Update development mode docs

2 files changed, 131 insertions, 31 deletions

diff --git a/docs/userguide/development_mode.rst b/docs/userguide/development_mode.rst
index 4130ab73..01c4de03 100644
--- a/docs/userguide/development_mode.rst
+++ b/docs/userguide/development_mode.rst
@@ -1,34 +1,132 @@
-Development Mode
-================
-
-Under normal circumstances, the ``setuptools`` assume that you are going to
-build a distribution of your project, not use it in its "raw" or "unbuilt"
-form.  However, if you were to use the ``setuptools`` to build a distribution,
-you would have to rebuild and reinstall your project every time you made a
-change to it during development.
-
-Another problem that sometimes comes is that you may
-need to do development on two related projects at the same time.  You may need
-to put both projects' packages in the same directory to run them, but need to
-keep them separate for revision control purposes.  How can you do this?
-
-Setuptools allows you to deploy your projects for use in a common directory or
-staging area, but without copying any files.  Thus, you can edit each project's
-code in its checkout directory, and only need to run build commands when you
-change files that need to be compiled or the provided metadata and setuptools configuration.
-
-You can perform a ``pip`` installation passing the ``-e/--editable``
-flag (e.g., ``pip install -e .``). It works very similarly to
-``pip install .``, except that it doesn't actually install anything.
-Instead, it creates a special ``.egg-link`` file in the target directory
-(usually ``site-packages``) that links to your project's source code.
-It may also update an existing ``easy-install.pth`` file
-to include your project's source code, thereby making
-it available on ``sys.path`` for all programs using that Python installation.
-
-You can deploy the same project to multiple staging areas, e.g., if you have
-multiple projects on the same machine that are sharing the same project you're
-doing development work.
+Development Mode (a.k.a. "Editable Installs")
+=============================================
+
+When creating a Python project, developers usually want to implement and test
+changes iteratively, before cutting a release and preparing a distribution archive.
+
+In normal circumstances this can be quite cumbersome and require the developers
+to manipulate the ``PATHONPATH`` environment variable or to continuous re-build
+and re-install the project.
+
+To facilitate iterative exploration and experimentation, setuptools allows
+users to instruct the Python interpreter and its import machinery to load the
+code under development directly from the project folder without having to
+copy the files to a different location in the disk.
+This means that changes in the Python source code can immediately take place
+without requiring a new installation.
+
+You can enter this "development mode" by performing an :doc:`editable installation
+<pip:topics/local-project-installs>` inside of a :term:`virtual environment`,
+using :doc:`pip's <pip:cli/pip_install>` ``-e/--editable`` flag, as shown bellow:
+
+.. code-block:: bash
+
+   $ cd your-python-project
+   $ python -m venv .venv
+   # Activate your environemt with:
+   #      `source .venv/bin/activate` on Unix/macOS
+   # or   `.venv\Scripts\activate` on Windows
+
+   $ pip install --editable .
+
+   # Now you have access to your package
+   # as if it was installed in .venv
+   $ python -c "import your_python_project"
+
+
+An "editable installation" works very similarly to a regular install with
+``pip install .``, except that it only installs your package dependencies,
+metadata and wrappers for :ref:`console and GUI scripts <console-scripts>`.
+Under the hood, setuptools will try to create a special :mod:`.pth file <site>`
+in the target directory (usually ``site-packages``) that extends the
+``PYTHONPATH`` or install a custom :doc:`import hook <python:reference/import>`.
 
 When you're done with a given development task, you can simply uninstall your
 package (as you would normally do with ``pip uninstall <package name>``).
+
+Please note that, by default an editable install will expose at least all the
+files that would be available in a regular installation. However, depending on
+the file and directory organization in your project, it might also expose
+as a side effect files that would not be normally available.
+This is allowed so you can create iteratively create new Python modules.
+Please have a look on the following section if you are looking for a different behaviour.
+
+.. admonition:: Virtual Environments
+
+   You can think virtual environments as "isolated Python runtime deployments"
+   that allow users to install different sets of libraries and tools without
+   messing with the global behaviour of the system.
+
+   They are the safest way of testing new projects and can be created easily
+   with the :mod:`venv` module from the standard library.
+
+   Please note however that depending on your operating system or distribution,
+   ``venv`` might not come installed by default with Python. For those cases,
+   you might need to use the OS package manager to install it.
+   For example, in Debian/Ubuntu-based systems you can obtain it via:
+
+   .. code-block:: bash
+
+       sudo apt install python3-venv
+
+   Alternatively, you can also try installing :pypi:`virtualenᴠ`.
+   More information is available on the Python Packaging User Guide on
+   :doc:`PyPUG:guides/installing-using-pip-and-virtual-environments`.
+
+.. note::
+    .. versionchanged:: v63.0.0
+       Editable installation hooks implemented according to :pep:`660`.
+       Support for :pep:`namespace packages <420>` is still **EXPERIMENTAL**.
+
+
+"Strict" editable installs
+--------------------------
+
+When thinking about editable installations, users might have the following
+expectations:
+
+1. It should allow developers to create add new files and have them
+   automatically exposed.
+2. It should behave as close as possible to a regular installation and help
+   users to detect problems (e.g. new files not being included in the distribution).
+
+Unfortunately these expectations are in conflict with each other.
+To solve this problem ``setuptools`` allows developers to choose a more
+*"strict"* mode for the editable installation. This can be done by passing
+a special *configuration setting* via :pypi:`pip`, as indicated bellow:
+
+.. code-block:: bash
+
+    pip install -e . --config-settings editable_mode=strict
+
+In this mode, new files **won't** be exposed and the editable installs will
+try to mimic as much as possible the behavior of a regular install.
+Under the hood, ``setuptools`` will create a tree of file links in an auxiliary
+directory (``$your_project_dir/build``) and add it to ``PYTHONPATH`` via a
+:mod:`.pth file <site>`. (Please be careful to not delete this repository
+by mistake otherwise your files may stop being accessible).
+
+
+.. note::
+    .. versionadded:: v63.0.0
+       *Strict* mode implemented as **EXPERIMENTAL**.
+
+
+Limitations
+-----------
+
+- The *editable* term is used to refer only to Python modules
+  inside the package directories. Non-Python files, external (data) files,
+  executable script files, binary extensions, headers and metadata may be
+  exposed as a *snapshot* of the version they were at the moment of the
+  installation.
+- Adding new dependencies or entry-points to your project require
+  a fresh "editable" re-installation.
+- Console scripts and GUI scripts **MUST** be specified via entry-points
+  to work properly.
+- *Strict* editable installs require the file system to support
+  either :wiki:`symbolic <symbolic link>` or :wiki:`hard links <hard link>`.
+- Editable installations may not work with
+  :doc:`namespaces created with pkgutil or pkg_resouces
+  <PyPUG:guides/packaging-namespace-packages>`.
+  Please use :pep:`420`-style implicit namespaces.
diff --git a/docs/userguide/entry_point.rst b/docs/userguide/entry_point.rst
index eff20cf0..6ba00287 100644
--- a/docs/userguide/entry_point.rst
+++ b/docs/userguide/entry_point.rst
@@ -21,6 +21,8 @@ highlighting tool :pypi:`pygments` allows specifying additional styles
 using the entry point ``pygments.styles``.
 
 
+.. _console-scripts:
+
 Console Scripts
 ===============