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<h1>Python Enhancement Proposals</h1>
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<h1 class="page-title">PEP 406 – Improved Encapsulation of Import State</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">Alyssa Coghlan <ncoghlan at gmail.com>, Greg Slodkowicz <jergosh 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="Removed from consideration by sponsor or authors">Withdrawn</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">04-Jul-2011</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.4</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even">31-Jul-2011, 13-Nov-2011, 04-Dec-2011</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="#pep-withdrawal">PEP Withdrawal</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#proposal">Proposal</a></li>
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<li><a class="reference internal" href="#specification">Specification</a><ul>
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<li><a class="reference internal" href="#importengine-api">ImportEngine API</a></li>
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<li><a class="reference internal" href="#global-variables">Global variables</a></li>
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<li><a class="reference internal" href="#no-changes-to-finder-loader-interfaces">No changes to finder/loader interfaces</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#open-issues">Open Issues</a><ul>
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<li><a class="reference internal" href="#api-design-for-falling-back-to-global-import-state">API design for falling back to global import state</a></li>
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<li><a class="reference internal" href="#builtin-and-extension-modules-must-be-process-global">Builtin and extension modules must be process global</a></li>
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<li><a class="reference internal" href="#scope-of-substitution">Scope of substitution</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li>
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<li><a class="reference internal" href="#references">References</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</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>This PEP proposes the introduction of a new ‘ImportEngine’ class as part of
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||
<code class="docutils literal notranslate"><span class="pre">importlib</span></code> which would encapsulate all state related to importing modules
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||
into a single object. Creating new instances of this object would then provide
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||
an alternative to completely replacing the built-in implementation of the
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||
import statement, by overriding the <code class="docutils literal notranslate"><span class="pre">__import__()</span></code> function. To work with
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the builtin import functionality and importing via import engine objects,
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this PEP proposes a context management based approach to temporarily replacing
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the global import state.</p>
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<p>The PEP also proposes inclusion of a <code class="docutils literal notranslate"><span class="pre">GlobalImportEngine</span></code> subclass and a
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globally accessible instance of that class, which “writes through” to the
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process global state. This provides a backwards compatible bridge between the
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proposed encapsulated API and the legacy process global state, and allows
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straightforward support for related state updates (e.g. selectively
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invalidating path cache entries when <code class="docutils literal notranslate"><span class="pre">sys.path</span></code> is modified).</p>
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</section>
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<section id="pep-withdrawal">
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<h2><a class="toc-backref" href="#pep-withdrawal" role="doc-backlink">PEP Withdrawal</a></h2>
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<p>The import system has seen substantial changes since this PEP was originally
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||
written, as part of <a class="pep reference internal" href="../pep-0420/" title="PEP 420 – Implicit Namespace Packages">PEP 420</a> in Python 3.3 and <a class="pep reference internal" href="../pep-0451/" title="PEP 451 – A ModuleSpec Type for the Import System">PEP 451</a> in Python 3.4.</p>
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<p>While providing an encapsulation of the import state is still highly
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||
desirable, it is better tackled in a new PEP using <a class="pep reference internal" href="../pep-0451/" title="PEP 451 – A ModuleSpec Type for the Import System">PEP 451</a> as a foundation,
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and permitting only the use of <a class="pep reference internal" href="../pep-0451/" title="PEP 451 – A ModuleSpec Type for the Import System">PEP 451</a> compatible finders and loaders (as
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those avoid many of the issues of direct manipulation of global state
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associated with the previous loader API).</p>
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</section>
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<section id="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<p>Currently, most state related to the import system is stored as module level
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attributes in the <code class="docutils literal notranslate"><span class="pre">sys</span></code> module. The one exception is the import lock, which
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||
is not accessible directly, but only via the related functions in the <code class="docutils literal notranslate"><span class="pre">imp</span></code>
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module. The current process global import state comprises:</p>
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<ul class="simple">
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||
<li>sys.modules</li>
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<li>sys.path</li>
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<li>sys.path_hooks</li>
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<li>sys.meta_path</li>
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<li>sys.path_importer_cache</li>
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<li>the import lock (imp.lock_held()/acquire_lock()/release_lock())</li>
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</ul>
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<p>Isolating this state would allow multiple import states to be
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||
conveniently stored within a process. Placing the import functionality
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||
in a self-contained object would also allow subclassing to add additional
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||
features (e.g. module import notifications or fine-grained control
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||
over which modules can be imported). The engine would also be
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||
subclassed to make it possible to use the import engine API to
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||
interact with the existing process-global state.</p>
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<p>The namespace PEPs (especially <a class="pep reference internal" href="../pep-0402/" title="PEP 402 – Simplified Package Layout and Partitioning">PEP 402</a>) raise a potential need for
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||
<em>additional</em> process global state, in order to correctly update package paths
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||
as <code class="docutils literal notranslate"><span class="pre">sys.path</span></code> is modified.</p>
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<p>Finally, providing a coherent object for all this state makes it feasible to
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||
also provide context management features that allow the import state to be
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||
temporarily substituted.</p>
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||
</section>
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||
<section id="proposal">
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||
<h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2>
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||
<p>We propose introducing an ImportEngine class to encapsulate import
|
||
functionality. This includes an <code class="docutils literal notranslate"><span class="pre">__import__()</span></code> method which can
|
||
be used as an alternative to the built-in <code class="docutils literal notranslate"><span class="pre">__import__()</span></code> when
|
||
desired and also an <code class="docutils literal notranslate"><span class="pre">import_module()</span></code> method, equivalent to
|
||
<code class="docutils literal notranslate"><span class="pre">importlib.import_module()</span></code> <a class="footnote-reference brackets" href="#id6" id="id1">[3]</a>.</p>
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||
<p>Since there are global import state invariants that are assumed and should be
|
||
maintained, we introduce a <code class="docutils literal notranslate"><span class="pre">GlobalImportState</span></code> class with an interface
|
||
identical to <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> but directly accessing the current global import
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state. This can be easily implemented using class properties.</p>
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||
</section>
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||
<section id="specification">
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||
<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
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<section id="importengine-api">
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<h3><a class="toc-backref" href="#importengine-api" role="doc-backlink">ImportEngine API</a></h3>
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<p>The proposed extension consists of the following objects:</p>
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||
<p><code class="docutils literal notranslate"><span class="pre">importlib.engine.ImportEngine</span></code></p>
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<blockquote>
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||
<div><code class="docutils literal notranslate"><span class="pre">from_engine(self,</span> <span class="pre">other)</span></code><blockquote>
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<div>Create a new import object from another ImportEngine instance. The
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||
new object is initialised with a copy of the state in <code class="docutils literal notranslate"><span class="pre">other</span></code>. When
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called on <code class="docutils literal notranslate"><span class="pre">importlib</span> <span class="pre">engine.sysengine</span></code>, <code class="docutils literal notranslate"><span class="pre">from_engine()</span></code> can be
|
||
used to create an <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> object with a <strong>copy</strong> of the
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global import state.</div></blockquote>
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<p><code class="docutils literal notranslate"><span class="pre">__import__(self,</span> <span class="pre">name,</span> <span class="pre">globals={},</span> <span class="pre">locals={},</span> <span class="pre">fromlist=[],</span> <span class="pre">level=0)</span></code></p>
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<blockquote>
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<div>Reimplementation of the builtin <code class="docutils literal notranslate"><span class="pre">__import__()</span></code> function. The
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import of a module will proceed using the state stored in the
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||
ImportEngine instance rather than the global import state. For full
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documentation of <code class="docutils literal notranslate"><span class="pre">__import__</span></code> functionality, see <a class="footnote-reference brackets" href="#id5" id="id2">[2]</a> .
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<code class="docutils literal notranslate"><span class="pre">__import__()</span></code> from <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> and its subclasses can be used
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to customise the behaviour of the <code class="docutils literal notranslate"><span class="pre">import</span></code> statement by replacing
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<code class="docutils literal notranslate"><span class="pre">__builtin__.__import__</span></code> with <code class="docutils literal notranslate"><span class="pre">ImportEngine().__import__</span></code>.</div></blockquote>
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<p><code class="docutils literal notranslate"><span class="pre">import_module(name,</span> <span class="pre">package=None)</span></code></p>
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<blockquote>
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<div>A reimplementation of <code class="docutils literal notranslate"><span class="pre">importlib.import_module()</span></code> which uses the
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import state stored in the ImportEngine instance. See <a class="footnote-reference brackets" href="#id6" id="id3">[3]</a> for a full
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reference.</div></blockquote>
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<p><code class="docutils literal notranslate"><span class="pre">modules,</span> <span class="pre">path,</span> <span class="pre">path_hooks,</span> <span class="pre">meta_path,</span> <span class="pre">path_importer_cache</span></code></p>
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<blockquote>
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<div>Instance-specific versions of their process global <code class="docutils literal notranslate"><span class="pre">sys</span></code> equivalents</div></blockquote>
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</div></blockquote>
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<p><code class="docutils literal notranslate"><span class="pre">importlib.engine.GlobalImportEngine(ImportEngine)</span></code></p>
|
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<blockquote>
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<div>Convenience class to provide engine-like access to the global state.
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||
Provides <code class="docutils literal notranslate"><span class="pre">__import__()</span></code>, <code class="docutils literal notranslate"><span class="pre">import_module()</span></code> and <code class="docutils literal notranslate"><span class="pre">from_engine()</span></code>
|
||
methods like <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> but writes through to the global state
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in <code class="docutils literal notranslate"><span class="pre">sys</span></code>.</div></blockquote>
|
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<p>To support various namespace package mechanisms, when <code class="docutils literal notranslate"><span class="pre">sys.path</span></code> is altered,
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||
tools like <code class="docutils literal notranslate"><span class="pre">pkgutil.extend_path</span></code> should be used to also modify other parts
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||
of the import state (in this case, package <code class="docutils literal notranslate"><span class="pre">__path__</span></code> attributes). The path
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importer cache should also be invalidated when a variety of changes are made.</p>
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<p>The <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> API will provide convenience methods that automatically
|
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make related import state updates as part of a single operation.</p>
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</section>
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<section id="global-variables">
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<h3><a class="toc-backref" href="#global-variables" role="doc-backlink">Global variables</a></h3>
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<p><code class="docutils literal notranslate"><span class="pre">importlib.engine.sysengine</span></code></p>
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<blockquote>
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<div>A precreated instance of <code class="docutils literal notranslate"><span class="pre">GlobalImportEngine</span></code>. Intended for use by
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importers and loaders that have been updated to accept optional <code class="docutils literal notranslate"><span class="pre">engine</span></code>
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parameters and with <code class="docutils literal notranslate"><span class="pre">ImportEngine.from_engine(sysengine)</span></code> to start with
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a copy of the process global import state.</div></blockquote>
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</section>
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<section id="no-changes-to-finder-loader-interfaces">
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<h3><a class="toc-backref" href="#no-changes-to-finder-loader-interfaces" role="doc-backlink">No changes to finder/loader interfaces</a></h3>
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||
<p>Rather than attempting to update the <a class="pep reference internal" href="../pep-0302/" title="PEP 302 – New Import Hooks">PEP 302</a> APIs to accept additional state,
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this PEP proposes that <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> support the content management
|
||
protocol (similar to the context substitution mechanisms in the <code class="docutils literal notranslate"><span class="pre">decimal</span></code>
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module).</p>
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<p>The context management mechanism for <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> would:</p>
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<ul class="simple">
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<li>On entry:
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* Acquire the import lock
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* Substitute the global import state with the import engine’s own state</li>
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<li>On exit:
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* Restore the previous global import state
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* Release the import lock</li>
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</ul>
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<p>The precise API for this is TBD (but will probably use a distinct context
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management object, along the lines of that created by
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<code class="docutils literal notranslate"><span class="pre">decimal.localcontext</span></code>).</p>
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</section>
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||
</section>
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<section id="open-issues">
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||
<h2><a class="toc-backref" href="#open-issues" role="doc-backlink">Open Issues</a></h2>
|
||
<section id="api-design-for-falling-back-to-global-import-state">
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||
<h3><a class="toc-backref" href="#api-design-for-falling-back-to-global-import-state" role="doc-backlink">API design for falling back to global import state</a></h3>
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||
<p>The current proposal relies on the <code class="docutils literal notranslate"><span class="pre">from_engine()</span></code> API to fall back to the
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global import state. It may be desirable to offer a variant that instead falls
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back to the global import state dynamically.</p>
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<p>However, one big advantage of starting with an “as isolated as possible”
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design is that it becomes possible to experiment with subclasses that blur
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||
the boundaries between the engine instance state and the process global state
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||
in various ways.</p>
|
||
</section>
|
||
<section id="builtin-and-extension-modules-must-be-process-global">
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||
<h3><a class="toc-backref" href="#builtin-and-extension-modules-must-be-process-global" role="doc-backlink">Builtin and extension modules must be process global</a></h3>
|
||
<p>Due to platform limitations, only one copy of each builtin and extension
|
||
module can readily exist in each process. Accordingly, it is impossible for
|
||
each <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> instance to load such modules independently.</p>
|
||
<p>The simplest solution is for <code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> to refuse to load such modules,
|
||
raising <code class="docutils literal notranslate"><span class="pre">ImportError</span></code>. <code class="docutils literal notranslate"><span class="pre">GlobalImportEngine</span></code> would be able to load them
|
||
normally.</p>
|
||
<p><code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> will still return such modules from a prepopulated module
|
||
cache - it’s only loading them directly which causes problems.</p>
|
||
</section>
|
||
<section id="scope-of-substitution">
|
||
<h3><a class="toc-backref" href="#scope-of-substitution" role="doc-backlink">Scope of substitution</a></h3>
|
||
<p>Related to the previous open issue is the question of what state to substitute
|
||
when using the context management API. It is currently the case that replacing
|
||
<code class="docutils literal notranslate"><span class="pre">sys.modules</span></code> can be unreliable due to cached references and there’s the
|
||
underlying fact that having independent copies of some modules is simply
|
||
impossible due to platform limitations.</p>
|
||
<p>As part of this PEP, it will be necessary to document explicitly:</p>
|
||
<ul class="simple">
|
||
<li>Which parts of the global import state can be substituted (and declare code
|
||
which caches references to that state without dealing with the substitution
|
||
case buggy)</li>
|
||
<li>Which parts must be modified in-place (and hence are not substituted by the
|
||
<code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> context management API, or otherwise scoped to
|
||
<code class="docutils literal notranslate"><span class="pre">ImportEngine</span></code> instances)</li>
|
||
</ul>
|
||
</section>
|
||
</section>
|
||
<section id="reference-implementation">
|
||
<h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2>
|
||
<p>A reference implementation <a class="footnote-reference brackets" href="#id7" id="id4">[4]</a> for an earlier draft of this PEP, based on
|
||
Brett Cannon’s importlib has been developed by Greg Slodkowicz as part of the
|
||
2011 Google Summer of Code. Note that the current implementation avoids
|
||
modifying existing code, and hence duplicates a lot of things unnecessarily.
|
||
An actual implementation would just modify any such affected code in place.</p>
|
||
<p>That earlier draft of the PEP proposed change the <a class="pep reference internal" href="../pep-0302/" title="PEP 302 – New Import Hooks">PEP 302</a> APIs to support passing
|
||
in an optional engine instance. This had the (serious) downside of not correctly
|
||
affecting further imports from the imported module, hence the change to the
|
||
context management based proposal for substituting the global state.</p>
|
||
</section>
|
||
<section id="references">
|
||
<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
|
||
<aside class="footnote-list brackets">
|
||
<aside class="footnote brackets" id="id5" role="doc-footnote">
|
||
<dt class="label" id="id5">[<a href="#id2">2</a>]</dt>
|
||
<dd>__import__() builtin function, The Python Standard Library documentation
|
||
(<a class="reference external" href="http://docs.python.org/library/functions.html#__import__">http://docs.python.org/library/functions.html#__import__</a>)</aside>
|
||
<aside class="footnote brackets" id="id6" role="doc-footnote">
|
||
<dt class="label" id="id6">[3]<em> (<a href='#id1'>1</a>, <a href='#id3'>2</a>) </em></dt>
|
||
<dd>Importlib documentation, Cannon
|
||
(<a class="reference external" href="http://docs.python.org/dev/library/importlib">http://docs.python.org/dev/library/importlib</a>)</aside>
|
||
<aside class="footnote brackets" id="id7" role="doc-footnote">
|
||
<dt class="label" id="id7">[<a href="#id4">4</a>]</dt>
|
||
<dd>Reference implementation
|
||
(<a class="reference external" href="https://bitbucket.org/jergosh/gsoc_import_engine/src/default/Lib/importlib/engine.py">https://bitbucket.org/jergosh/gsoc_import_engine/src/default/Lib/importlib/engine.py</a>)</aside>
|
||
</aside>
|
||
</section>
|
||
<section id="copyright">
|
||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document has been placed in the public domain.</p>
|
||
</section>
|
||
</section>
|
||
<hr class="docutils" />
|
||
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0406.rst">https://github.com/python/peps/blob/main/peps/pep-0406.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0406.rst">2023-10-11 12:05:51 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="#pep-withdrawal">PEP Withdrawal</a></li>
|
||
<li><a class="reference internal" href="#rationale">Rationale</a></li>
|
||
<li><a class="reference internal" href="#proposal">Proposal</a></li>
|
||
<li><a class="reference internal" href="#specification">Specification</a><ul>
|
||
<li><a class="reference internal" href="#importengine-api">ImportEngine API</a></li>
|
||
<li><a class="reference internal" href="#global-variables">Global variables</a></li>
|
||
<li><a class="reference internal" href="#no-changes-to-finder-loader-interfaces">No changes to finder/loader interfaces</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#open-issues">Open Issues</a><ul>
|
||
<li><a class="reference internal" href="#api-design-for-falling-back-to-global-import-state">API design for falling back to global import state</a></li>
|
||
<li><a class="reference internal" href="#builtin-and-extension-modules-must-be-process-global">Builtin and extension modules must be process global</a></li>
|
||
<li><a class="reference internal" href="#scope-of-substitution">Scope of substitution</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></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-0406.rst">Page Source (GitHub)</a>
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