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Python Enhancement Proposals

PEP 3003 – Python Language Moratorium

Brett Cannon, Jesse Noller, Guido van Rossum

Table of Contents


This PEP proposes a temporary moratorium (suspension) of all changes to the Python language syntax, semantics, and built-ins for a period of at least two years from the release of Python 3.1. In particular, the moratorium would include Python 3.2 (to be released 18-24 months after 3.1) but allow Python 3.3 (assuming it is not released prematurely) to once again include language changes.

This suspension of features is designed to allow non-CPython implementations to “catch up” to the core implementation of the language, help ease adoption of Python 3.x, and provide a more stable base for the community.


This idea was proposed by Guido van Rossum on the python-ideas [1] mailing list. The premise of his email was to slow the alteration of the Python core syntax, builtins and semantics to allow non-CPython implementations to catch up to the current state of Python, both 2.x and 3.x.

Python, as a language is more than the core implementation – CPython – with a rich, mature and vibrant community of implementations, such as Jython [2], IronPython [3] and PyPy [4] that are a benefit not only to the community, but to the language itself.

Still others, such as Unladen Swallow [5] (a branch of CPython) seek not to create an alternative implementation, but rather they seek to enhance the performance and implementation of CPython itself.

Python 3.x was a large part of the last several years of Python’s development. Its release, as well as a bevy of changes to the language introduced by it and the previous 2.6.x releases, puts alternative implementations at a severe disadvantage in “keeping pace” with core python development.

Additionally, many of the changes put into the recent releases of the language as implemented by CPython have not yet seen widespread usage by the general user population. For example, most users are limited to the version of the interpreter (typically CPython) which comes pre-installed with their operating system. Most OS vendors are just barely beginning to ship Python 2.6 – even fewer are shipping Python 3.x.

As it is expected that Python 2.7 be the effective “end of life” of the Python 2.x code line, with Python 3.x being the future, it is in the best interest of Python core development to temporarily suspend the alteration of the language itself to allow all of these external entities to catch up and to assist in the adoption of, and migration to, Python 3.x

Finally, the moratorium is intended to free up cycles within core development to focus on other issues, such as the CPython interpreter and improvements therein, the standard library, etc.

This moratorium does not allow for exceptions – once accepted, any pending changes to the syntax or semantics of the language will be postponed until the moratorium is lifted.

This moratorium does not attempt to apply to any other Python implementation meaning that if desired other implementations may add features which deviate from the standard implementation.


Cannot Change

  • New built-ins
  • Language syntax
    The grammar file essentially becomes immutable apart from ambiguity fixes.
  • General language semantics
    The language operates as-is with only specific exemptions (see below).
  • New __future__ imports
    These are explicitly forbidden, as they effectively change the language syntax and/or semantics (albeit using a compiler directive).

Case-by-Case Exemptions

  • New methods on built-ins
    The case for adding a method to a built-in object can be made.
  • Incorrect language semantics
    If the language semantics turn out to be ambiguous or improperly implemented based on the intention of the original design then the semantics may change.
  • Language semantics that are difficult to implement
    Because other VMs have not begun implementing Python 3.x semantics there is a possibility that certain semantics are too difficult to replicate. In those cases they can be changed to ease adoption of Python 3.x by the other VMs.

Allowed to Change

  • C API
    It is entirely acceptable to change the underlying C code of CPython as long as other restrictions of this moratorium are not broken. E.g. removing the GIL would be fine assuming certain operations that are currently atomic remain atomic.
  • The standard library
    As the standard library is not directly tied to the language definition it is not covered by this moratorium.
  • Backports of 3.x features to 2.x
    The moratorium only affects features that would be new in 3.x.
  • Import semantics
    For example, PEP 382. After all, import semantics vary between Python implementations anyway.


It is important to note that the moratorium covers all changes since the release of Python 3.1. This rule is intended to avoid features being rushed or smuggled into the CPython source tree while the moratorium is being discussed. A review of the NEWS file for the py3k development branch showed no commits would need to be rolled back in order to meet this goal.


The time period of the moratorium can only be extended through a new PEP.



Last modified: 2023-09-09 17:39:29 GMT