Added version policy (pandas-dev#28415)

Author: TomAugspurger

doc/source/development/index.rst

@@ -16,4 +16,5 @@ Development
     internals
     extending
     developer
+    policies
     roadmap

doc/source/development/policies.rst

@@ -0,0 +1,57 @@
+.. _develop.policies:
+
+********
+Policies
+********
+
+.. _policies.version:
+
+Version Policy
+~~~~~~~~~~~~~~
+
+.. versionchanged:: 1.0.0
+
+Pandas uses a loose variant of semantic versioning (`SemVer`_) to govern
+deprecations, API compatibility, and version numbering.
+
+A pandas release number is made up of ``MAJOR.MINOR.PATCH``.
+
+API breaking changes should only occur in **major** releases. Theses changes
+will be documented, with clear guidance on what is changing, why it's changing,
+and how to migrate existing code to the new behavior.
+
+Whenever possible, a deprecation path will be provided rather than an outright
+breaking change.
+
+Pandas will introduce deprecations in **minor** releases. These deprecations
+will preserve the existing behavior while emitting a warning that provide
+guidance on:
+
+* How to achieve similar behavior if an alternative is available
+* The pandas version in which the deprecation will be enforced.
+
+We will not introduce new deprecations in patch releases.
+
+Deprecations will only be enforced in **major** releases. For example, if a
+behavior is deprecated in pandas 1.2.0, it will continue to work, with a
+warning, for all releases in the 1.x series. The behavior will change and the
+deprecation removed in the next next major release (2.0.0).
+
+.. note::
+
+   Pandas will sometimes make *behavior changing* bug fixes, as part of
+   minor or patch releases. Whether or not a change is a bug fix or an
+   API-breaking change is a judgement call. We'll do our best, and we
+   invite you to participate in development discussion on the issue
+   tracker or mailing list.
+
+These policies do not apply to features marked as **experimental** in the documentation.
+Pandas may change the behavior of experimental features at any time.
+
+Python Support
+~~~~~~~~~~~~~~
+
+Pandas will only drop support for specific Python versions (e.g. 3.5.x, 3.6.x) in
+pandas **major** releases.
+
+.. _SemVer: https://semver.org

doc/source/whatsnew/v1.0.0.rst

@@ -3,10 +3,39 @@
 What's new in 1.0.0 (??)
 ------------------------
 
-.. warning::
-
-   Starting with the 0.25.x series of releases, pandas only supports Python 3.5.3 and higher.
-   See `Dropping Python 2.7 <https://pandas.pydata.org/pandas-docs/version/0.24/install.html#install-dropping-27>`_ for more details.
+New Deprecation Policy
+~~~~~~~~~~~~~~~~~~~~~~
+
+Starting with Pandas 1.0.0, pandas will adopt a version of `SemVer`_.
+
+Historically, pandas has used a "rolling" deprecation policy, with occasional
+outright breaking API changes. Where possible, we would deprecate the behavior
+we'd like to change, giving an option to adopt the new behavior (via a keyword
+or an alternative method), and issuing a warning for users of the old behavior.
+Sometimes, a deprecation was not possible, and we would make an outright API
+breaking change.
+
+We'll continue to *introduce* deprecations in major and minor releases (e.g.
+1.0.0, 1.1.0, ...). Those deprecations will be *enforced* in the next major
+release.
+
+Note that *behavior changes* and *API breaking changes* are not identical. API
+breaking changes will only be released in major versions. If we consider a
+behavior to be a bug, and fixing that bug induces a behavior change, we'll
+release that change in a minor release. This is a sometimes difficult judgment
+call that we'll do our best on.
+
+This doesn't mean that pandas' pace of development will slow down. In the `2019
+Pandas User Survey`_, about 95% of the respondents said they considered pandas
+"stable enough". This indicates there's an appetite for new features, even if it
+comes at the cost of break API. The difference is that now API breaking changes
+will be accompanied with a bump in the major version number (e.g. pandas 1.5.1
+-> 2.0.0).
+
+See :ref:`policies.version` for more.
+
+.. _2019 Pandas User Survey: http://dev.pandas.io/pandas-blog/2019-pandas-user-survey.html
+.. _SemVer: https://semver.org
 
 .. warning::