PDEP-17: Backwards compatibility and deprecation policy (#59125)

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+# PDEP-17: Backwards compatibility and deprecation policy
+
+- Created: 27 June 2024
+- Status: Accepted
+- Discussion: [#59125](https://github.com/pandas-dev/pandas/issues/59125)
+- Author: [Abdulaziz Aloqeely](https://github.com/Aloqeely)
+- Revision: 1
+
+## Abstract
+
+This PDEP defines pandas' backwards compatibility and deprecation policy.
+
+The main additions to [pandas' current version policy](https://pandas.pydata.org/pandas-docs/version/2.2/development/policies.html) are:
+- Deprecated functionality should remain unchanged in at least 2 minor releases before being changed or removed.
+- Deprecations should initially use DeprecationWarning, and then be switched to FutureWarning in the last minor release before the major release they are planned to be removed in
+
+## Motivation
+
+Having a clear backwards compatibility and deprecation policy is crucial to having a healthy ecosystem. We want to ensure users can rely on pandas being stable while still allowing the library to evolve.
+
+This policy will ensure that users have enough time to deal with deprecations while also minimizing disruptions on downstream packages' users.
+
+## Scope
+
+This PDEP covers pandas' approach to backwards compatibility and the deprecation and removal process.
+
+## Background
+
+pandas uses a loose variant of semantic versioning.  
+A pandas release number is written in the format of ``MAJOR.MINOR.PATCH``.
+
+## General policy
+
+This policy applies to the [public API][1]. Anything not part of the [public API][1] or is marked as "Experimental" may be changed or removed at anytime.
+
+- Breaking backwards compatibility should benefit more than it harms users.
+- Breaking changes should go through a deprecation cycle before being implemented if possible.
+- Breaking changes should only occur in major releases.
+- No deprecations should be introduced in patch releases.
+- Deprecated functionality should remain unchanged in at least 2 minor releases before being changed or removed.
+
+Some bug fixes may require breaking backwards compatibility. In these cases, a deprecation cycle is not necessary. However, bug fixes which have a large impact on users might be treated as a breaking change. Whether or not a change is a bug fix or an API breaking change is a judgement call.
+
+## Deprecation process
+
+Deprecation provides a way to warn developers and give them time to adapt their code to the new functionality before the old behavior is eventually removed.
+
+A deprecation's warning message should:
+- Provide information on what is changing.
+- Mention how to achieve similar behavior if an alternative is available.
+- For large-scale deprecations, it is recommended to include a reason for the deprecation, alongside a discussion link to get user feedback.
+
+Additionally, when one introduces a deprecation, they should:
+- Use the appropriate warning class. More info on this can be found below.
+- Add the GitHub issue/PR number as a comment above the warning line.
+- Add an entry in the release notes.
+- Mention that the functionality is deprecated in the documentation using the ``.. deprecated::`` directive.
+
+### Which warning class to use
+
+Deprecations should initially use ``DeprecationWarning``, and then be switched to ``FutureWarning`` for broader visibility in the last minor release before the major release they are planned to be removed in.  
+This implementation detail can be ignored by using the appropriate ``PandasDeprecationWarning`` variable, which will be aliased to the proper warning class based on the pandas version.
+
+### Enforcement of deprecations
+
+When one enforces a deprecation, they should:
+- Add an entry in the release notes.
+- For API changes, replace the ``.. deprecated::`` directive in the documentation with a ``.. versionchanged::`` directive.
+
+### PDEP-17 History
+
+- 27 June 2024: Initial version.
+
+[1]: https://pandas.pydata.org/docs/reference/index.html