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Release: <span class="version-num">0.9.8</span> | Release Date: October 13, 2014
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<h1>SQLAlchemy 0.9 Documentation</h1>
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<h3><a href="#">
Events
</a></h3>
<ul>
<li><a class="reference internal" href="#">Events</a><ul>
<li><a class="reference internal" href="#event-registration">Event Registration</a></li>
<li><a class="reference internal" href="#named-argument-styles">Named Argument Styles</a></li>
<li><a class="reference internal" href="#targets">Targets</a></li>
<li><a class="reference internal" href="#modifiers">Modifiers</a></li>
<li><a class="reference internal" href="#event-reference">Event Reference</a></li>
<li><a class="reference internal" href="#api-reference">API Reference</a></li>
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<div class="section" id="events">
<span id="event-toplevel"></span><h1>Events<a class="headerlink" href="#events" title="Permalink to this headline">¶</a></h1>
<p>SQLAlchemy includes an event API which publishes a wide variety of hooks into
the internals of both SQLAlchemy Core and ORM.</p>
<div class="versionadded">
<p><span>New in version 0.7: </span>The system supersedes the previous system of “extension”, “proxy”,
and “listener” classes.</p>
</div>
<div class="section" id="event-registration">
<h2>Event Registration<a class="headerlink" href="#event-registration" title="Permalink to this headline">¶</a></h2>
<p>Subscribing to an event occurs through a single API point, the <a class="reference internal" href="#sqlalchemy.event.listen" title="sqlalchemy.event.listen"><tt class="xref py py-func docutils literal"><span class="pre">listen()</span></tt></a> function,
or alternatively the <a class="reference internal" href="#sqlalchemy.event.listens_for" title="sqlalchemy.event.listens_for"><tt class="xref py py-func docutils literal"><span class="pre">listens_for()</span></tt></a> decorator. These functions
accept a user-defined listening function, a string identifier which identifies the event to be
intercepted, and a target. Additional positional and keyword arguments to these
two functions may be supported by
specific types of events, which may specify alternate interfaces for the given event function, or provide
instructions regarding secondary event targets based on the given target.</p>
<p>The name of an event and the argument signature of a corresponding listener function is derived from
a class bound specification method, which exists bound to a marker class that’s described in the documentation.
For example, the documentation for <a class="reference internal" href="events.html#sqlalchemy.events.PoolEvents.connect" title="sqlalchemy.events.PoolEvents.connect"><tt class="xref py py-meth docutils literal"><span class="pre">PoolEvents.connect()</span></tt></a> indicates that the event name is <tt class="docutils literal"><span class="pre">"connect"</span></tt>
and that a user-defined listener function should receive two positional arguments:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.event</span> <span class="kn">import</span> <span class="n">listen</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">import</span> <span class="n">Pool</span>
<span class="k">def</span> <span class="nf">my_on_connect</span><span class="p">(</span><span class="n">dbapi_con</span><span class="p">,</span> <span class="n">connection_record</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"New DBAPI connection:"</span><span class="p">,</span> <span class="n">dbapi_con</span>
<span class="n">listen</span><span class="p">(</span><span class="n">Pool</span><span class="p">,</span> <span class="s">'connect'</span><span class="p">,</span> <span class="n">my_on_connect</span><span class="p">)</span></pre></div>
</div>
<p>To listen with the <a class="reference internal" href="#sqlalchemy.event.listens_for" title="sqlalchemy.event.listens_for"><tt class="xref py py-func docutils literal"><span class="pre">listens_for()</span></tt></a> decorator looks like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.event</span> <span class="kn">import</span> <span class="n">listens_for</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">import</span> <span class="n">Pool</span>
<span class="nd">@listens_for</span><span class="p">(</span><span class="n">Pool</span><span class="p">,</span> <span class="s">"connect"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_on_connect</span><span class="p">(</span><span class="n">dbapi_con</span><span class="p">,</span> <span class="n">connection_record</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"New DBAPI connection:"</span><span class="p">,</span> <span class="n">dbapi_con</span></pre></div>
</div>
</div>
<div class="section" id="named-argument-styles">
<h2>Named Argument Styles<a class="headerlink" href="#named-argument-styles" title="Permalink to this headline">¶</a></h2>
<p>There are some varieties of argument styles which can be accepted by listener
functions. Taking the example of <a class="reference internal" href="events.html#sqlalchemy.events.PoolEvents.connect" title="sqlalchemy.events.PoolEvents.connect"><tt class="xref py py-meth docutils literal"><span class="pre">PoolEvents.connect()</span></tt></a>, this function
is documented as receiving <tt class="docutils literal"><span class="pre">dbapi_connection</span></tt> and <tt class="docutils literal"><span class="pre">connection_record</span></tt> arguments.
We can opt to receive these arguments by name, by establishing a listener function
that accepts <tt class="docutils literal"><span class="pre">**keyword</span></tt> arguments, by passing <tt class="docutils literal"><span class="pre">named=True</span></tt> to either
<a class="reference internal" href="#sqlalchemy.event.listen" title="sqlalchemy.event.listen"><tt class="xref py py-func docutils literal"><span class="pre">listen()</span></tt></a> or <a class="reference internal" href="#sqlalchemy.event.listens_for" title="sqlalchemy.event.listens_for"><tt class="xref py py-func docutils literal"><span class="pre">listens_for()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.event</span> <span class="kn">import</span> <span class="n">listens_for</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">import</span> <span class="n">Pool</span>
<span class="nd">@listens_for</span><span class="p">(</span><span class="n">Pool</span><span class="p">,</span> <span class="s">"connect"</span><span class="p">,</span> <span class="n">named</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_on_connect</span><span class="p">(</span><span class="o">**</span><span class="n">kw</span><span class="p">):</span>
<span class="k">print</span><span class="p">(</span><span class="s">"New DBAPI connection:"</span><span class="p">,</span> <span class="n">kw</span><span class="p">[</span><span class="s">'dbapi_connection'</span><span class="p">])</span></pre></div>
</div>
<p>When using named argument passing, the names listed in the function argument
specification will be used as keys in the dictionary.</p>
<p>Named style passes all arguments by name regardless of the function
signature, so specific arguments may be listed as well, in any order,
as long as the names match up:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.event</span> <span class="kn">import</span> <span class="n">listens_for</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">import</span> <span class="n">Pool</span>
<span class="nd">@listens_for</span><span class="p">(</span><span class="n">Pool</span><span class="p">,</span> <span class="s">"connect"</span><span class="p">,</span> <span class="n">named</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_on_connect</span><span class="p">(</span><span class="n">dbapi_connection</span><span class="p">,</span> <span class="o">**</span><span class="n">kw</span><span class="p">):</span>
<span class="k">print</span><span class="p">(</span><span class="s">"New DBAPI connection:"</span><span class="p">,</span> <span class="n">dbapi_connection</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="s">"Connection record:"</span><span class="p">,</span> <span class="n">kw</span><span class="p">[</span><span class="s">'connection_record'</span><span class="p">])</span></pre></div>
</div>
<p>Above, the presence of <tt class="docutils literal"><span class="pre">**kw</span></tt> tells <a class="reference internal" href="#sqlalchemy.event.listens_for" title="sqlalchemy.event.listens_for"><tt class="xref py py-func docutils literal"><span class="pre">listens_for()</span></tt></a> that
arguments should be passed to the function by name, rather than positionally.</p>
<div class="versionadded">
<p><span>New in version 0.9.0: </span>Added optional <tt class="docutils literal"><span class="pre">named</span></tt> argument dispatch to
event calling.</p>
</div>
</div>
<div class="section" id="targets">
<h2>Targets<a class="headerlink" href="#targets" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#sqlalchemy.event.listen" title="sqlalchemy.event.listen"><tt class="xref py py-func docutils literal"><span class="pre">listen()</span></tt></a> function is very flexible regarding targets. It
generally accepts classes, instances of those classes, and related
classes or objects from which the appropriate target can be derived.
For example, the above mentioned <tt class="docutils literal"><span class="pre">"connect"</span></tt> event accepts
<a class="reference internal" href="connections.html#sqlalchemy.engine.Engine" title="sqlalchemy.engine.Engine"><tt class="xref py py-class docutils literal"><span class="pre">Engine</span></tt></a> classes and objects as well as <a class="reference internal" href="pooling.html#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a> classes
and objects:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.event</span> <span class="kn">import</span> <span class="n">listen</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">import</span> <span class="n">Pool</span><span class="p">,</span> <span class="n">QueuePool</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="kn">import</span> <span class="n">create_engine</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.engine</span> <span class="kn">import</span> <span class="n">Engine</span>
<span class="kn">import</span> <span class="nn">psycopg2</span>
<span class="k">def</span> <span class="nf">connect</span><span class="p">():</span>
<span class="k">return</span> <span class="n">psycopg2</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">username</span><span class="o">=</span><span class="s">'ed'</span><span class="p">,</span> <span class="n">host</span><span class="o">=</span><span class="s">'127.0.0.1'</span><span class="p">,</span> <span class="n">dbname</span><span class="o">=</span><span class="s">'test'</span><span class="p">)</span>
<span class="n">my_pool</span> <span class="o">=</span> <span class="n">QueuePool</span><span class="p">(</span><span class="n">connect</span><span class="p">)</span>
<span class="n">my_engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'postgresql://ed@localhost/test'</span><span class="p">)</span>
<span class="c"># associate listener with all instances of Pool</span>
<span class="n">listen</span><span class="p">(</span><span class="n">Pool</span><span class="p">,</span> <span class="s">'connect'</span><span class="p">,</span> <span class="n">my_on_connect</span><span class="p">)</span>
<span class="c"># associate listener with all instances of Pool</span>
<span class="c"># via the Engine class</span>
<span class="n">listen</span><span class="p">(</span><span class="n">Engine</span><span class="p">,</span> <span class="s">'connect'</span><span class="p">,</span> <span class="n">my_on_connect</span><span class="p">)</span>
<span class="c"># associate listener with my_pool</span>
<span class="n">listen</span><span class="p">(</span><span class="n">my_pool</span><span class="p">,</span> <span class="s">'connect'</span><span class="p">,</span> <span class="n">my_on_connect</span><span class="p">)</span>
<span class="c"># associate listener with my_engine.pool</span>
<span class="n">listen</span><span class="p">(</span><span class="n">my_engine</span><span class="p">,</span> <span class="s">'connect'</span><span class="p">,</span> <span class="n">my_on_connect</span><span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="modifiers">
<h2>Modifiers<a class="headerlink" href="#modifiers" title="Permalink to this headline">¶</a></h2>
<p>Some listeners allow modifiers to be passed to <a class="reference internal" href="#sqlalchemy.event.listen" title="sqlalchemy.event.listen"><tt class="xref py py-func docutils literal"><span class="pre">listen()</span></tt></a>. These
modifiers sometimes provide alternate calling signatures for
listeners. Such as with ORM events, some event listeners can have a
return value which modifies the subsequent handling. By default, no
listener ever requires a return value, but by passing <tt class="docutils literal"><span class="pre">retval=True</span></tt>
this value can be supported:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">validate_phone</span><span class="p">(</span><span class="n">target</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">oldvalue</span><span class="p">,</span> <span class="n">initiator</span><span class="p">):</span>
<span class="sd">"""Strip non-numeric characters from a phone number"""</span>
<span class="k">return</span> <span class="n">re</span><span class="o">.</span><span class="n">sub</span><span class="p">(</span><span class="s">r'(?![0-9])'</span><span class="p">,</span> <span class="s">''</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span>
<span class="c"># setup listener on UserContact.phone attribute, instructing</span>
<span class="c"># it to use the return value</span>
<span class="n">listen</span><span class="p">(</span><span class="n">UserContact</span><span class="o">.</span><span class="n">phone</span><span class="p">,</span> <span class="s">'set'</span><span class="p">,</span> <span class="n">validate_phone</span><span class="p">,</span> <span class="n">retval</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="event-reference">
<h2>Event Reference<a class="headerlink" href="#event-reference" title="Permalink to this headline">¶</a></h2>
<p>Both SQLAlchemy Core and SQLAlchemy ORM feature a wide variety of event hooks:</p>
<ul class="simple">
<li><strong>Core Events</strong> - these are described in
<a class="reference internal" href="events.html"><em>Core Events</em></a> and include event hooks specific to
connection pool lifecycle, SQL statement execution,
transaction lifecycle, and schema creation and teardown.</li>
<li><strong>ORM Events</strong> - these are described in
<a class="reference internal" href="../orm/events.html"><em>ORM Events</em></a>, and include event hooks specific to
class and attribute instrumentation, object initialization
hooks, attribute on-change hooks, session state, flush, and
commit hooks, mapper initialization, object/result population,
and per-instance persistence hooks.</li>
</ul>
</div>
<div class="section" id="api-reference">
<h2>API Reference<a class="headerlink" href="#api-reference" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="sqlalchemy.event.listen">
<tt class="descclassname">sqlalchemy.event.</tt><tt class="descname">listen</tt><big>(</big><em>target</em>, <em>identifier</em>, <em>fn</em>, <em>*args</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.event.listen" title="Permalink to this definition">¶</a></dt>
<dd><p>Register a listener function for the given target.</p>
<p>e.g.:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="kn">import</span> <span class="n">event</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.schema</span> <span class="kn">import</span> <span class="n">UniqueConstraint</span>
<span class="k">def</span> <span class="nf">unique_constraint_name</span><span class="p">(</span><span class="n">const</span><span class="p">,</span> <span class="n">table</span><span class="p">):</span>
<span class="n">const</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s">"uq_</span><span class="si">%s</span><span class="s">_</span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="p">(</span>
<span class="n">table</span><span class="o">.</span><span class="n">name</span><span class="p">,</span>
<span class="nb">list</span><span class="p">(</span><span class="n">const</span><span class="o">.</span><span class="n">columns</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">name</span>
<span class="p">)</span>
<span class="n">event</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span>
<span class="n">UniqueConstraint</span><span class="p">,</span>
<span class="s">"after_parent_attach"</span><span class="p">,</span>
<span class="n">unique_constraint_name</span><span class="p">)</span></pre></div>
</div>
<p>A given function can also be invoked for only the first invocation
of the event using the <tt class="docutils literal"><span class="pre">once</span></tt> argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">on_config</span><span class="p">():</span>
<span class="n">do_config</span><span class="p">()</span>
<span class="n">event</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="n">Mapper</span><span class="p">,</span> <span class="s">"before_configure"</span><span class="p">,</span> <span class="n">on_config</span><span class="p">,</span> <span class="n">once</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span></pre></div>
</div>
<div class="versionadded">
<p><span>New in version 0.9.4: </span>Added <tt class="docutils literal"><span class="pre">once=True</span></tt> to <a class="reference internal" href="#sqlalchemy.event.listen" title="sqlalchemy.event.listen"><tt class="xref py py-func docutils literal"><span class="pre">event.listen()</span></tt></a>
and <a class="reference internal" href="#sqlalchemy.event.listens_for" title="sqlalchemy.event.listens_for"><tt class="xref py py-func docutils literal"><span class="pre">event.listens_for()</span></tt></a>.</p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.event.listens_for">
<tt class="descclassname">sqlalchemy.event.</tt><tt class="descname">listens_for</tt><big>(</big><em>target</em>, <em>identifier</em>, <em>*args</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.event.listens_for" title="Permalink to this definition">¶</a></dt>
<dd><p>Decorate a function as a listener for the given target + identifier.</p>
<p>e.g.:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="kn">import</span> <span class="n">event</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.schema</span> <span class="kn">import</span> <span class="n">UniqueConstraint</span>
<span class="nd">@event.listens_for</span><span class="p">(</span><span class="n">UniqueConstraint</span><span class="p">,</span> <span class="s">"after_parent_attach"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">unique_constraint_name</span><span class="p">(</span><span class="n">const</span><span class="p">,</span> <span class="n">table</span><span class="p">):</span>
<span class="n">const</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s">"uq_</span><span class="si">%s</span><span class="s">_</span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="p">(</span>
<span class="n">table</span><span class="o">.</span><span class="n">name</span><span class="p">,</span>
<span class="nb">list</span><span class="p">(</span><span class="n">const</span><span class="o">.</span><span class="n">columns</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">name</span>
<span class="p">)</span></pre></div>
</div>
<p>A given function can also be invoked for only the first invocation
of the event using the <tt class="docutils literal"><span class="pre">once</span></tt> argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@event.listens_for</span><span class="p">(</span><span class="n">Mapper</span><span class="p">,</span> <span class="s">"before_configure"</span><span class="p">,</span> <span class="n">once</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">on_config</span><span class="p">():</span>
<span class="n">do_config</span><span class="p">()</span></pre></div>
</div>
<div class="versionadded">
<p><span>New in version 0.9.4: </span>Added <tt class="docutils literal"><span class="pre">once=True</span></tt> to <a class="reference internal" href="#sqlalchemy.event.listen" title="sqlalchemy.event.listen"><tt class="xref py py-func docutils literal"><span class="pre">event.listen()</span></tt></a>
and <a class="reference internal" href="#sqlalchemy.event.listens_for" title="sqlalchemy.event.listens_for"><tt class="xref py py-func docutils literal"><span class="pre">event.listens_for()</span></tt></a>.</p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.event.remove">
<tt class="descclassname">sqlalchemy.event.</tt><tt class="descname">remove</tt><big>(</big><em>target</em>, <em>identifier</em>, <em>fn</em><big>)</big><a class="headerlink" href="#sqlalchemy.event.remove" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove an event listener.</p>
<p>The arguments here should match exactly those which were sent to
<a class="reference internal" href="#sqlalchemy.event.listen" title="sqlalchemy.event.listen"><tt class="xref py py-func docutils literal"><span class="pre">listen()</span></tt></a>; all the event registration which proceeded as a result
of this call will be reverted by calling <a class="reference internal" href="#sqlalchemy.event.remove" title="sqlalchemy.event.remove"><tt class="xref py py-func docutils literal"><span class="pre">remove()</span></tt></a> with the same
arguments.</p>
<p>e.g.:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># if a function was registered like this...</span>
<span class="nd">@event.listens_for</span><span class="p">(</span><span class="n">SomeMappedClass</span><span class="p">,</span> <span class="s">"before_insert"</span><span class="p">,</span> <span class="n">propagate</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_listener_function</span><span class="p">(</span><span class="o">*</span><span class="n">arg</span><span class="p">):</span>
<span class="k">pass</span>
<span class="c"># ... it's removed like this</span>
<span class="n">event</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">SomeMappedClass</span><span class="p">,</span> <span class="s">"before_insert"</span><span class="p">,</span> <span class="n">my_listener_function</span><span class="p">)</span></pre></div>
</div>
<p>Above, the listener function associated with <tt class="docutils literal"><span class="pre">SomeMappedClass</span></tt> was also
propagated to subclasses of <tt class="docutils literal"><span class="pre">SomeMappedClass</span></tt>; the <a class="reference internal" href="#sqlalchemy.event.remove" title="sqlalchemy.event.remove"><tt class="xref py py-func docutils literal"><span class="pre">remove()</span></tt></a>
function will revert all of these operations.</p>
<div class="versionadded">
<p><span>New in version 0.9.0.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.event.contains">
<tt class="descclassname">sqlalchemy.event.</tt><tt class="descname">contains</tt><big>(</big><em>target</em>, <em>identifier</em>, <em>fn</em><big>)</big><a class="headerlink" href="#sqlalchemy.event.contains" title="Permalink to this definition">¶</a></dt>
<dd><p>Return True if the given target/ident/fn is set up to listen.</p>
<div class="versionadded">
<p><span>New in version 0.9.0.</span></p>
</div>
</dd></dl>
</div>
</div>
</div>
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