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<h1 class="page-title">PEP 5 – Guidelines for Language Evolution</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Paul Prescod <paul at prescod.net></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Replaced by another succeeding PEP">Superseded</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP describing or proposing a change to a Python community process, workflow or governance">Process</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">26-Oct-2000</dd>
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<dt class="field-odd">Post-History<span class="colon">:</span></dt>
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<dd class="field-odd"><p></p></dd>
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<dt class="field-even">Superseded-By<span class="colon">:</span></dt>
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<dd class="field-even"><a class="reference external" href="../pep-0387/">387</a></dd>
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<section id="contents">
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#implementation-details">Implementation Details</a></li>
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<li><a class="reference internal" href="#scope">Scope</a></li>
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<li><a class="reference internal" href="#steps-for-introducing-backwards-incompatible-features">Steps For Introducing Backwards-Incompatible Features</a></li>
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</ul>
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</details></section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
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<p>In the natural evolution of programming languages it is sometimes
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necessary to make changes that modify the behavior of older programs.
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This PEP proposes a policy for implementing these changes in a manner
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respectful of the installed base of Python users.</p>
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</section>
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<section id="implementation-details">
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<h2><a class="toc-backref" href="#implementation-details" role="doc-backlink">Implementation Details</a></h2>
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<p>Implementation of this PEP requires the addition of a formal warning
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and deprecation facility that will be described in another proposal.</p>
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</section>
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<section id="scope">
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<h2><a class="toc-backref" href="#scope" role="doc-backlink">Scope</a></h2>
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<p>These guidelines apply to future versions of Python that introduce
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backward-incompatible behavior. Backward incompatible behavior is a
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major deviation in Python interpretation from an earlier behavior
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described in the standard Python documentation. Removal of a feature
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also constitutes a change of behavior.</p>
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<p>This PEP does not replace or preclude other compatibility strategies
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such as dynamic loading of backwards-compatible parsers. On the other
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hand, if execution of “old code” requires a special switch or pragma
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then that is indeed a change of behavior from the point of view of the
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user and that change should be implemented according to these
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guidelines.</p>
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<p>In general, common sense must prevail in the implementation of these
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guidelines. For instance changing “sys.copyright” does not constitute
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a backwards-incompatible change of behavior!</p>
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</section>
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<section id="steps-for-introducing-backwards-incompatible-features">
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<h2><a class="toc-backref" href="#steps-for-introducing-backwards-incompatible-features" role="doc-backlink">Steps For Introducing Backwards-Incompatible Features</a></h2>
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<ol class="arabic simple">
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<li>Propose backwards-incompatible behavior in a PEP. The PEP must
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include a section on backwards compatibility that describes in
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detail a plan to complete the remainder of these steps.</li>
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<li>Once the PEP is accepted as a productive direction, implement an
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alternate way to accomplish the task previously provided by the
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feature that is being removed or changed. For instance if the
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addition operator were scheduled for removal, a new version of
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Python could implement an “add()” built-in function.</li>
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<li>Formally deprecate the obsolete construct in the Python
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documentation.</li>
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<li>Add an optional warning mode to the parser that will inform users
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when the deprecated construct is used. In other words, all
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programs that will behave differently in the future must trigger
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warnings in this mode. Compile-time warnings are preferable to
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runtime warnings. The warning messages should steer people from
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the deprecated construct to the alternative construct.</li>
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<li>There must be at least a one-year transition period between the
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release of the transitional version of Python and the release of
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the backwards incompatible version. Users will have at least a
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year to test their programs and migrate them from use of the
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deprecated construct to the alternative one.</li>
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</ol>
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</section>
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</section>
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<hr class="docutils" />
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<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0005.rst">https://github.com/python/peps/blob/main/peps/pep-0005.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0005.rst">2023-11-28 14:46:07 GMT</a></p>
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</article>
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<nav id="pep-sidebar">
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<h2>Contents</h2>
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<ul>
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#implementation-details">Implementation Details</a></li>
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<li><a class="reference internal" href="#scope">Scope</a></li>
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<li><a class="reference internal" href="#steps-for-introducing-backwards-incompatible-features">Steps For Introducing Backwards-Incompatible Features</a></li>
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</ul>
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<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0005.rst">Page Source (GitHub)</a>
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