone 6.2, you can specify that you don't want this version bump by adding the `--no-breaking` option.
+```
+
+
+## Update the test matrix
+
+```{note}
+This step is relevant only for Plone core packages.
+```
+
+Because the switch to the native namespace must be coordinated, all Python distributions need to be only for the same Plone version.
+In this case, it was decided to do it for the Plone 6.2 version.
+Thus, we need to ensure that the test matrix only tests against this Plone version.
+For that, update {file}`.meta.toml` with the following changes.
+
+```toml
+[tox]
+test_matrix = {"6.2" = ["*"]}
+```
+
+Update the scaffolding files with `plone.meta`.
+
+```shell
+uvx --from plone.meta config-package --branch current .
+```
+
+Review the changes and ensure all changes are sound.
+
+```{note}
+If the diff is quite big, then run `config-package` before all the changes.
+Get the result into a pull request, approved, and merged.
+Then do a follow up pull request to move it to a native namespace.
+```
+
+
+(compare-counts-after-change)=
+
+## Compare counts after change
+
+Get the list of tests, and compare this result to the earlier one to ensure the same test counts.
+
+```shell
+tox run -e test -- --list-tests | wc -l
+```
+
+Similarly, create the distribution files and compare the numbers with the previous run.
+
+```shell
+rm -rf dist/
+uvx --from build pyproject-build
+python -c "import glob; import tarfile; print(len(tarfile.open(glob.glob('dist/*.tar.gz')[0], 'r:gz').getnames()))"
+python -c "import glob; from zipfile import ZipFile; print(len(ZipFile(glob.glob('dist/*.whl')[0]).namelist()))"
+```
+
+It is okay if the numbers are slightly lower.
+For obvious reasons, the old source distribution will have one or more extra {file}`__init__.py` files.
+Both old distributions are expected to have an extra {file}`namespace_packages.txt` file and possibly an {file}`x-nspkg.pth` file.
+If you lose more than a few files though, something is wrong.
+
+If the numbers are close enough, review the changes once more, and push the branch with the changes for others to review it.