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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 606 – Python Compatibility Version | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0606/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 606 – Python Compatibility Version | peps.python.org'> <meta property="og:description" content="Add sys.set_python_compat_version(version) to enable partial compatibility with requested Python version. 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Add sys.get_python_compat_version()."> <meta name="theme-color" content="#3776ab"> </head> <body> <svg xmlns="http://www.w3.org/2000/svg" style="display: none;"> <symbol id="svg-sun-half" viewBox="0 0 24 24" pointer-events="all"> <title>Following system colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <circle cx="12" cy="12" r="9"></circle> <path d="M12 3v18m0-12l4.65-4.65M12 14.3l7.37-7.37M12 19.6l8.85-8.85"></path> </svg> </symbol> <symbol id="svg-moon" viewBox="0 0 24 24" pointer-events="all"> <title>Selected dark colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <path stroke="none" d="M0 0h24v24H0z" fill="none"></path> <path d="M12 3c.132 0 .263 0 .393 0a7.5 7.5 0 0 0 7.92 12.446a9 9 0 1 1 -8.313 -12.454z"></path> </svg> </symbol> <symbol id="svg-sun" viewBox="0 0 24 24" pointer-events="all"> <title>Selected light colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <circle cx="12" cy="12" r="5"></circle> <line x1="12" y1="1" x2="12" y2="3"></line> <line x1="12" y1="21" x2="12" y2="23"></line> <line x1="4.22" y1="4.22" x2="5.64" y2="5.64"></line> <line x1="18.36" y1="18.36" x2="19.78" y2="19.78"></line> <line x1="1" y1="12" x2="3" y2="12"></line> <line x1="21" y1="12" x2="23" y2="12"></line> <line x1="4.22" y1="19.78" x2="5.64" y2="18.36"></line> <line x1="18.36" y1="5.64" x2="19.78" y2="4.22"></line> </svg> </symbol> </svg> <script> document.documentElement.dataset.colour_scheme = localStorage.getItem("colour_scheme") || "auto" </script> <section id="pep-page-section"> <header> <h1>Python Enhancement Proposals</h1> <ul class="breadcrumbs"> <li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 606</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> <span class="visually-hidden">Toggle light / dark / auto colour theme</span> </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 606 – Python Compatibility Version</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Victor Stinner <vstinner at python.org></dd> <dt class="field-even">Status<span class="colon">:</span></dt> <dd class="field-even"><abbr title="Formally declined and will not be accepted">Rejected</abbr></dd> <dt class="field-odd">Type<span class="colon">:</span></dt> <dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd> <dt class="field-even">Created<span class="colon">:</span></dt> <dd class="field-even">18-Oct-2019</dd> <dt class="field-odd">Python-Version<span class="colon">:</span></dt> <dd class="field-odd">3.9</dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale">Rationale</a><ul> <li><a class="reference internal" href="#the-need-to-evolve-frequently">The need to evolve frequently</a></li> <li><a class="reference internal" href="#partial-compatibility-to-minimize-the-python-maintenance-burden">Partial compatibility to minimize the Python maintenance burden</a></li> <li><a class="reference internal" href="#cases-excluded-from-backward-compatibility">Cases excluded from backward compatibility</a></li> <li><a class="reference internal" href="#upgrading-a-project-to-a-newer-python">Upgrading a project to a newer Python</a></li> <li><a class="reference internal" href="#cleaning-up-python-and-deprecationwarning">Cleaning up Python and DeprecationWarning</a></li> <li><a class="reference internal" href="#redistribute-the-maintenance-burden">Redistribute the maintenance burden</a></li> </ul> </li> <li><a class="reference internal" href="#examples-of-backward-compatibility">Examples of backward compatibility</a><ul> <li><a class="reference internal" href="#collections-abc-aliases">collections ABC aliases</a></li> <li><a class="reference internal" href="#deprecated-open-u-mode">Deprecated open() “U” mode</a></li> </ul> </li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#sys-functions">sys functions</a></li> <li><a class="reference internal" href="#command-line">Command line</a></li> </ul> </li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li> <li><a class="reference internal" href="#security-implications">Security Implications</a></li> <li><a class="reference internal" href="#alternatives">Alternatives</a><ul> <li><a class="reference internal" href="#provide-a-workaround-for-each-incompatible-change">Provide a workaround for each incompatible change</a></li> <li><a class="reference internal" href="#handle-backward-compatibility-in-the-parser">Handle backward compatibility in the parser</a><ul> <li><a class="reference internal" href="#from-future-import-python38-syntax">from __future__ import python38_syntax</a></li> <li><a class="reference internal" href="#update-cache-tag">Update cache_tag</a></li> </ul> </li> <li><a class="reference internal" href="#temporary-moratorium-on-incompatible-changes">Temporary moratorium on incompatible changes</a></li> <li><a class="reference internal" href="#pep-387">PEP 387</a></li> <li><a class="reference internal" href="#pep-497">PEP 497</a></li> </ul> </li> <li><a class="reference internal" href="#examples-of-incompatible-changes">Examples of incompatible changes</a><ul> <li><a class="reference internal" href="#python-3-8">Python 3.8</a></li> <li><a class="reference internal" href="#python-3-7">Python 3.7</a></li> <li><a class="reference internal" href="#micro-releases">Micro releases</a></li> </ul> </li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> <p>Add <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version(version)</span></code> to enable partial compatibility with requested Python version. Add <code class="docutils literal notranslate"><span class="pre">sys.get_python_compat_version()</span></code>.</p> <p>Modify a few functions in the standard library to implement partial compatibility with Python 3.8.</p> <p>Add <code class="docutils literal notranslate"><span class="pre">sys.set_python_min_compat_version(version)</span></code> to deny backward compatibility with Python versions older than <em>version</em>.</p> <p>Add <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">compat_version=VERSION</span></code> and <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">min_compat_version=VERSION</span></code> command line options. Add <code class="docutils literal notranslate"><span class="pre">PYTHONCOMPATVERSION</span></code> and <code class="docutils literal notranslate"><span class="pre">PYTHONCOMPATMINVERSION</span></code> environment variables.</p> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <section id="the-need-to-evolve-frequently"> <h3><a class="toc-backref" href="#the-need-to-evolve-frequently" role="doc-backlink">The need to evolve frequently</a></h3> <p>To remain relevant and useful, Python has to evolve frequently; some enhancements require incompatible changes. Any incompatible change can break an unknown number of Python projects. Developers can decide to not implement a feature because of that.</p> <p>Users want to get the latest Python version to obtain new features and better performance. A few incompatible changes can prevent them from using their applications on the latest Python version.</p> <p>This PEP proposes to add a partial compatibility with old Python versions as a tradeoff to fit both use cases.</p> <p>The main issue with the migration from Python 2 to Python 3 is not that Python 3 is backward incompatible, but how incompatible changes were introduced.</p> </section> <section id="partial-compatibility-to-minimize-the-python-maintenance-burden"> <h3><a class="toc-backref" href="#partial-compatibility-to-minimize-the-python-maintenance-burden" role="doc-backlink">Partial compatibility to minimize the Python maintenance burden</a></h3> <p>While technically it would be possible to provide full compatibility with old Python versions, this PEP proposes to minimize the number of functions handling backward compatibility to reduce the maintenance burden of the Python project (CPython).</p> <p>Each change introducing backport compatibility to a function should be properly discussed to estimate the maintenance cost in the long-term.</p> <p>Backward compatibility code will be dropped on each Python release, on a case-by-case basis. Each compatibility function can be supported for a different number of Python releases depending on its maintenance cost and the estimated risk (number of broken projects) if it’s removed.</p> <p>The maintenance cost does not only come from the code implementing the backward compatibility, but also comes from the additional tests.</p> </section> <section id="cases-excluded-from-backward-compatibility"> <h3><a class="toc-backref" href="#cases-excluded-from-backward-compatibility" role="doc-backlink">Cases excluded from backward compatibility</a></h3> <p>The performance overhead of any compatibility code must be low when <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version()</span></code> is not called.</p> <p>The C API is out of the scope of this PEP: <code class="docutils literal notranslate"><span class="pre">Py_LIMITED_API</span></code> macro and the stable ABI are solving this problem differently, see the <a class="pep reference internal" href="../pep-0384/" title="PEP 384 – Defining a Stable ABI">PEP 384: Defining a Stable ABI</a>.</p> <p>Security fixes which break backward compatibility on purpose will not get a compatibility layer; security matters more than compatibility. For example, <code class="docutils literal notranslate"><span class="pre">http.client.HTTPSConnection</span></code> was modified in Python 3.4.3 to performs all the necessary certificate and hostname checks by default. It was a deliberate change motivated by <a class="pep reference internal" href="../pep-0476/" title="PEP 476 – Enabling certificate verification by default for stdlib http clients">PEP 476: Enabling certificate verification by default for stdlib http clients</a> (<a class="reference external" href="https://bugs.python.org/issue22417">bpo-22417</a>).</p> <p>The Python language does not provide backward compatibility.</p> <p>Changes which are not clearly incompatible are not covered by this PEP. For example, Python 3.9 changed the default protocol in the <code class="docutils literal notranslate"><span class="pre">pickle</span></code> module to Protocol 4 which was first introduced in Python 3.4. This change is backward compatible up to Python 3.4. There is no need to use the Protocol 3 by default when compatibility with Python 3.8 is requested.</p> <p>The new <code class="docutils literal notranslate"><span class="pre">DeprecationWarning</span></code> and <code class="docutils literal notranslate"><span class="pre">PendingDeprecatingWarning</span></code> warnings in Python 3.9 will not be disabled in Python 3.8 compatibility mode. If a project runs its test suite using <code class="docutils literal notranslate"><span class="pre">-Werror</span></code> (treat any warning as an error), these warnings must be fixed, or specific deprecation warnings must be ignored on a case-by-case basis.</p> </section> <section id="upgrading-a-project-to-a-newer-python"> <h3><a class="toc-backref" href="#upgrading-a-project-to-a-newer-python" role="doc-backlink">Upgrading a project to a newer Python</a></h3> <p>Without backward compatibility, all incompatible changes must be fixed at once, which can be a blocker issue. It is even worse when a project is upgraded to a newer Python which is separated by multiple releases from the old Python.</p> <p>Postponing an upgrade only makes things worse: each skipped release adds more incompatible changes. The technical debt only steadily increases over time.</p> <p>With backward compatibility, it becomes possible to upgrade Python incrementally in a project, without having to fix all of the issues at once.</p> <p>The “all-or-nothing” is a showstopper to port large Python 2 code bases to Python 3. The list of incompatible changes between Python 2 and Python 3 is long, and it’s getting longer with each Python 3.x release.</p> </section> <section id="cleaning-up-python-and-deprecationwarning"> <h3><a class="toc-backref" href="#cleaning-up-python-and-deprecationwarning" role="doc-backlink">Cleaning up Python and DeprecationWarning</a></h3> <p>One of the <a class="pep reference internal" href="../pep-0020/" title="PEP 20 – The Zen of Python">Zen of Python (PEP 20)</a> motto is:</p> <blockquote> <div>There should be one– and preferably only one –obvious way to do it.</div></blockquote> <p>When Python evolves, new ways inevitably emerge. <code class="docutils literal notranslate"><span class="pre">DeprecationWarning</span></code>s are emitted to suggest using the new way, but many developers ignore these warnings, which are silent by default (except in the <code class="docutils literal notranslate"><span class="pre">__main__</span></code> module: see the <a class="pep reference internal" href="../pep-0565/" title="PEP 565 – Show DeprecationWarning in __main__">PEP 565</a>). Some developers simply ignore all warnings when there are too many warnings, thus only bother with exceptions when the deprecated code is removed.</p> <p>Sometimes, supporting both ways has a minor maintenance cost, but developers prefer to drop the old way to clean up their code. These kinds of changes are backward incompatible.</p> <p>Some developers can take the end of the Python 2 support as an opportunity to push even more incompatible changes than usual.</p> <p>Adding an opt-in backward compatibility prevents the breaking of applications and allows developers to continue doing these cleanups.</p> </section> <section id="redistribute-the-maintenance-burden"> <h3><a class="toc-backref" href="#redistribute-the-maintenance-burden" role="doc-backlink">Redistribute the maintenance burden</a></h3> <p>The backward compatibility involves authors of incompatible changes more in the upgrade path.</p> </section> </section> <section id="examples-of-backward-compatibility"> <h2><a class="toc-backref" href="#examples-of-backward-compatibility" role="doc-backlink">Examples of backward compatibility</a></h2> <section id="collections-abc-aliases"> <h3><a class="toc-backref" href="#collections-abc-aliases" role="doc-backlink">collections ABC aliases</a></h3> <p><code class="docutils literal notranslate"><span class="pre">collections.abc</span></code> aliases to ABC classes have been removed from the <code class="docutils literal notranslate"><span class="pre">collections</span></code> module in Python 3.9, after being deprecated since Python 3.3. For example, <code class="docutils literal notranslate"><span class="pre">collections.Mapping</span></code> no longer exists.</p> <p>In Python 3.6, aliases were created in <code class="docutils literal notranslate"><span class="pre">collections/__init__.py</span></code> by <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">_collections_abc</span> <span class="pre">import</span> <span class="pre">*</span></code>.</p> <p>In Python 3.7, a <code class="docutils literal notranslate"><span class="pre">__getattr__()</span></code> has been added to the <code class="docutils literal notranslate"><span class="pre">collections</span></code> module to emit a DeprecationWarning upon first access to an attribute:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="fm">__getattr__</span><span class="p">(</span><span class="n">name</span><span class="p">):</span> <span class="c1"># For backwards compatibility, continue to make the collections ABCs</span> <span class="c1"># through Python 3.6 available through the collections module.</span> <span class="c1"># Note: no new collections ABCs were added in Python 3.7</span> <span class="k">if</span> <span class="n">name</span> <span class="ow">in</span> <span class="n">_collections_abc</span><span class="o">.</span><span class="n">__all__</span><span class="p">:</span> <span class="n">obj</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">_collections_abc</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span> <span class="kn">import</span><span class="w"> </span><span class="nn">warnings</span> <span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s2">"Using or importing the ABCs from 'collections' instead "</span> <span class="s2">"of from 'collections.abc' is deprecated since Python 3.3, "</span> <span class="s2">"and in 3.9 it will be removed."</span><span class="p">,</span> <span class="ne">DeprecationWarning</span><span class="p">,</span> <span class="n">stacklevel</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span> <span class="nb">globals</span><span class="p">()[</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="n">obj</span> <span class="k">return</span> <span class="n">obj</span> <span class="k">raise</span> <span class="ne">AttributeError</span><span class="p">(</span><span class="sa">f</span><span class="s1">'module </span><span class="si">{</span><span class="vm">__name__</span><span class="si">!r}</span><span class="s1"> has no attribute </span><span class="si">{</span><span class="n">name</span><span class="si">!r}</span><span class="s1">'</span><span class="p">)</span> </pre></div> </div> <p>Compatibility with Python 3.8 can be restored in Python 3.9 by adding back the <code class="docutils literal notranslate"><span class="pre">__getattr__()</span></code> function, but only when backward compatibility is requested:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="fm">__getattr__</span><span class="p">(</span><span class="n">name</span><span class="p">):</span> <span class="k">if</span> <span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">get_python_compat_version</span><span class="p">()</span> <span class="o"><</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">9</span><span class="p">)</span> <span class="ow">and</span> <span class="n">name</span> <span class="ow">in</span> <span class="n">_collections_abc</span><span class="o">.</span><span class="n">__all__</span><span class="p">):</span> <span class="o">...</span> <span class="k">raise</span> <span class="ne">AttributeError</span><span class="p">(</span><span class="sa">f</span><span class="s1">'module </span><span class="si">{</span><span class="vm">__name__</span><span class="si">!r}</span><span class="s1"> has no attribute </span><span class="si">{</span><span class="n">name</span><span class="si">!r}</span><span class="s1">'</span><span class="p">)</span> </pre></div> </div> </section> <section id="deprecated-open-u-mode"> <h3><a class="toc-backref" href="#deprecated-open-u-mode" role="doc-backlink">Deprecated open() “U” mode</a></h3> <p>The <code class="docutils literal notranslate"><span class="pre">"U"</span></code> mode of <code class="docutils literal notranslate"><span class="pre">open()</span></code> is deprecated since Python 3.4 and emits a <code class="docutils literal notranslate"><span class="pre">DeprecationWarning</span></code>. <a class="reference external" href="https://bugs.python.org/issue37330">bpo-37330</a> proposes to drop this mode: <code class="docutils literal notranslate"><span class="pre">open(filename,</span> <span class="pre">"rU")</span></code> would raise an exception.</p> <p>This change falls into the “cleanup” category: it is not required to implement a feature.</p> <p>A backward compatibility mode would be trivial to implement and would be welcomed by users.</p> </section> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> <section id="sys-functions"> <h3><a class="toc-backref" href="#sys-functions" role="doc-backlink">sys functions</a></h3> <p>Add 3 functions to the <code class="docutils literal notranslate"><span class="pre">sys</span></code> module:</p> <ul class="simple"> <li><code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version(version)</span></code>: set the Python compatibility version. If it has been called previously, use the minimum of requested versions. Raise an exception if <code class="docutils literal notranslate"><span class="pre">sys.set_python_min_compat_version(min_version)</span></code> has been called and <code class="docutils literal notranslate"><span class="pre">version</span> <span class="pre"><</span> <span class="pre">min_version</span></code>. <em>version</em> must be greater than or equal to <code class="docutils literal notranslate"><span class="pre">(3,</span> <span class="pre">0)</span></code>.</li> <li><code class="docutils literal notranslate"><span class="pre">sys.set_python_min_compat_version(min_version)</span></code>: set the <strong>minimum</strong> compatibility version. Raise an exception if <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version(old_version)</span></code> has been called previously and <code class="docutils literal notranslate"><span class="pre">old_version</span> <span class="pre"><</span> <span class="pre">min_version</span></code>. <em>min_version</em> must be greater than or equal to <code class="docutils literal notranslate"><span class="pre">(3,</span> <span class="pre">0)</span></code>.</li> <li><code class="docutils literal notranslate"><span class="pre">sys.get_python_compat_version()</span></code>: get the Python compatibility version. Return a <code class="docutils literal notranslate"><span class="pre">tuple</span></code> of 3 integers.</li> </ul> <p>A <em>version</em> must a tuple of 2 or 3 integers. <code class="docutils literal notranslate"><span class="pre">(major,</span> <span class="pre">minor)</span></code> version is equivalent to <code class="docutils literal notranslate"><span class="pre">(major,</span> <span class="pre">minor,</span> <span class="pre">0)</span></code>.</p> <p>By default, <code class="docutils literal notranslate"><span class="pre">sys.get_python_compat_version()</span></code> returns the current Python version.</p> <p>For example, to request compatibility with Python 3.8.0:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">collections</span> <span class="n">sys</span><span class="o">.</span><span class="n">set_python_compat_version</span><span class="p">((</span><span class="mi">3</span><span class="p">,</span> <span class="mi">8</span><span class="p">))</span> <span class="c1"># collections.Mapping alias, removed from Python 3.9, is available</span> <span class="c1"># again, even if collections has been imported before calling</span> <span class="c1"># set_python_compat_version().</span> <span class="n">parent</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">Mapping</span> </pre></div> </div> <p>Obviously, calling <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version(version)</span></code> has no effect on code executed before the call. Use <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">compat_version=VERSION</span></code> command line option or <code class="docutils literal notranslate"><span class="pre">PYTHONCOMPATVERSIONVERSION=VERSION</span></code> environment variable to set the compatibility version at Python startup.</p> </section> <section id="command-line"> <h3><a class="toc-backref" href="#command-line" role="doc-backlink">Command line</a></h3> <p>Add <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">compat_version=VERSION</span></code> and <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">min_compat_version=VERSION</span></code> command line options: call respectively <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version()</span></code> and <code class="docutils literal notranslate"><span class="pre">sys.set_python_min_compat_version()</span></code>. <code class="docutils literal notranslate"><span class="pre">VERSION</span></code> is a version string with 2 or 3 numbers (<code class="docutils literal notranslate"><span class="pre">major.minor.micro</span></code> or <code class="docutils literal notranslate"><span class="pre">major.minor</span></code>). For example, <code class="docutils literal notranslate"><span class="pre">-X</span> <span class="pre">compat_version=3.8</span></code> calls <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version((3,</span> <span class="pre">8))</span></code>.</p> <p>Add <code class="docutils literal notranslate"><span class="pre">PYTHONCOMPATVERSIONVERSION=VERSION</span></code> and <code class="docutils literal notranslate"><span class="pre">PYTHONCOMPATMINVERSION=VERSION=VERSION</span></code> environment variables: call respectively <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version()</span></code> and <code class="docutils literal notranslate"><span class="pre">sys.set_python_min_compat_version()</span></code>. <code class="docutils literal notranslate"><span class="pre">VERSION</span></code> is a version string with the same format as the command line options.</p> </section> </section> <section id="backwards-compatibility"> <h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2> <p>Introducing the <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version()</span></code> function means that an application will behave differently depending on the compatibility version. Moreover, since the version can be decreased multiple times, the application can behave differently depending on the import order.</p> <p>Python 3.9 with <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version((3,</span> <span class="pre">8))</span></code> is not fully compatible with Python 3.8: the compatibility is only partial.</p> </section> <section id="security-implications"> <h2><a class="toc-backref" href="#security-implications" role="doc-backlink">Security Implications</a></h2> <p><code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version()</span></code> must not disable security fixes.</p> </section> <section id="alternatives"> <h2><a class="toc-backref" href="#alternatives" role="doc-backlink">Alternatives</a></h2> <section id="provide-a-workaround-for-each-incompatible-change"> <h3><a class="toc-backref" href="#provide-a-workaround-for-each-incompatible-change" role="doc-backlink">Provide a workaround for each incompatible change</a></h3> <p>An application can work around most incompatible changes which impacts it.</p> <p>For example, <code class="docutils literal notranslate"><span class="pre">collections</span></code> aliases can be added back using:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">collections.abc</span> <span class="n">collections</span><span class="o">.</span><span class="n">Mapping</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">abc</span><span class="o">.</span><span class="n">Mapping</span> <span class="n">collections</span><span class="o">.</span><span class="n">Sequence</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">abc</span><span class="o">.</span><span class="n">Sequence</span> </pre></div> </div> </section> <section id="handle-backward-compatibility-in-the-parser"> <h3><a class="toc-backref" href="#handle-backward-compatibility-in-the-parser" role="doc-backlink">Handle backward compatibility in the parser</a></h3> <p>The parser is modified to support multiple versions of the Python language (grammar).</p> <p>The current Python parser cannot be easily modified for that. AST and grammar are hardcoded to a single Python version.</p> <p>In Python 3.8, <code class="docutils literal notranslate"><span class="pre">compile()</span></code> has an undocumented <code class="docutils literal notranslate"><span class="pre">_feature_version</span></code> to not consider <code class="docutils literal notranslate"><span class="pre">async</span></code> and <code class="docutils literal notranslate"><span class="pre">await</span></code> as keywords.</p> <p>The latest major language backward incompatible change was Python 3.7 which made <code class="docutils literal notranslate"><span class="pre">async</span></code> and <code class="docutils literal notranslate"><span class="pre">await</span></code> real keywords. It seems like Twisted was the only affected project, and Twisted had a single affected function (it used a parameter called <code class="docutils literal notranslate"><span class="pre">async</span></code>).</p> <p>Handling backward compatibility in the parser seems quite complex, not only to modify the parser, but also for developers who have to check which version of the Python language is used.</p> <section id="from-future-import-python38-syntax"> <h4><a class="toc-backref" href="#from-future-import-python38-syntax" role="doc-backlink">from __future__ import python38_syntax</a></h4> <p>Add <code class="docutils literal notranslate"><span class="pre">pythonXY_syntax</span></code> to the <code class="docutils literal notranslate"><span class="pre">__future__</span></code> module. It would enable backward compatibility with Python X.Y syntax, but only for the current file.</p> <p>With this option, there is no need to change <code class="docutils literal notranslate"><span class="pre">sys.implementation.cache_tag</span></code> to use a different <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> filename, since the parser will always produce the same output for the same input (except for the optimization level).</p> <p>For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">__future__</span><span class="w"> </span><span class="kn">import</span> <span class="n">python35_syntax</span> <span class="k">async</span> <span class="o">=</span> <span class="mi">1</span> <span class="k">await</span> <span class="o">=</span> <span class="mi">2</span> </pre></div> </div> </section> <section id="update-cache-tag"> <h4><a class="toc-backref" href="#update-cache-tag" role="doc-backlink">Update cache_tag</a></h4> <p>Modify the parser to use <code class="docutils literal notranslate"><span class="pre">sys.get_python_compat_version()</span></code> to choose the version of the Python language.</p> <p><code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version()</span></code> updates <code class="docutils literal notranslate"><span class="pre">sys.implementation.cache_tag</span></code> to include the compatibility version without the micro version as a suffix. For example, Python 3.9 uses <code class="docutils literal notranslate"><span class="pre">'cpython-39'</span></code> by default, but <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version((3,</span> <span class="pre">7,</span> <span class="pre">2))</span></code> sets <code class="docutils literal notranslate"><span class="pre">cache_tag</span></code> to <code class="docutils literal notranslate"><span class="pre">'cpython-39-37'</span></code>. Changes to the Python language are now allowed in micro releases.</p> <p>One problem is that <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">asyncio</span></code> is likely to fail if <code class="docutils literal notranslate"><span class="pre">sys.set_python_compat_version((3,</span> <span class="pre">6))</span></code> has been called previously. The code of the <code class="docutils literal notranslate"><span class="pre">asyncio</span></code> module requires <code class="docutils literal notranslate"><span class="pre">async</span></code> and <code class="docutils literal notranslate"><span class="pre">await</span></code> to be real keywords (change done in Python 3.7).</p> <p>Another problem is that regular users cannot write <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files into system directories, and so cannot create them on demand. It means that <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> optimization cannot be used in the backward compatibility mode.</p> <p>One solution for that is to modify the Python installer and Python package installers to precompile <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files not only for the current Python version, but also for multiple older Python versions (up to Python 3.0?).</p> <p>Each <code class="docutils literal notranslate"><span class="pre">.py</span></code> file would have 3n <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files (3 optimization levels), where <code class="docutils literal notranslate"><span class="pre">n</span></code> is the number of supported Python versions. For example, it means 6 <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> files, instead of 3, to support Python 3.8 and Python 3.9.</p> </section> </section> <section id="temporary-moratorium-on-incompatible-changes"> <h3><a class="toc-backref" href="#temporary-moratorium-on-incompatible-changes" role="doc-backlink">Temporary moratorium on incompatible changes</a></h3> <p>In 2009, <a class="pep reference internal" href="../pep-3003/" title="PEP 3003 – Python Language Moratorium">PEP 3003</a> “Python Language Moratorium” proposed a temporary moratorium (suspension) of all changes to the Python language syntax, semantics, and built-ins for Python 3.1 and Python 3.2.</p> <p>In May 2018, during the <a class="pep reference internal" href="../pep-0572/" title="PEP 572 – Assignment Expressions">PEP 572</a> discussions, it was also proposed to slow down Python changes: see the python-dev thread <a class="reference external" href="https://mail.python.org/archives/list/python-dev@python.org/thread/HHKRXOMRJQH75VNM3JMSQIOOU6MIUB24/#PHA35EAPNONZMTOYBINGFR6XXNMCDPFQ">Slow down…</a></p> <p><a class="reference external" href="https://mail.python.org/archives/list/python-dev@python.org/message/XR7IF2OB3S72KBP3PEQ3IKBOERE4FV2I/">Barry Warsaw’s call on this</a>:</p> <blockquote> <div>I don’t believe that the way for Python to remain relevant and useful for the next 10 years is to cease all language evolution. Who knows what the computing landscape will look like in 5 years, let alone 10? Something as arbitrary as a 10-year moratorium is (again, IMHO) a death sentence for the language.</div></blockquote> </section> <section id="pep-387"> <h3><a class="toc-backref" href="#pep-387" role="doc-backlink">PEP 387</a></h3> <p><a class="pep reference internal" href="../pep-0387/" title="PEP 387 – Backwards Compatibility Policy">PEP 387 – Backwards Compatibility Policy</a> proposes a process to make incompatible changes. The main point is the 4th step of the process:</p> <blockquote> <div>See if there’s any feedback. Users not involved in the original discussions may comment now after seeing the warning. Perhaps reconsider.</div></blockquote> </section> <section id="pep-497"> <h3><a class="toc-backref" href="#pep-497" role="doc-backlink">PEP 497</a></h3> <p><a class="pep reference internal" href="../pep-0497/" title="PEP 497 – A standard mechanism for backward compatibility">PEP 497 – A standard mechanism for backward compatibility</a> proposes different solutions to provide backward compatibility.</p> <p>Except for the <code class="docutils literal notranslate"><span class="pre">__past__</span></code> mechanism idea, <a class="pep reference internal" href="../pep-0497/" title="PEP 497 – A standard mechanism for backward compatibility">PEP 497</a> does not propose concrete solutions:</p> <blockquote> <div>When an incompatible change to core language syntax or semantics is being made, Python-dev’s policy is to prefer and expect that, wherever possible, a mechanism for backward compatibility be considered and provided for future Python versions after the breaking change is adopted by default, in addition to any mechanisms proposed for forward compatibility such as new future_statements.</div></blockquote> </section> </section> <section id="examples-of-incompatible-changes"> <h2><a class="toc-backref" href="#examples-of-incompatible-changes" role="doc-backlink">Examples of incompatible changes</a></h2> <section id="python-3-8"> <h3><a class="toc-backref" href="#python-3-8" role="doc-backlink">Python 3.8</a></h3> <p>Examples of Python 3.8 incompatible changes:</p> <ul class="simple"> <li>(During beta phase) <code class="docutils literal notranslate"><span class="pre">PyCode_New()</span></code> required a new parameter: it broke all Cython extensions (all projects distributing precompiled Cython code). This change has been reverted during the 3.8 beta phase and a new <code class="docutils literal notranslate"><span class="pre">PyCode_NewWithPosOnlyArgs()</span></code> function was added instead.</li> <li><code class="docutils literal notranslate"><span class="pre">types.CodeType</span></code> requires an additional mandatory parameter. The <code class="docutils literal notranslate"><span class="pre">CodeType.replace()</span></code> function was added to help projects to no longer depend on the exact signature of the <code class="docutils literal notranslate"><span class="pre">CodeType</span></code> constructor.</li> <li>C extensions are no longer linked to libpython.</li> <li><code class="docutils literal notranslate"><span class="pre">sys.abiflags</span></code> changed from <code class="docutils literal notranslate"><span class="pre">'m'</span></code> to an empty string. For example, <code class="docutils literal notranslate"><span class="pre">python3.8m</span></code> program is gone.</li> <li>The C structure <code class="docutils literal notranslate"><span class="pre">PyInterpreterState</span></code> was made opaque.<ul> <li>Blender:<ul> <li><a class="reference external" href="https://bugzilla.redhat.com/show_bug.cgi?id=1734980#c6">https://bugzilla.redhat.com/show_bug.cgi?id=1734980#c6</a></li> <li><a class="reference external" href="https://developer.blender.org/D6038">https://developer.blender.org/D6038</a></li> </ul> </li> </ul> </li> <li>XML attribute order: <a class="reference external" href="https://bugs.python.org/issue34160">bpo-34160</a>. Broken projects:<ul> <li><a class="reference external" href="https://bugs.python.org/issue34160#msg329612">coverage</a></li> <li><a class="reference external" href="https://sourceforge.net/p/docutils/bugs/359/">docutils</a></li> <li><a class="reference external" href="https://bugzilla.redhat.com/show_bug.cgi?id=1705475">pcs</a></li> <li><a class="reference external" href="https://bugzilla.redhat.com/show_bug.cgi?id=1705391">python-glyphsLib</a></li> </ul> </li> </ul> <p>Backward compatibility cannot be added for all these changes. For example, changes in the C API and in the build system are out of the scope of this PEP.</p> <p>See <a class="reference external" href="https://docs.python.org/dev/whatsnew/3.8.html#api-and-feature-removals">What’s New In Python 3.8: API and Feature Removals</a> for all changes.</p> <p>See also the <a class="reference external" href="https://docs.python.org/dev/whatsnew/3.8.html#porting-to-python-3-8">Porting to Python 3.8</a> section of What’s New In Python 3.8.</p> </section> <section id="python-3-7"> <h3><a class="toc-backref" href="#python-3-7" role="doc-backlink">Python 3.7</a></h3> <p>Examples of Python 3.7 incompatible changes:</p> <ul class="simple"> <li><code class="docutils literal notranslate"><span class="pre">async</span></code> and <code class="docutils literal notranslate"><span class="pre">await</span></code> are now reserved keywords.</li> <li>Several undocumented internal imports were removed. One example is that <code class="docutils literal notranslate"><span class="pre">os.errno</span></code> is no longer available; use <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">errno</span></code> directly instead. Note that such undocumented internal imports may be removed any time without notice, even in micro version releases.</li> <li>Unknown escapes consisting of <code class="docutils literal notranslate"><span class="pre">'\'</span></code> and an ASCII letter in replacement templates for <code class="docutils literal notranslate"><span class="pre">re.sub()</span></code> were deprecated in Python 3.5, and will now cause an error.</li> <li>The <code class="docutils literal notranslate"><span class="pre">asyncio.windows_utils.socketpair()</span></code> function has been removed: it was an alias to <code class="docutils literal notranslate"><span class="pre">socket.socketpair()</span></code>.</li> <li><code class="docutils literal notranslate"><span class="pre">asyncio</span></code> no longer exports the <code class="docutils literal notranslate"><span class="pre">selectors</span></code> and <code class="docutils literal notranslate"><span class="pre">_overlapped</span></code> modules as <code class="docutils literal notranslate"><span class="pre">asyncio.selectors</span></code> and <code class="docutils literal notranslate"><span class="pre">asyncio._overlapped</span></code>. Replace <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">asyncio</span> <span class="pre">import</span> <span class="pre">selectors</span></code> with <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">selectors</span></code>.</li> <li><a class="pep reference internal" href="../pep-0479/" title="PEP 479 – Change StopIteration handling inside generators">PEP 479</a> is enabled for all code in Python 3.7, meaning that <code class="docutils literal notranslate"><span class="pre">StopIteration</span></code> exceptions raised directly or indirectly in coroutines and generators are transformed into <code class="docutils literal notranslate"><span class="pre">RuntimeError</span></code> exceptions.</li> <li><code class="docutils literal notranslate"><span class="pre">socketserver.ThreadingMixIn.server_close()</span></code> now waits until all non-daemon threads complete. Set the new <code class="docutils literal notranslate"><span class="pre">block_on_close</span></code> class attribute to <code class="docutils literal notranslate"><span class="pre">False</span></code> to get the pre-3.7 behaviour.</li> <li>The <code class="docutils literal notranslate"><span class="pre">struct.Struct.format</span></code> type is now <code class="docutils literal notranslate"><span class="pre">str</span></code> instead of <code class="docutils literal notranslate"><span class="pre">bytes</span></code>.</li> <li><code class="docutils literal notranslate"><span class="pre">repr</span></code> for <code class="docutils literal notranslate"><span class="pre">datetime.timedelta</span></code> has changed to include the keyword arguments in the output.</li> <li><code class="docutils literal notranslate"><span class="pre">tracemalloc.Traceback</span></code> frames are now sorted from oldest to most recent to be more consistent with <code class="docutils literal notranslate"><span class="pre">traceback</span></code>.</li> </ul> <p>Adding backward compatibility for most of these changes would be easy.</p> <p>See also the <a class="reference external" href="https://docs.python.org/dev/whatsnew/3.7.html#porting-to-python-3-7">Porting to Python 3.7</a> section of What’s New In Python 3.7.</p> </section> <section id="micro-releases"> <h3><a class="toc-backref" href="#micro-releases" role="doc-backlink">Micro releases</a></h3> <p>Sometimes, incompatible changes are introduced in micro releases (<code class="docutils literal notranslate"><span class="pre">micro</span></code> in <code class="docutils literal notranslate"><span class="pre">major.minor.micro</span></code>) to fix bugs or security vulnerabilities. Examples include:</p> <ul class="simple"> <li>Python 3.7.2, <code class="docutils literal notranslate"><span class="pre">compileall</span></code> and <code class="docutils literal notranslate"><span class="pre">py_compile</span></code> module: the <em>invalidation_mode</em> parameter’s default value is updated to <code class="docutils literal notranslate"><span class="pre">None</span></code>; the <code class="docutils literal notranslate"><span class="pre">SOURCE_DATE_EPOCH</span></code> environment variable no longer overrides the value of the <em>invalidation_mode</em> argument, and determines its default value instead.</li> <li>Python 3.7.1, <code class="docutils literal notranslate"><span class="pre">xml</span></code> modules: the SAX parser no longer processes general external entities by default to increase security by default.</li> <li>Python 3.5.2, <code class="docutils literal notranslate"><span class="pre">os.urandom()</span></code>: on Linux, if the <code class="docutils literal notranslate"><span class="pre">getrandom()</span></code> syscall blocks (the urandom entropy pool is not initialized yet), fall back on reading <code class="docutils literal notranslate"><span class="pre">/dev/urandom</span></code>.</li> <li>Python 3.5.1, <code class="docutils literal notranslate"><span class="pre">sys.setrecursionlimit()</span></code>: a <code class="docutils literal notranslate"><span class="pre">RecursionError</span></code> exception is now raised if the new limit is too low at the current recursion depth.</li> <li>Python 3.4.4, <code class="docutils literal notranslate"><span class="pre">ssl.create_default_context()</span></code>: RC4 was dropped from the default cipher string.</li> <li>Python 3.4.3, <code class="docutils literal notranslate"><span class="pre">http.client</span></code>: <code class="docutils literal notranslate"><span class="pre">HTTPSConnection</span></code> now performs all the necessary certificate and hostname checks by default.</li> <li>Python 3.4.2, <code class="docutils literal notranslate"><span class="pre">email.message</span></code>: <code class="docutils literal notranslate"><span class="pre">EmailMessage.is_attachment()</span></code> is now a method instead of a property, for consistency with <code class="docutils literal notranslate"><span class="pre">Message.is_multipart()</span></code>.</li> <li>Python 3.4.1, <code class="docutils literal notranslate"><span class="pre">os.makedirs(name,</span> <span class="pre">mode=0o777,</span> <span class="pre">exist_ok=False)</span></code>: Before Python 3.4.1, if <em>exist_ok</em> was <code class="docutils literal notranslate"><span class="pre">True</span></code> and the directory existed, <code class="docutils literal notranslate"><span class="pre">makedirs()</span></code> would still raise an error if <em>mode</em> did not match the mode of the existing directory. Since this behavior was impossible to implement safely, it was removed in Python 3.4.1 (<a class="reference external" href="https://bugs.python.org/issue21082">bpo-21082</a>).</li> </ul> <p>Examples of changes made in micro releases which are not backward incompatible:</p> <ul class="simple"> <li><code class="docutils literal notranslate"><span class="pre">ssl.OP_NO_TLSv1_3</span></code> constant was added to 2.7.15, 3.6.3 and 3.7.0 for backwards compatibility with OpenSSL 1.0.2.</li> <li><code class="docutils literal notranslate"><span class="pre">typing.AsyncContextManager</span></code> was added to Python 3.6.2.</li> <li>The <code class="docutils literal notranslate"><span class="pre">zipfile</span></code> module accepts a path-like object since Python 3.6.2.</li> <li><code class="docutils literal notranslate"><span class="pre">loop.create_future()</span></code> was added to Python 3.5.2 in the <code class="docutils literal notranslate"><span class="pre">asyncio</span></code> module.</li> </ul> <p>No backward compatibility code is needed for these kinds of changes.</p> </section> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <p>Accepted PEPs:</p> <ul class="simple"> <li><a class="pep reference internal" href="../pep-0005/" title="PEP 5 – Guidelines for Language Evolution">PEP 5 – Guidelines for Language Evolution</a></li> <li><a class="pep reference internal" href="../pep-0236/" title="PEP 236 – Back to the __future__">PEP 236 – Back to the __future__</a></li> <li><a class="pep reference internal" href="../pep-0411/" title="PEP 411 – Provisional packages in the Python standard library">PEP 411 – Provisional packages in the Python standard library</a></li> <li><a class="pep reference internal" href="../pep-3002/" title="PEP 3002 – Procedure for Backwards-Incompatible Changes">PEP 3002 – Procedure for Backwards-Incompatible Changes</a></li> </ul> <p>Draft PEPs:</p> <ul class="simple"> <li><a class="pep reference internal" href="../pep-0602/" title="PEP 602 – Annual Release Cycle for Python">PEP 602 – Annual Release Cycle for Python</a></li> <li><a class="pep reference internal" href="../pep-0605/" title="PEP 605 – A rolling feature release stream for CPython">PEP 605 – A rolling feature release stream for CPython</a></li> <li>See also withdrawn <a class="pep reference internal" href="../pep-0598/" title="PEP 598 – Introducing incremental feature releases">PEP 598 – Introducing incremental feature releases</a></li> </ul> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> <p>This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.</p> </section> </section> <hr class="docutils" /> <p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0606.rst">https://github.com/python/peps/blob/main/peps/pep-0606.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0606.rst">2025-02-01 08:55:40 GMT</a></p> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale">Rationale</a><ul> <li><a class="reference internal" href="#the-need-to-evolve-frequently">The need to evolve frequently</a></li> <li><a class="reference internal" href="#partial-compatibility-to-minimize-the-python-maintenance-burden">Partial compatibility to minimize the Python maintenance burden</a></li> <li><a class="reference internal" href="#cases-excluded-from-backward-compatibility">Cases excluded from backward compatibility</a></li> <li><a class="reference internal" href="#upgrading-a-project-to-a-newer-python">Upgrading a project to a newer Python</a></li> <li><a class="reference internal" href="#cleaning-up-python-and-deprecationwarning">Cleaning up Python and DeprecationWarning</a></li> <li><a class="reference internal" href="#redistribute-the-maintenance-burden">Redistribute the maintenance burden</a></li> </ul> </li> <li><a class="reference internal" href="#examples-of-backward-compatibility">Examples of backward compatibility</a><ul> <li><a class="reference internal" href="#collections-abc-aliases">collections ABC aliases</a></li> <li><a class="reference internal" href="#deprecated-open-u-mode">Deprecated open() “U” mode</a></li> </ul> </li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#sys-functions">sys functions</a></li> <li><a class="reference internal" href="#command-line">Command line</a></li> </ul> </li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li> <li><a class="reference internal" href="#security-implications">Security Implications</a></li> <li><a class="reference internal" href="#alternatives">Alternatives</a><ul> <li><a class="reference internal" href="#provide-a-workaround-for-each-incompatible-change">Provide a workaround for each incompatible change</a></li> <li><a class="reference internal" href="#handle-backward-compatibility-in-the-parser">Handle backward compatibility in the parser</a><ul> <li><a class="reference internal" href="#from-future-import-python38-syntax">from __future__ import python38_syntax</a></li> <li><a class="reference internal" href="#update-cache-tag">Update cache_tag</a></li> </ul> </li> <li><a class="reference internal" href="#temporary-moratorium-on-incompatible-changes">Temporary moratorium on incompatible changes</a></li> <li><a class="reference internal" href="#pep-387">PEP 387</a></li> <li><a class="reference internal" href="#pep-497">PEP 497</a></li> </ul> </li> <li><a class="reference internal" href="#examples-of-incompatible-changes">Examples of incompatible changes</a><ul> <li><a class="reference internal" href="#python-3-8">Python 3.8</a></li> <li><a class="reference internal" href="#python-3-7">Python 3.7</a></li> <li><a class="reference internal" href="#micro-releases">Micro releases</a></li> </ul> </li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <br> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0606.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>