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<h1 class="page-title">PEP 235 – Import on Case-Insensitive Platforms</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">Tim Peters <tim.peters at gmail.com></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abbr title="Accepted and implementation complete, or no longer active">Final</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 with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">21-Feb-2001</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.1</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even">16-Feb-2001</dd>
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</dl>
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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="#note">Note</a></li>
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<li><a class="reference internal" href="#motivation">Motivation</a></li>
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<li><a class="reference internal" href="#current-lower-left-semantics">Current Lower-Left Semantics</a></li>
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<li><a class="reference internal" href="#proposed-semantics">Proposed Semantics</a></li>
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</ul>
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</details></section>
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<section id="note">
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<h2><a class="toc-backref" href="#note" role="doc-backlink">Note</a></h2>
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<p>This is essentially a retroactive PEP: the issue came up too late
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in the 2.1 release process to solicit wide opinion before deciding
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what to do, and can’t be put off until 2.2 without also delaying
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the Cygwin and MacOS X ports.</p>
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</section>
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<section id="motivation">
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<h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2>
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<p>File systems vary across platforms in whether or not they preserve
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the case of filenames, and in whether or not the platform C
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library file-opening functions do or don’t insist on
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case-sensitive matches:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="k">case</span><span class="o">-</span><span class="n">preserving</span> <span class="n">case</span><span class="o">-</span><span class="n">destroying</span>
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<span class="o">+-------------------+------------------+</span>
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<span class="k">case</span><span class="o">-</span><span class="n">sensitive</span> <span class="o">|</span> <span class="n">most</span> <span class="n">Unix</span> <span class="n">flavors</span> <span class="o">|</span> <span class="n">brrrrrrrrrr</span> <span class="o">|</span>
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<span class="o">+-------------------+------------------+</span>
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<span class="k">case</span><span class="o">-</span><span class="n">insensitive</span> <span class="o">|</span> <span class="n">Windows</span> <span class="o">|</span> <span class="n">some</span> <span class="n">unfortunate</span> <span class="o">|</span>
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<span class="o">|</span> <span class="n">MacOSX</span> <span class="n">HFS</span><span class="o">+</span> <span class="o">|</span> <span class="n">network</span> <span class="n">schemes</span> <span class="o">|</span>
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<span class="o">|</span> <span class="n">Cygwin</span> <span class="o">|</span> <span class="o">|</span>
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<span class="o">|</span> <span class="o">|</span> <span class="n">OpenVMS</span> <span class="o">|</span>
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<span class="o">+-------------------+------------------+</span>
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</pre></div>
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</div>
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<p>In the upper left box, if you create “fiLe” it’s stored as “fiLe”,
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and only <code class="docutils literal notranslate"><span class="pre">open("fiLe")</span></code> will open it (<code class="docutils literal notranslate"><span class="pre">open("file")</span></code> will not, nor
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will the 14 other variations on that theme).</p>
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<p>In the lower right box, if you create “fiLe”, there’s no telling
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||
what it’s stored as – but most likely as “FILE” – and any of the
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16 obvious variations on <code class="docutils literal notranslate"><span class="pre">open("FilE")</span></code> will open it.</p>
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<p>The lower left box is a mix: creating “fiLe” stores “fiLe” in the
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platform directory, but you don’t have to match case when opening
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it; any of the 16 obvious variations on <code class="docutils literal notranslate"><span class="pre">open("FILe")</span></code> work.</p>
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<p>NONE OF THAT IS CHANGING! Python will continue to follow platform
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conventions w.r.t. whether case is preserved when creating a file,
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and w.r.t. whether <code class="docutils literal notranslate"><span class="pre">open()</span></code> requires a case-sensitive match. In
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practice, you should always code as if matches were
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case-sensitive, else your program won’t be portable.</p>
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<p>What’s proposed is to change the semantics of Python “import”
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statements, and there <em>only</em> in the lower left box.</p>
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</section>
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<section id="current-lower-left-semantics">
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<h2><a class="toc-backref" href="#current-lower-left-semantics" role="doc-backlink">Current Lower-Left Semantics</a></h2>
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<p>Support for MacOSX HFS+, and for Cygwin, is new in 2.1, so nothing
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is changing there. What’s changing is Windows behavior. Here are
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the current rules for import on Windows:</p>
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<ol class="arabic">
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<li>Despite that the filesystem is case-insensitive, Python insists
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||
on a case-sensitive match. But not in the way the upper left
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box works: if you have two files, <code class="docutils literal notranslate"><span class="pre">FiLe.py</span></code> and <code class="docutils literal notranslate"><span class="pre">file.py</span></code> on
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<code class="docutils literal notranslate"><span class="pre">sys.path</span></code>, and do<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">file</span>
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</pre></div>
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</div>
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<p>then if Python finds <code class="docutils literal notranslate"><span class="pre">FiLe.py</span></code> first, it raises a <code class="docutils literal notranslate"><span class="pre">NameError</span></code>.
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It does <em>not</em> go on to find <code class="docutils literal notranslate"><span class="pre">file.py</span></code>; indeed, it’s impossible to
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import any but the first case-insensitive match on <code class="docutils literal notranslate"><span class="pre">sys.path</span></code>,
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and then only if case matches exactly in the first
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||
case-insensitive match.</p>
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</li>
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<li>An ugly exception: if the first case-insensitive match on
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<code class="docutils literal notranslate"><span class="pre">sys.path</span></code> is for a file whose name is entirely in upper case
|
||
(<code class="docutils literal notranslate"><span class="pre">FILE.PY</span></code> or <code class="docutils literal notranslate"><span class="pre">FILE.PYC</span></code> or <code class="docutils literal notranslate"><span class="pre">FILE.PYO</span></code>), then the import silently
|
||
grabs that, no matter what mixture of case was used in the
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||
import statement. This is apparently to cater to miserable old
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||
filesystems that really fit in the lower right box. But this
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||
exception is unique to Windows, for reasons that may or may not
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||
exist.</li>
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||
<li>And another exception: if the environment variable <code class="docutils literal notranslate"><span class="pre">PYTHONCASEOK</span></code>
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||
exists, Python silently grabs the first case-insensitive match
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||
of any kind.</li>
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||
</ol>
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<p>So these Windows rules are pretty complicated, and neither match
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||
the Unix rules nor provide semantics natural for the native
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filesystem. That makes them hard to explain to Unix <em>or</em> Windows
|
||
users. Nevertheless, they’ve worked fine for years, and in
|
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isolation there’s no compelling reason to change them.</p>
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||
<p>However, that was before the MacOSX HFS+ and Cygwin ports arrived.
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||
They also have case-preserving case-insensitive filesystems, but
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||
the people doing the ports despised the Windows rules. Indeed, a
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||
patch to make HFS+ act like Unix for imports got past a reviewer
|
||
and into the code base, which incidentally made Cygwin also act
|
||
like Unix (but this met the unbounded approval of the Cygwin
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||
folks, so they sure didn’t complain – they had patches of their
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||
own pending to do this, but the reviewer for those balked).</p>
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<p>At a higher level, we want to keep Python consistent, by following
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the same rules on <em>all</em> platforms with case-preserving
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case-insensitive filesystems.</p>
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</section>
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<section id="proposed-semantics">
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<h2><a class="toc-backref" href="#proposed-semantics" role="doc-backlink">Proposed Semantics</a></h2>
|
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<p>The proposed new semantics for the lower left box:</p>
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<ol class="upperalpha simple">
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<li>If the <code class="docutils literal notranslate"><span class="pre">PYTHONCASEOK</span></code> environment variable exists, same as
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before: silently accept the first case-insensitive match of any
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kind; raise <code class="docutils literal notranslate"><span class="pre">ImportError</span></code> if none found.</li>
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<li>Else search <code class="docutils literal notranslate"><span class="pre">sys.path</span></code> for the first case-sensitive match; raise
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<code class="docutils literal notranslate"><span class="pre">ImportError</span></code> if none found.</li>
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</ol>
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<p>#B is the same rule as is used on Unix, so this will improve
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cross-platform portability. That’s good. #B is also the rule the Mac
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and Cygwin folks want (and wanted enough to implement themselves,
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multiple times, which is a powerful argument in PythonLand). It
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can’t cause any existing non-exceptional Windows import to fail,
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because any existing non-exceptional Windows import finds a
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case-sensitive match first in the path – and it still will. An
|
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exceptional Windows import currently blows up with a <code class="docutils literal notranslate"><span class="pre">NameError</span></code> or
|
||
<code class="docutils literal notranslate"><span class="pre">ImportError</span></code>, in which latter case it still will, or in which
|
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former case will continue searching, and either succeed or blow up
|
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with an <code class="docutils literal notranslate"><span class="pre">ImportError</span></code>.</p>
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<p>#A is needed to cater to case-destroying filesystems mounted on Windows,
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and <em>may</em> also be used by people so enamored of “natural” Windows
|
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behavior that they’re willing to set an environment variable to
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get it. I don’t intend to implement #A for Unix too, but that’s
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just because I’m not clear on how I <em>could</em> do so efficiently (I’m
|
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not going to slow imports under Unix just for theoretical purity).</p>
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<p>The potential damage is here: #2 (matching on <code class="docutils literal notranslate"><span class="pre">ALLCAPS.PY</span></code>) is
|
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proposed to be dropped. Case-destroying filesystems are a
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vanishing breed, and support for them is ugly. We’re already
|
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supporting (and will continue to support) <code class="docutils literal notranslate"><span class="pre">PYTHONCASEOK</span></code> for their
|
||
benefit, but they don’t deserve multiple hacks in 2001.</p>
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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-0235.rst">https://github.com/python/peps/blob/main/peps/pep-0235.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0235.rst">2023-09-09 17:39:29 GMT</a></p>
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<li><a class="reference internal" href="#note">Note</a></li>
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<li><a class="reference internal" href="#motivation">Motivation</a></li>
|
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<li><a class="reference internal" href="#current-lower-left-semantics">Current Lower-Left Semantics</a></li>
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<li><a class="reference internal" href="#proposed-semantics">Proposed Semantics</a></li>
|
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</ul>
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