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<li><a class="reference internal" href="#">eventloop.stack_context</a><ul>
<li><a class="reference internal" href="#module-zmq.eventloop.stack_context">Module: <tt class="docutils literal"><span class="pre">eventloop.stack_context</span></tt></a></li>
<li><a class="reference internal" href="#classes">Classes</a><ul>
<li><a class="reference internal" href="#exceptionstackcontext"><tt class="docutils literal"><span class="pre">ExceptionStackContext</span></tt></a></li>
<li><a class="reference internal" href="#nullcontext"><tt class="docutils literal"><span class="pre">NullContext</span></tt></a></li>
<li><a class="reference internal" href="#stackcontext"><tt class="docutils literal"><span class="pre">StackContext</span></tt></a></li>
</ul>
</li>
<li><a class="reference internal" href="#function">Function</a></li>
</ul>
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<div class="section" id="eventloop-stack-context">
<h1>eventloop.stack_context<a class="headerlink" href="#eventloop-stack-context" title="Permalink to this headline">¶</a></h1>
<div class="section" id="module-zmq.eventloop.stack_context">
<span id="module-eventloop-stack-context"></span><h2>Module: <tt class="xref py py-mod docutils literal"><span class="pre">eventloop.stack_context</span></tt><a class="headerlink" href="#module-zmq.eventloop.stack_context" title="Permalink to this headline">¶</a></h2>
<p>StackContext allows applications to maintain threadlocal-like state
that follows execution as it moves to other execution contexts.</p>
<p>The motivating examples are to eliminate the need for explicit
async_callback wrappers (as in tornado.web.RequestHandler), and to
allow some additional context to be kept for logging.</p>
<p>This is slightly magic, but it’s an extension of the idea that an exception
handler is a kind of stack-local state and when that stack is suspended
and resumed in a new context that state needs to be preserved. StackContext
shifts the burden of restoring that state from each call site (e.g.
wrapping each AsyncHTTPClient callback in async_callback) to the mechanisms
that transfer control from one context to another (e.g. AsyncHTTPClient
itself, IOLoop, thread pools, etc).</p>
<p>Example usage:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@contextlib.contextmanager</span>
<span class="k">def</span> <span class="nf">die_on_error</span><span class="p">():</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span>
<span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">error</span><span class="p">(</span><span class="s">"exception in asynchronous operation"</span><span class="p">,</span><span class="n">exc_info</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">with</span> <span class="n">StackContext</span><span class="p">(</span><span class="n">die_on_error</span><span class="p">):</span>
<span class="c"># Any exception thrown here *or in callback and its desendents*</span>
<span class="c"># will cause the process to exit instead of spinning endlessly</span>
<span class="c"># in the ioloop.</span>
<span class="n">http_client</span><span class="o">.</span><span class="n">fetch</span><span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="n">callback</span><span class="p">)</span>
<span class="n">ioloop</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>Most applications shouln’t have to work with <cite>StackContext</cite> directly.
Here are a few rules of thumb for when it’s necessary:</p>
<ul class="simple">
<li>If you’re writing an asynchronous library that doesn’t rely on a
stack_context-aware library like <cite>tornado.ioloop</cite> or <cite>tornado.iostream</cite>
(for example, if you’re writing a thread pool), use
<cite>stack_context.wrap()</cite> before any asynchronous operations to capture the
stack context from where the operation was started.</li>
<li>If you’re writing an asynchronous library that has some shared
resources (such as a connection pool), create those shared resources
within a <tt class="docutils literal"><span class="pre">with</span> <span class="pre">stack_context.NullContext():</span></tt> block. This will prevent
<tt class="docutils literal"><span class="pre">StackContexts</span></tt> from leaking from one request to another.</li>
<li>If you want to write something like an exception handler that will
persist across asynchronous calls, create a new <cite>StackContext</cite> (or
<cite>ExceptionStackContext</cite>), and make your asynchronous calls in a <tt class="docutils literal"><span class="pre">with</span></tt>
block that references your <cite>StackContext</cite>.</li>
</ul>
</div>
<div class="section" id="classes">
<h2>Classes<a class="headerlink" href="#classes" title="Permalink to this headline">¶</a></h2>
<div class="section" id="exceptionstackcontext">
<h3><a class="reference internal" href="#zmq.eventloop.stack_context.ExceptionStackContext" title="zmq.eventloop.stack_context.ExceptionStackContext"><tt class="xref py py-class docutils literal"><span class="pre">ExceptionStackContext</span></tt></a><a class="headerlink" href="#exceptionstackcontext" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="zmq.eventloop.stack_context.ExceptionStackContext">
<em class="property">class </em><tt class="descclassname">zmq.eventloop.stack_context.</tt><tt class="descname">ExceptionStackContext</tt><big>(</big><em>exception_handler</em><big>)</big><a class="headerlink" href="#zmq.eventloop.stack_context.ExceptionStackContext" title="Permalink to this definition">¶</a></dt>
<dd><p>Specialization of StackContext for exception handling.</p>
<p>The supplied exception_handler function will be called in the
event of an uncaught exception in this context. The semantics are
similar to a try/finally clause, and intended use cases are to log
an error, close a socket, or similar cleanup actions. The
exc_info triple (type, value, traceback) will be passed to the
exception_handler function.</p>
<p>If the exception handler returns true, the exception will be
consumed and will not be propagated to other exception handlers.</p>
</dd></dl>
</div>
<div class="section" id="nullcontext">
<h3><a class="reference internal" href="#zmq.eventloop.stack_context.NullContext" title="zmq.eventloop.stack_context.NullContext"><tt class="xref py py-class docutils literal"><span class="pre">NullContext</span></tt></a><a class="headerlink" href="#nullcontext" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="zmq.eventloop.stack_context.NullContext">
<em class="property">class </em><tt class="descclassname">zmq.eventloop.stack_context.</tt><tt class="descname">NullContext</tt><a class="headerlink" href="#zmq.eventloop.stack_context.NullContext" title="Permalink to this definition">¶</a></dt>
<dd><p>Resets the StackContext.</p>
<p>Useful when creating a shared resource on demand (e.g. an AsyncHTTPClient)
where the stack that caused the creating is not relevant to future
operations.</p>
</dd></dl>
</div>
<div class="section" id="stackcontext">
<h3><a class="reference internal" href="#zmq.eventloop.stack_context.StackContext" title="zmq.eventloop.stack_context.StackContext"><tt class="xref py py-class docutils literal"><span class="pre">StackContext</span></tt></a><a class="headerlink" href="#stackcontext" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="zmq.eventloop.stack_context.StackContext">
<em class="property">class </em><tt class="descclassname">zmq.eventloop.stack_context.</tt><tt class="descname">StackContext</tt><big>(</big><em>context_factory</em><big>)</big><a class="headerlink" href="#zmq.eventloop.stack_context.StackContext" title="Permalink to this definition">¶</a></dt>
<dd><p>Establishes the given context as a StackContext that will be transferred.</p>
<p>Note that the parameter is a callable that returns a context
manager, not the context itself. That is, where for a
non-transferable context manager you would say:</p>
<div class="highlight-python"><pre>with my_context():</pre>
</div>
<p>StackContext takes the function itself rather than its result:</p>
<div class="highlight-python"><pre>with StackContext(my_context):</pre>
</div>
</dd></dl>
</div>
</div>
<div class="section" id="function">
<h2>Function<a class="headerlink" href="#function" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="zmq.eventloop.stack_context.wrap">
<tt class="descclassname">zmq.eventloop.stack_context.</tt><tt class="descname">wrap</tt><big>(</big><em>fn</em><big>)</big><a class="headerlink" href="#zmq.eventloop.stack_context.wrap" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns a callable object that will restore the current StackContext
when executed.</p>
<p>Use this whenever saving a callback to be executed later in a
different execution context (either in a different thread or
asynchronously in the same thread).</p>
</dd></dl>
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