docs: Add guidelines for public interface management

These guidelines sets out some approaches for managing our public
interface, it presents options for integrating code related to breaking
changes into the main branch while still allowing control as to when
those changes are released to users.

Happy for any suggestions, there may be better options or problems with
these options that I'm overlooking.

docs/index.rst

@@ -84,6 +84,7 @@ For developers
    :maxdepth: 1
 
    developers
+   interface_management
    CODE_OF_CONDUCT
    docs
    persisting_n3_terms

docs/interface_guidelines.rst

@@ -0,0 +1,240 @@
+Public interface guidelines
+===========================
+
+Overview
+--------
+
+This document provides guidelines for the management of RDFLib's public
+interface.
+
+These guidelines attempt to balance the following:
+
+* The need for stable public interface that do not frequently change in ways that are
+  backwards incompatible.
+* The need for modern, easy to use, well-designed public interface free of defects.
+* The need to change the public interface in a backwards incompatible way to
+  modernize it, eliminate defects, and improve usability and design.
+
+Versioning
+----------
+
+RDFLib follows semantic versioning [SEMVER]_, which can be summarized as:
+
+   Given a version number MAJOR.MINOR.PATCH, increment the:
+
+   #. MAJOR version when you make incompatible API changes
+   #. MINOR version when you add functionality in a backwards compatible
+      manner
+   #. PATCH version when you make backwards compatible bug fixes
+
+   Additional labels for pre-release and build metadata are available as
+   extensions to the MAJOR.MINOR.PATCH format.
+
+Guidelines for backwards compatible changes
+---------------------------------------------
+
+Examples of backwards compatible changes include:
+
+* Adding entirely new modules, functions, variables, or classes.
+* Adding new parameters to existing functions with defaults that maintains the
+  existing behavior of the function.
+
+Non-breaking changes can be made directly in the main RDFLib module, i.e.
+:mod:`rdflib`.
+
+In some cases it may be appropriate to place new identifiers under a
+``_provisional`` module (see `Provisional Python modules`_), especially if they
+provide complex functionality, this gives the users of RDFLib some opportunity
+to try them in non-critical settings while still allowing RDFLib developers to
+change the behavior before committing to them and making them part of the public
+interface.
+
+Guidelines for backwards incompatible changes
+---------------------------------------------
+
+Examples of backwards incompatible changes include:
+
+* Altering a function to behave differently in a way that cannot be considered a
+  bug fix, some more specific examples:
+
+  * Changing the return type to an incompatible type.
+  * Changing the supported argument types to incompatible types.
+  * Changing the required arguments.
+* Changing the class hierarchy so that class relations that held previously no
+  longer holds.
+
+When backwards incompatible changes will be introduced users should be provided
+with at least one release advanced notice, and, if possible, users should be
+given the opportunity to adapt their code to the new behavior before the old
+behavior is removed.
+
+At the same time it is preferable to integrate code that implements the new
+behavior of backwards incompatible changes into the main branch of RDFLib as
+soon as possible, so that long-lived pull requests or branches can be avoided.
+
+This section details various options for integrating code that implements
+backwards incompatible changes into the main branch while allowing maintainers
+explicit choice in when the backwards incompatible changes are released to users
+and allowing users of RDFLib the opportunity to opt into the new behavior.
+
+Simple deprecation
+^^^^^^^^^^^^^^^^^^
+
+In the simplest case if a function, class, or variable will be removed or
+replaced with a new function with a different name, the old identifier should be
+marked as deprecated and the deprecation notice should indicate the new function
+that should be used instead.
+
+For functions `warnings.warn` should be used with `DeprecationWarning`, and for
+variables and classes the `Sphinx deprecated directive
+<https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-deprecated>`_
+should be used.
+
+When deprecating whole classes a deprecation warning should be raised from
+`object.__init__` or `object.__new__`.
+
+The deprecation notice should indicate in what version of RDFLib the identifiers
+will be removed, and in most cases this should be the next major version.
+
+Release preparation for major versions of RDFLib should remove all code that was
+marked as deprecated for removal in the new major version.
+
+Use versioned top level module
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In some cases critical parts of the class hierarchy like `rdflib.graph.Graph`
+and `rdflib.term.Node` must be changed. For changes like this versioned top
+level modules can be used (see `Top-level Python modules`_).
+
+For example, if a new `rdflib.graph.Graph` class hierarchy needs to be created
+for RDFLib 7, the process would be:
+
+* Some time before releasing RDFLib 7:
+
+  * Create ``rdflib.v7.graph.Graph``.
+  * Mark `rdflib.graph.Graph` as deprecated.
+* When releasing RDFLib 7:
+
+  * Remove `rdflib.graph.Graph`.
+  * Import ``rdflib.v7.graph.Graph`` to `rdflib.graph.Graph`
+
+With this approach users can start using the new class before RDFLib 7 is
+released, and the class does not have to be kept in a separate branch or pull
+request until RDFLib 7 is ready for release. 
+
+Some care must be taken to avoid duplicated code, some options available to do
+this is:
+
+* Move shared functionality out to another class or function that can be used
+  from the old and new classes.
+* Import the old class and override the needed/relevant functionality.
+
+Release preparation for major versions should import the identifiers from the
+versioned python modules (e.g. :mod:`rdflib.v7`) into the main python module
+(i.e. :mod:`rdflib`) and remove the identifiers that were marked as deprecated.
+
+Use the ``rdflib._version._ENABLE_V{major_version}`` flags
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+With each released of RDFLib the corresponding
+``rdflib._version._ENABLE_V{major_version}`` variable will be set to `True`.
+
+These variables can be used to select default values or change other behavior of
+functions, for example changing the default format for `rdflib.graph.Graph.serialize`.
+
+Boolean flags are used as they can be used with the ``always_true`` and
+``always_false`` `directives from mypy
+<https://mypy.readthedocs.io/en/stable/config_file.html#confval-always_true>`_.
+
+Python modules
+--------------
+
+This section describes Python modules that are relevant to the management of
+RDFLib's public interface.
+
+Top-level Python modules
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+:mod:`rdflib`, the main Python module:
+    This is the main Python module for RDFLib and represents the public
+    interface of the current version of RDFLib.
+
+``rdflib{major_version}`` (e.g. :mod:`rdflib.v7`), versioned modules:
+    These modules are for parts of RDFLib public interface specific to a major
+    version, and is intended to be imported into the main RDFLib module (i.e.
+    :mod:`rdflib`) for RDFLib releases with the same or later major version.
+
+    So for example, identifiers from the :mod:`rdflib.v7` module should not be
+    imported as public identifiers into :mod:`rdflib` until version 7 of RDFLib
+    is released but may be symlinked into :mod:`rdflib` from version 7 and
+    onward.
+
+    These modules exist to facilitate integrating code into the main branch that
+    should go into future major versions of RDFLib, and in most cases will be
+    used for things that replace parts of the older interface.
+
+    Versioned modules part of the public RDFLib interface, and once something
+    appears in a versioned module it should remain part of the public interface
+    until the next major version of RDFLib.
+
+    That is, things from :mod:`rdflib.v7` should not be removed until version 8 of
+    RDFLib is released.
+
+
+Provisional Python modules
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``rdflib._provisional`` and ``rdflib.v{major_version}._provisional`` (e.g. ``rdflib.v7._provisional``), provisional modules:
+    These modules contain code that is expected to become part of :mod:`rdflib`
+    and ``rdflib.v{major_version}`` respectively but may change before this
+    happens.
+
+    Given that the provisional modules are marked for internal use (i.e.
+    prefixed with an underscore ``_``) they are not considered part of RDFLib's
+    public interface and therefore the guarantees associated with RDFLib's
+    public interface does not extend to content of these modules, however, there
+    is some expectation that the provisional module will be used and to minimize
+    the impact to users of the provisional module anything that is moved out of
+    the provisional module into the main or versioned RDFLib modules will be
+    imported back into the provisional module so that code that used the
+    provisional interface can potentially still remain operational.
+
+Practical Examples
+------------------
+
+Glossary
+--------
+
+`Python identifier <https://docs.python.org/3/reference/lexical_analysis.html#identifiers>`_
+    The name of a module, variable, class, function, or function argument. In
+    some cases this is also referred to as a Python name.
+
+Backwards incompatible change
+    A change to RDFLib that require changes to code that use RDFLib in order for
+    that code to keep performing the same function. Also referred to as a
+    breaking change.
+
+`Public interface <https://en.wikipedia.org/wiki/Public_interface>`_
+    The parts of RDFLib that are intended for use by the users of RDFLib. This
+    more or less corresponds to the identifiers that are not directly or
+    indirectly marked as internal by prefixing them or packages containing them
+    with a single leading underscore [PEP8]_.
+
+Public Python identifier
+    A Python identifier that qualifies as part of a public interface.
+
+`Python module <https://docs.python.org/3/glossary.html#term-module>`_
+    "An object that serves as an organizational unit of Python code. Modules
+    have a namespace containing arbitrary Python objects. Modules are loaded
+    into Python by the process of importing."
+
+    The phrase "Python module" is used preferentially over `Python package
+    <https://docs.python.org/3/glossary.html#term-package>`_ as it is less
+    ambiguous.
+
+References
+----------
+
+.. [PEP8] `PEP 8 – Style Guide for Python Code
+    <https://peps.python.org/pep-0008/>`_
+.. [SEMVER] `Semantic Versioning 2.0.0 <https://semver.org/spec/v2.0.0.html>`_

rdflib/_provisional/__init__.py

@@ -0,0 +1,6 @@
+"""
+This module contains code that may become part of :mod:`rdflib` though is
+expected to change further before that happens. Once code here becomes part of
+:mod:`rdflib` it should be imported back into :mod:`rdflib._provisional` so that
+code that used :mod:`rdflib._provisional` can continue working.
+"""

rdflib/_version.py

@@ -0,0 +1,10 @@
+"""
+This module contains information about the version of RDFLib.
+"""
+
+
+# _ENABLE_V? flags are used to control version specific behaviour
+_ENABLE_V6 = True
+_ENABLE_V7 = False
+_ENABLE_V8 = False
+_ENABLE_V9 = False

rdflib/v7/__init__.py

@@ -0,0 +1,4 @@
+"""
+This module contains code that will become part of :mod:`rdflib` once RDFLib 7
+is released.
+"""

rdflib/v7/_provisional/__init__.py

@@ -0,0 +1,6 @@
+"""
+This module contains code that may become part of :mod:`rdflib.v7` though is
+expected to change further before that happens. Once code here becomes part of
+:mod:`rdflib.v7` it should be imported back into :mod:`rdflib.v7._provisional`
+so that code that used :mod:`rdflib.v7._provisional` can continue working.
+"""

setup.cfg

@@ -58,6 +58,9 @@ disallow_subclassing_any = False
 warn_unreachable = True
 warn_unused_ignores = True
 
+always_true = rdflib._version._ENABLE_V6
+always_false = rdflib._version._ENABLE_V7,rdflib._version._ENABLE_V8,rdflib._version._ENABLE_V9
+
 # This is here to exclude the setup.py files in test plugins because these
 # confuse mypy as mypy think they are the same module.
 exclude = (?x)(