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<li><a class="reference internal" href="#concurrency-and-multithreading">並行性和多執行緒 (Concurrency and Multithreading)</a></li>
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<li><a class="reference internal" href="#asynchronous-generators-best-practices">Asynchronous generators best practices</a><ul>
<li><a class="reference internal" href="#close-asynchronous-generators-explicitly">Close asynchronous generators explicitly</a></li>
<li><a class="reference internal" href="#create-asynchronous-generators-only-when-the-event-loop-is-running">Create asynchronous generators only when the event loop is running</a></li>
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<section id="developing-with-asyncio">
<span id="asyncio-dev"></span><h1>使用 asyncio 開發<a class="headerlink" href="#developing-with-asyncio" title="連結到這個標頭">¶</a></h1>
<p>非同步程式設計 (asynchronous programming) 與傳統的"順序"程式設計 (sequential programming) 不同。</p>
<p>本頁列出常見的錯誤和陷阱，並解釋如何避免它們。</p>
<section id="debug-mode">
<span id="asyncio-debug-mode"></span><h2>除錯模式<a class="headerlink" href="#debug-mode" title="連結到這個標頭">¶</a></h2>
<p>在預設情況下 asyncio 以正式生產模式 (production mode) 執行。為了讓開發更輕鬆，asyncio 還有一種<em>除錯模式 (debug mode)</em>。</p>
<p>有幾種方法可以啟用 asyncio 除錯模式：</p>
<ul class="simple">
<li><p>將 <span class="target" id="index-2"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONASYNCIODEBUG"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONASYNCIODEBUG</span></code></a> 環境變數設定為 <code class="docutils literal notranslate"><span class="pre">1</span></code>。</p></li>
<li><p>使用 <a class="reference internal" href="devmode.html#devmode"><span class="std std-ref">Python 開發模式 (Development Mode)</span></a>。</p></li>
<li><p>將 <code class="docutils literal notranslate"><span class="pre">debug=True</span></code> 傳遞給 <a class="reference internal" href="asyncio-runner.html#asyncio.run" title="asyncio.run"><code class="xref py py-func docutils literal notranslate"><span class="pre">asyncio.run()</span></code></a>。</p></li>
<li><p>呼叫 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.set_debug" title="asyncio.loop.set_debug"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.set_debug()</span></code></a>。</p></li>
</ul>
<p>除了啟用除錯模式外，還要考慮：</p>
<ul>
<li><p>將 <a class="reference internal" href="#asyncio-logger"><span class="std std-ref">asyncio logger（日誌記錄器）</span></a>的日誌級别設置為 <a class="reference internal" href="logging.html#logging.DEBUG" title="logging.DEBUG"><code class="xref py py-const docutils literal notranslate"><span class="pre">logging.DEBUG</span></code></a>，例如下面的程式片段可以在應用程式啟動時運行：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p>配置 <a class="reference internal" href="warnings.html#module-warnings" title="warnings: Issue warning messages and control their disposition."><code class="xref py py-mod docutils literal notranslate"><span class="pre">warnings</span></code></a> 模組以顯示 <a class="reference internal" href="exceptions.html#ResourceWarning" title="ResourceWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ResourceWarning</span></code></a> 警告。一種方法是使用 <a class="reference internal" href="../using/cmdline.html#cmdoption-W"><code class="xref std std-option docutils literal notranslate"><span class="pre">-W</span></code></a> <code class="docutils literal notranslate"><span class="pre">default</span></code> 命令列選項。</p></li>
</ul>
<p>啟用除錯模式時：</p>
<ul class="simple">
<li><p>許多非執行緒安全 (non-threadsafe) 的 asyncio APIs（例如 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_soon" title="asyncio.loop.call_soon"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_soon()</span></code></a> 和 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_at" title="asyncio.loop.call_at"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_at()</span></code></a> 方法），如果從錯誤的執行緒呼叫就會引發例外。</p></li>
<li><p>如果執行一個 I/O 操作花費的時間太長，則將 I/O 選擇器 (selector) 的執行時間記錄到日誌中。</p></li>
<li><p>執行時間超過 100 毫秒的回呼 (callback) 將會被記錄於日誌。屬性 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.slow_callback_duration" title="asyncio.loop.slow_callback_duration"><code class="xref py py-attr docutils literal notranslate"><span class="pre">loop.slow_callback_duration</span></code></a> 可用於設定以秒為單位的最小執行持續時間，超過這個值執行時間就會被視為"緩慢"。</p></li>
</ul>
</section>
<section id="concurrency-and-multithreading">
<span id="asyncio-multithreading"></span><h2>並行性和多執行緒 (Concurrency and Multithreading)<a class="headerlink" href="#concurrency-and-multithreading" title="連結到這個標頭">¶</a></h2>
<p>事件迴圈在執行緒中運行（通常是主執行緒），並在其執行緒中執行所有回呼和 Tasks（任務）。當一個 Task 在事件迴圈中運行時，沒有其他 Task 可以在同一個執行緒中運行。當一個 Task 執行一個 <code class="docutils literal notranslate"><span class="pre">await</span></code> 運算式時，正在執行的 Task 會被暫停，而事件迴圈會執行下一個 Task。</p>
<p>要從不同的 OS 執行緒為一個 <a class="reference internal" href="../glossary.html#term-callback"><span class="xref std std-term">callback</span></a> 排程，應該使用 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_soon_threadsafe" title="asyncio.loop.call_soon_threadsafe"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_soon_threadsafe()</span></code></a> 方法。例如：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">loop</span><span class="o">.</span><span class="n">call_soon_threadsafe</span><span class="p">(</span><span class="n">callback</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">)</span>
</pre></div>
</div>
<p>幾乎所有 asyncio 物件都不支援執行緒安全 (thread safe)，這通常不是問題，除非在 Task 或回呼函式之外有程式需要和它們一起運作。如果需要這樣的程式來呼叫低階 asyncio API，應該使用 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_soon_threadsafe" title="asyncio.loop.call_soon_threadsafe"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_soon_threadsafe()</span></code></a> 方法，例如：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">loop</span><span class="o">.</span><span class="n">call_soon_threadsafe</span><span class="p">(</span><span class="n">fut</span><span class="o">.</span><span class="n">cancel</span><span class="p">)</span>
</pre></div>
</div>
<p>要從不同的 OS 執行緒為一個協程物件排程，應該使用 <a class="reference internal" href="asyncio-task.html#asyncio.run_coroutine_threadsafe" title="asyncio.run_coroutine_threadsafe"><code class="xref py py-func docutils literal notranslate"><span class="pre">run_coroutine_threadsafe()</span></code></a> 函式。它會回傳一個 <a class="reference internal" href="concurrent.futures.html#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.Future</span></code></a> 以存取結果：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">coro_func</span><span class="p">():</span>
<span class="k">return</span> <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">42</span><span class="p">)</span>
<span class="c1"># 之後在另一個 OS 執行緒中：</span>
<span class="n">future</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">run_coroutine_threadsafe</span><span class="p">(</span><span class="n">coro_func</span><span class="p">(),</span> <span class="n">loop</span><span class="p">)</span>
<span class="c1"># 等待結果：</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">future</span><span class="o">.</span><span class="n">result</span><span class="p">()</span>
</pre></div>
</div>
<p>為了能夠處理訊號，事件迴圈必須於主執行緒中運行。</p>
<p><a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.run_in_executor" title="asyncio.loop.run_in_executor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.run_in_executor()</span></code></a> 方法可以和 <a class="reference internal" href="concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.ThreadPoolExecutor</span></code></a> 或 <a class="reference internal" href="concurrent.futures.html#concurrent.futures.InterpreterPoolExecutor" title="concurrent.futures.InterpreterPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">InterpreterPoolExecutor</span></code></a> 一起使用，這能夠在作業系統上另一個不同的執行緒中執行阻塞程式，且避免阻塞執行事件迴圈的執行緒。</p>
<p>目前沒有什麼辦法能直接從另一個行程（例如透過 <a class="reference internal" href="multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> 啟動的程序）來為協程或回呼排程。<a class="reference internal" href="asyncio-eventloop.html#asyncio-event-loop-methods"><span class="std std-ref">事件迴圈方法</span></a>小節列出了可以從 pipes（管道）讀取並監視 file descriptor（檔案描述器）而不會阻塞事件迴圈的 API。此外，asyncio 的<a class="reference internal" href="asyncio-subprocess.html#asyncio-subprocess"><span class="std std-ref">子行程</span></a> API 提供了一種啟動行程並從事件迴圈與其通訊的辦法。最後，之前提到的 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.run_in_executor" title="asyncio.loop.run_in_executor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.run_in_executor()</span></code></a> 方法也可和 <a class="reference internal" href="concurrent.futures.html#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.ProcessPoolExecutor</span></code></a> 搭配使用，以在另一個行程中執行程式。</p>
</section>
<section id="running-blocking-code">
<span id="asyncio-handle-blocking"></span><h2>執行阻塞的程式<a class="headerlink" href="#running-blocking-code" title="連結到這個標頭">¶</a></h2>
<p>不應該直接呼叫阻塞（CPU 密集型）程式。例如一個執行 1 秒 CPU 密集型計算的函式，那麼所有並行非同步 Tasks 和 IO 操作都會被延遲 1 秒。</p>
<p>一個 executor（執行器）可以被用來在不同的執行緒（包含不同直譯器）、或甚至不同的行程中執行任務，以避免事件迴圈阻塞了 OS 執行緒。詳情請見 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.run_in_executor" title="asyncio.loop.run_in_executor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.run_in_executor()</span></code></a> 方法。</p>
</section>
<section id="logging">
<span id="asyncio-logger"></span><h2>日誌記錄<a class="headerlink" href="#logging" title="連結到這個標頭">¶</a></h2>
<p>asyncio 使用 <a class="reference internal" href="logging.html#module-logging" title="logging: Flexible event logging system for applications."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging</span></code></a> 模組，所有日誌記錄都是透過 <code class="docutils literal notranslate"><span class="pre">"asyncio"</span></code> logger 執行的。</p>
<p>日誌級別被預設為 <a class="reference internal" href="logging.html#logging.INFO" title="logging.INFO"><code class="xref py py-const docutils literal notranslate"><span class="pre">logging.INFO</span></code></a>，它可以很容易地被調整：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s2">"asyncio"</span><span class="p">)</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">WARNING</span><span class="p">)</span>
</pre></div>
</div>
<p>網路日誌記錄可能會阻塞事件迴圈。建議使用獨立的執行緒來處理日誌或使用非阻塞 IO，範例請參見 <a class="reference internal" href="../howto/logging-cookbook.html#blocking-handlers"><span class="std std-ref">Dealing with handlers that block</span></a>。</p>
</section>
<section id="detect-never-awaited-coroutines">
<span id="asyncio-coroutine-not-scheduled"></span><h2>偵測從未被等待的 (never-awaited) 協程<a class="headerlink" href="#detect-never-awaited-coroutines" title="連結到這個標頭">¶</a></h2>
<p>當協程函式被呼叫而不是被等待時（即執行 <code class="docutils literal notranslate"><span class="pre">coro()</span></code> 而不是 <code class="docutils literal notranslate"><span class="pre">await</span> <span class="pre">coro()</span></code>）或者協程沒有透過 <a class="reference internal" href="asyncio-task.html#asyncio.create_task" title="asyncio.create_task"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.create_task()</span></code></a> 被排程，asyncio 將會發出 <a class="reference internal" href="exceptions.html#RuntimeWarning" title="RuntimeWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeWarning</span></code></a>：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">test</span><span class="p">():</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"never scheduled"</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
<span class="n">test</span><span class="p">()</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
</div>
<p>輸出：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">7</span><span class="p">:</span> <span class="ne">RuntimeWarning</span><span class="p">:</span> <span class="n">coroutine</span> <span class="s1">'test'</span> <span class="n">was</span> <span class="n">never</span> <span class="n">awaited</span>
<span class="n">test</span><span class="p">()</span>
</pre></div>
</div>
<p>除錯模式中的輸出：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">7</span><span class="p">:</span> <span class="ne">RuntimeWarning</span><span class="p">:</span> <span class="n">coroutine</span> <span class="s1">'test'</span> <span class="n">was</span> <span class="n">never</span> <span class="n">awaited</span>
<span class="n">Coroutine</span> <span class="n">created</span> <span class="n">at</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">)</span>
<span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">9</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">(),</span> <span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="o"><</span> <span class="o">..</span> <span class="o">></span>
<span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">7</span><span class="p">,</span> <span class="ow">in</span> <span class="n">main</span>
<span class="n">test</span><span class="p">()</span>
<span class="n">test</span><span class="p">()</span>
</pre></div>
</div>
<p>常用的修復方法是去等待協程或者呼叫 <a class="reference internal" href="asyncio-task.html#asyncio.create_task" title="asyncio.create_task"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.create_task()</span></code></a> 函式：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
<span class="k">await</span> <span class="n">test</span><span class="p">()</span>
</pre></div>
</div>
</section>
<section id="detect-never-retrieved-exceptions">
<h2>偵測從未被取得的 (never-retrieved) 例外<a class="headerlink" href="#detect-never-retrieved-exceptions" title="連結到這個標頭">¶</a></h2>
<p>如果呼叫 <a class="reference internal" href="asyncio-future.html#asyncio.Future.set_exception" title="asyncio.Future.set_exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Future.set_exception()</span></code></a>，但 Future 物件從未被等待，例外將無法被傳播 (propagate) 到使用者程式。在這種情況下，當 Future 物件被垃圾回收 (garbage collected) 時，asyncio 將發出一則日誌訊息。</p>
<p>未處理例外的例子：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">bug</span><span class="p">():</span>
<span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">bug</span><span class="p">())</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
</div>
<p>輸出：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">Task</span> <span class="n">exception</span> <span class="n">was</span> <span class="n">never</span> <span class="n">retrieved</span>
<span class="n">future</span><span class="p">:</span> <span class="o"><</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">coro</span><span class="o">=<</span><span class="n">bug</span><span class="p">()</span> <span class="n">done</span><span class="p">,</span> <span class="n">defined</span> <span class="n">at</span> <span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">3</span><span class="o">></span>
<span class="n">exception</span><span class="o">=</span><span class="ne">Exception</span><span class="p">(</span><span class="s1">'not consumed'</span><span class="p">)</span><span class="o">></span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"test.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">4</span><span class="p">,</span> <span class="ow">in</span> <span class="n">bug</span>
<span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>
<span class="ne">Exception</span><span class="p">:</span> <span class="ow">not</span> <span class="n">consumed</span>
</pre></div>
</div>
<p><a class="reference internal" href="#asyncio-debug-mode"><span class="std std-ref">啟用除錯模式</span></a>以取得任務建立處的追蹤資訊 (traceback)：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">(),</span> <span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
</pre></div>
</div>
<p>除錯模式中的輸出：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">Task</span> <span class="n">exception</span> <span class="n">was</span> <span class="n">never</span> <span class="n">retrieved</span>
<span class="n">future</span><span class="p">:</span> <span class="o"><</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">coro</span><span class="o">=<</span><span class="n">bug</span><span class="p">()</span> <span class="n">done</span><span class="p">,</span> <span class="n">defined</span> <span class="n">at</span> <span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">3</span><span class="o">></span>
<span class="n">exception</span><span class="o">=</span><span class="ne">Exception</span><span class="p">(</span><span class="s1">'not consumed'</span><span class="p">)</span> <span class="n">created</span> <span class="n">at</span> <span class="n">asyncio</span><span class="o">/</span><span class="n">tasks</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">321</span><span class="o">></span>
<span class="n">source_traceback</span><span class="p">:</span> <span class="n">Object</span> <span class="n">created</span> <span class="n">at</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">9</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">(),</span> <span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="o"><</span> <span class="o">..</span> <span class="o">></span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">4</span><span class="p">,</span> <span class="ow">in</span> <span class="n">bug</span>
<span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>
<span class="ne">Exception</span><span class="p">:</span> <span class="ow">not</span> <span class="n">consumed</span>
</pre></div>
</div>
</section>
<section id="asynchronous-generators-best-practices">
<h2>Asynchronous generators best practices<a class="headerlink" href="#asynchronous-generators-best-practices" title="連結到這個標頭">¶</a></h2>
<p>Writing correct and efficient asyncio code requires awareness of certain pitfalls.
This section outlines essential best practices that can save you hours of debugging.</p>
<section id="close-asynchronous-generators-explicitly">
<h3>Close asynchronous generators explicitly<a class="headerlink" href="#close-asynchronous-generators-explicitly" title="連結到這個標頭">¶</a></h3>
<p>It is recommended to manually close the
<a class="reference internal" href="../glossary.html#term-asynchronous-generator-iterator"><span class="xref std std-term">asynchronous generator</span></a>. If a generator
exits early - for example, due to an exception raised in the body of
an <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code> loop - its asynchronous cleanup code may run in an
unexpected context. This can occur after the tasks it depends on have completed,
or during the event loop shutdown when the async-generator's garbage collection
hook is called.</p>
<p>To avoid this, explicitly close the generator by calling its
<a class="reference internal" href="../reference/expressions.html#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> method, or use the <a class="reference internal" href="contextlib.html#contextlib.aclosing" title="contextlib.aclosing"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextlib.aclosing()</span></code></a>
context manager:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="kn">import</span><span class="w"> </span><span class="nn">contextlib</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">gen</span><span class="p">():</span>
<span class="k">yield</span> <span class="mi">1</span>
<span class="k">yield</span> <span class="mi">2</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">func</span><span class="p">():</span>
<span class="k">async</span> <span class="k">with</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">aclosing</span><span class="p">(</span><span class="n">gen</span><span class="p">())</span> <span class="k">as</span> <span class="n">g</span><span class="p">:</span>
<span class="k">async</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">g</span><span class="p">:</span>
<span class="k">break</span> <span class="c1"># Don't iterate until the end</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">func</span><span class="p">())</span>
</pre></div>
</div>
<p>As noted above, the cleanup code for these asynchronous generators is deferred.
The following example demonstrates that the finalization of an asynchronous
generator can occur in an unexpected order:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="n">work_done</span> <span class="o">=</span> <span class="kc">False</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">cursor</span><span class="p">():</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="mi">1</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">assert</span> <span class="n">work_done</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">rows</span><span class="p">():</span>
<span class="k">global</span> <span class="n">work_done</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="mi">2</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.1</span><span class="p">)</span> <span class="c1"># immitate some async work</span>
<span class="n">work_done</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
<span class="k">async</span> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">cursor</span><span class="p">():</span>
<span class="k">async</span> <span class="k">for</span> <span class="n">r</span> <span class="ow">in</span> <span class="n">rows</span><span class="p">():</span>
<span class="k">break</span>
<span class="k">break</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
</div>
<p>For this example, we get the following output:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">unhandled</span> <span class="n">exception</span> <span class="n">during</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">()</span> <span class="n">shutdown</span>
<span class="n">task</span><span class="p">:</span> <span class="o"><</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">name</span><span class="o">=</span><span class="s1">'Task-3'</span> <span class="n">coro</span><span class="o">=<<</span><span class="n">async_generator_athrow</span> <span class="n">without</span> <span class="vm">__name__</span><span class="o">></span><span class="p">()</span><span class="o">></span> <span class="n">exception</span><span class="o">=</span><span class="ne">AssertionError</span><span class="p">()</span><span class="o">></span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"example.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">6</span><span class="p">,</span> <span class="ow">in</span> <span class="n">cursor</span>
<span class="k">yield</span> <span class="mi">1</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">exceptions</span><span class="o">.</span><span class="n">CancelledError</span>
<span class="n">During</span> <span class="n">handling</span> <span class="n">of</span> <span class="n">the</span> <span class="n">above</span> <span class="n">exception</span><span class="p">,</span> <span class="n">another</span> <span class="n">exception</span> <span class="n">occurred</span><span class="p">:</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"example.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">8</span><span class="p">,</span> <span class="ow">in</span> <span class="n">cursor</span>
<span class="k">assert</span> <span class="n">work_done</span>
<span class="o">^^^^^^^^^</span>
<span class="ne">AssertionError</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">cursor()</span></code> asynchronous generator was finalized before the <code class="docutils literal notranslate"><span class="pre">rows</span></code>
generator - an unexpected behavior.</p>
<p>The example can be fixed by explicitly closing the
<code class="docutils literal notranslate"><span class="pre">cursor</span></code> and <code class="docutils literal notranslate"><span class="pre">rows</span></code> async-generators:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
<span class="k">async</span> <span class="k">with</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">aclosing</span><span class="p">(</span><span class="n">cursor</span><span class="p">())</span> <span class="k">as</span> <span class="n">cursor_gen</span><span class="p">:</span>
<span class="k">async</span> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">cursor_gen</span><span class="p">:</span>
<span class="k">async</span> <span class="k">with</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">aclosing</span><span class="p">(</span><span class="n">rows</span><span class="p">())</span> <span class="k">as</span> <span class="n">rows_gen</span><span class="p">:</span>
<span class="k">async</span> <span class="k">for</span> <span class="n">r</span> <span class="ow">in</span> <span class="n">rows_gen</span><span class="p">:</span>
<span class="k">break</span>
<span class="k">break</span>
</pre></div>
</div>
</section>
<section id="create-asynchronous-generators-only-when-the-event-loop-is-running">
<h3>Create asynchronous generators only when the event loop is running<a class="headerlink" href="#create-asynchronous-generators-only-when-the-event-loop-is-running" title="連結到這個標頭">¶</a></h3>
<p>It is recommended to create
<a class="reference internal" href="../glossary.html#term-asynchronous-generator-iterator"><span class="xref std std-term">asynchronous generators</span></a> only after
the event loop has been created.</p>
<p>To ensure that asynchronous generators close reliably, the event loop uses the
<a class="reference internal" href="sys.html#sys.set_asyncgen_hooks" title="sys.set_asyncgen_hooks"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.set_asyncgen_hooks()</span></code></a> function to register callback functions. These
callbacks update the list of running asynchronous generators to keep it in a
consistent state.</p>
<p>When the <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.shutdown_asyncgens" title="asyncio.loop.shutdown_asyncgens"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.shutdown_asyncgens()</span></code></a>
function is called, the running generators are stopped gracefully and the
list is cleared.</p>
<p>The asynchronous generator invokes the corresponding system hook during its
first iteration. At the same time, the generator records that the hook has
been called and does not call it again.</p>
<p>Therefore, if iteration begins before the event loop is created,
the event loop will not be able to add the generator to its list of active
generators because the hooks are set after the generator attempts to call them.
Consequently, the event loop will not be able to terminate the generator
if necessary.</p>
<p>Consider the following example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">agenfn</span><span class="p">():</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="mi">10</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="k">with</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Runner</span><span class="p">()</span> <span class="k">as</span> <span class="n">runner</span><span class="p">:</span>
<span class="n">agen</span> <span class="o">=</span> <span class="n">agenfn</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="n">runner</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">anext</span><span class="p">(</span><span class="n">agen</span><span class="p">)))</span>
<span class="k">del</span> <span class="n">agen</span>
</pre></div>
</div>
<p>輸出：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mi">10</span>
<span class="ne">Exception</span> <span class="n">ignored</span> <span class="k">while</span> <span class="n">closing</span> <span class="n">generator</span> <span class="o"><</span><span class="n">async_generator</span> <span class="nb">object</span> <span class="n">agenfn</span> <span class="n">at</span> <span class="mh">0x000002F71CD10D70</span><span class="o">></span><span class="p">:</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"example.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">13</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="k">del</span> <span class="n">agen</span>
<span class="o">^^^^</span>
<span class="ne">RuntimeError</span><span class="p">:</span> <span class="k">async</span> <span class="n">generator</span> <span class="n">ignored</span> <span class="ne">GeneratorExit</span>
</pre></div>
</div>
<p>This example can be fixed as follows:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">agenfn</span><span class="p">():</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="mi">10</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
<span class="n">agen</span> <span class="o">=</span> <span class="n">agenfn</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="k">await</span> <span class="n">anext</span><span class="p">(</span><span class="n">agen</span><span class="p">))</span>
<span class="k">del</span> <span class="n">agen</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
</div>
</section>
<section id="avoid-concurrent-iteration-and-closure-of-the-same-generator">
<h3>Avoid concurrent iteration and closure of the same generator<a class="headerlink" href="#avoid-concurrent-iteration-and-closure-of-the-same-generator" title="連結到這個標頭">¶</a></h3>
<p>Async generators may be reentered while another
<a class="reference internal" href="../reference/expressions.html#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__anext__()</span></code></a> / <a class="reference internal" href="../reference/expressions.html#agen.athrow" title="agen.athrow"><code class="xref py py-meth docutils literal notranslate"><span class="pre">athrow()</span></code></a> / <a class="reference internal" href="../reference/expressions.html#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> call is in
progress. This may lead to an inconsistent state of the async generator and can
cause errors.</p>
<p>Let's consider the following example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">consumer</span><span class="p">():</span>
<span class="k">for</span> <span class="n">idx</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">100</span><span class="p">):</span>
<span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="n">message</span> <span class="o">=</span> <span class="k">yield</span> <span class="n">idx</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'received'</span><span class="p">,</span> <span class="n">message</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">amain</span><span class="p">():</span>
<span class="n">agenerator</span> <span class="o">=</span> <span class="n">consumer</span><span class="p">()</span>
<span class="k">await</span> <span class="n">agenerator</span><span class="o">.</span><span class="n">asend</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="n">fa</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">agenerator</span><span class="o">.</span><span class="n">asend</span><span class="p">(</span><span class="s1">'A'</span><span class="p">))</span>
<span class="n">fb</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">agenerator</span><span class="o">.</span><span class="n">asend</span><span class="p">(</span><span class="s1">'B'</span><span class="p">))</span>
<span class="k">await</span> <span class="n">fa</span>
<span class="k">await</span> <span class="n">fb</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">amain</span><span class="p">())</span>
</pre></div>
</div>
<p>輸出：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">received</span> <span class="n">A</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"test.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">38</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">amain</span><span class="p">())</span>
<span class="o">~~~~~~~~~~~^^^^^^^^^</span>
<span class="n">File</span> <span class="s2">"Lib/asyncio/runners.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">204</span><span class="p">,</span> <span class="ow">in</span> <span class="n">run</span>
<span class="k">return</span> <span class="n">runner</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">)</span>
<span class="o">~~~~~~~~~~^^^^^^</span>
<span class="n">File</span> <span class="s2">"Lib/asyncio/runners.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">127</span><span class="p">,</span> <span class="ow">in</span> <span class="n">run</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_loop</span><span class="o">.</span><span class="n">run_until_complete</span><span class="p">(</span><span class="n">task</span><span class="p">)</span>
<span class="o">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^</span>
<span class="n">File</span> <span class="s2">"Lib/asyncio/base_events.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">719</span><span class="p">,</span> <span class="ow">in</span> <span class="n">run_until_complete</span>
<span class="k">return</span> <span class="n">future</span><span class="o">.</span><span class="n">result</span><span class="p">()</span>
<span class="o">~~~~~~~~~~~~~^^</span>
<span class="n">File</span> <span class="s2">"test.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">36</span><span class="p">,</span> <span class="ow">in</span> <span class="n">amain</span>
<span class="k">await</span> <span class="n">fb</span>
<span class="ne">RuntimeError</span><span class="p">:</span> <span class="n">anext</span><span class="p">():</span> <span class="n">asynchronous</span> <span class="n">generator</span> <span class="ow">is</span> <span class="n">already</span> <span class="n">running</span>
</pre></div>
</div>
<p>Therefore, it is recommended to avoid using asynchronous generators in parallel
tasks or across multiple event loops.</p>
</section>
</section>
</section>
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<li><a class="reference internal" href="#debug-mode">除錯模式</a></li>
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<li><a class="reference internal" href="#avoid-concurrent-iteration-and-closure-of-the-same-generator">Avoid concurrent iteration and closure of the same generator</a></li>
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