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<li><a class="reference internal" href="#future-functions">Future 函数</a></li>
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<section id="futures">
<span id="asyncio-futures"></span><h1>Futures<a class="headerlink" href="#futures" title="永久链接至标题"></a></h1>
<p><strong>源代码:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.8/Lib/asyncio/futures.py">Lib/asyncio/futures.py</a>, <a class="reference external" href="https://github.com/python/cpython/tree/3.8/Lib/asyncio/base_futures.py">Lib/asyncio/base_futures.py</a></p>
<hr class="docutils" />
<p><em>Future</em> 对象用来链接 <strong>底层回调式代码</strong> 和高层异步/等待式代码。</p>
<section id="future-functions">
<h2>Future 函数<a class="headerlink" href="#future-functions" title="永久链接至标题"></a></h2>
<dl class="function">
<dt id="asyncio.isfuture">
<code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">isfuture</code><span class="sig-paren">(</span><em class="sig-param">obj</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.isfuture" title="永久链接至目标"></a></dt>
<dd><p>如果 <em>obj</em> 为下面任意对象，返回 <code class="docutils literal notranslate"><span class="pre">True</span></code>：</p>
<ul class="simple">
<li><p>一个 <a class="reference internal" href="#asyncio.Future" title="asyncio.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">asyncio.Future</span></code></a> 类的实例，</p></li>
<li><p>一个 <a class="reference internal" href="asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate"><span class="pre">asyncio.Task</span></code></a> 类的实例，</p></li>
<li><p>带有 <code class="docutils literal notranslate"><span class="pre">_asyncio_future_blocking</span></code> 属性的类似 Future 的对象。</p></li>
</ul>
<div class="versionadded">
<p><span class="versionmodified added">3.5 新版功能.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="asyncio.ensure_future">
<code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">ensure_future</code><span class="sig-paren">(</span><em class="sig-param">obj</em>, <em class="sig-param">*</em>, <em class="sig-param">loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.ensure_future" title="永久链接至目标"></a></dt>
<dd><p>返回：</p>
<ul class="simple">
<li><p><em>obj</em> 参数会是保持原样，如果 <em>obj</em> 是 <a class="reference internal" href="#asyncio.Future" title="asyncio.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a>、 <a class="reference internal" href="asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate"><span class="pre">Task</span></code></a> 或 类似 Future 的对象( <a class="reference internal" href="#asyncio.isfuture" title="asyncio.isfuture"><code class="xref py py-func docutils literal notranslate"><span class="pre">isfuture()</span></code></a> 用于测试。)</p></li>
<li><p>封装了 <em>obj</em> 的 <a class="reference internal" href="asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate"><span class="pre">Task</span></code></a> 对象，如果 <em>obj</em> 是一个协程 (使用 <a class="reference internal" href="asyncio-task.html#asyncio.iscoroutine" title="asyncio.iscoroutine"><code class="xref py py-func docutils literal notranslate"><span class="pre">iscoroutine()</span></code></a> 进行检测)；在此情况下该协程将通过 <code class="docutils literal notranslate"><span class="pre">ensure_future()</span></code> 加入执行计划。</p></li>
<li><p>等待 <em>obj</em> 的 <a class="reference internal" href="asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate"><span class="pre">Task</span></code></a> 对象，如果 <em>obj</em> 是一个可等待对象( <a class="reference internal" href="inspect.html#inspect.isawaitable" title="inspect.isawaitable"><code class="xref py py-func docutils literal notranslate"><span class="pre">inspect.isawaitable()</span></code></a> 用于测试)</p></li>
</ul>
<p>如果 <em>obj</em> 不是上述对象会引发一个 <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> 异常。</p>
<div class="admonition important">
<p class="admonition-title">重要</p>
<p>查看 <a class="reference internal" href="asyncio-task.html#asyncio.create_task" title="asyncio.create_task"><code class="xref py py-func docutils literal notranslate"><span class="pre">create_task()</span></code></a> 函数，它是创建新任务的首选途径。</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">在 3.5.1 版更改: </span>这个函数接受任意 <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> 对象。</p>
</div>
</dd></dl>
<dl class="function">
<dt id="asyncio.wrap_future">
<code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">wrap_future</code><span class="sig-paren">(</span><em class="sig-param">future</em>, <em class="sig-param">*</em>, <em class="sig-param">loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.wrap_future" title="永久链接至目标"></a></dt>
<dd><p>将一个 <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> 对象封装到 <a class="reference internal" href="#asyncio.Future" title="asyncio.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">asyncio.Future</span></code></a> 对象中。</p>
</dd></dl>
</section>
<section id="future-object">
<h2>Future 对象<a class="headerlink" href="#future-object" title="永久链接至标题"></a></h2>
<dl class="class">
<dt id="asyncio.Future">
<em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">Future</code><span class="sig-paren">(</span><em class="sig-param">*</em>, <em class="sig-param">loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future" title="永久链接至目标"></a></dt>
<dd><p>一个 Future 代表一个异步运算的最终结果。线程不安全。</p>
<p>Future 是一个 <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> 对象。协程可以等待 Future 对象直到它们有结果或异常集合或被取消。</p>
<p>通常 Future 用于支持底层回调式代码(例如在协议实现中使用asyncio <a class="reference internal" href="asyncio-protocol.html#asyncio-transports-protocols"><span class="std std-ref">transports</span></a>) 与高层异步/等待式代码交互。</p>
<p>经验告诉我们永远不要面向用户的接口暴露 Future 对象，同时建议使用 <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.create_future" title="asyncio.loop.create_future"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.create_future()</span></code></a> 来创建 Future 对象。这种方法可以让 Future 对象使用其它的事件循环实现，它可以注入自己的优化实现。</p>
<div class="versionchanged">
<p><span class="versionmodified changed">在 3.7 版更改: </span>加入对 <a class="reference internal" href="contextvars.html#module-contextvars" title="contextvars: Context Variables"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextvars</span></code></a> 模块的支持。</p>
</div>
<dl class="method">
<dt id="asyncio.Future.result">
<code class="sig-name descname">result</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.result" title="永久链接至目标"></a></dt>
<dd><p>返回 Future 的结果。</p>
<p>如果 Future 状态为 <em>完成</em> ，并由 <a class="reference internal" href="#asyncio.Future.set_result" title="asyncio.Future.set_result"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_result()</span></code></a> 方法设置一个结果，则返回这个结果。</p>
<p>如果 Future 状态为 <em>完成</em> ，并由 <a class="reference internal" href="#asyncio.Future.set_exception" title="asyncio.Future.set_exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_exception()</span></code></a> 方法设置一个异常，那么这个方法会引发异常。</p>
<p>如果 Future 已 <em>取消</em>，方法会引发一个 <a class="reference internal" href="asyncio-exceptions.html#asyncio.CancelledError" title="asyncio.CancelledError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">CancelledError</span></code></a> 异常。</p>
<p>如果 Future 的结果还不可用，此方法会引发一个 <a class="reference internal" href="asyncio-exceptions.html#asyncio.InvalidStateError" title="asyncio.InvalidStateError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InvalidStateError</span></code></a> 异常。</p>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.set_result">
<code class="sig-name descname">set_result</code><span class="sig-paren">(</span><em class="sig-param">result</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.set_result" title="永久链接至目标"></a></dt>
<dd><p>将 Future 标记为 <em>完成</em> 并设置结果。</p>
<p>如果 Future 已经 <em>完成</em> 则抛出一个 <a class="reference internal" href="asyncio-exceptions.html#asyncio.InvalidStateError" title="asyncio.InvalidStateError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InvalidStateError</span></code></a> 错误。</p>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.set_exception">
<code class="sig-name descname">set_exception</code><span class="sig-paren">(</span><em class="sig-param">exception</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.set_exception" title="永久链接至目标"></a></dt>
<dd><p>将 Future 标记为 <em>完成</em> 并设置一个异常。</p>
<p>如果 Future 已经 <em>完成</em> 则抛出一个 <a class="reference internal" href="asyncio-exceptions.html#asyncio.InvalidStateError" title="asyncio.InvalidStateError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InvalidStateError</span></code></a> 错误。</p>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.done">
<code class="sig-name descname">done</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.done" title="永久链接至目标"></a></dt>
<dd><p>如果 Future 为已 <em>完成</em> 则返回 <code class="docutils literal notranslate"><span class="pre">True</span></code> 。</p>
<p>如果 Future 为 <em>取消</em> 或调用 <a class="reference internal" href="#asyncio.Future.set_result" title="asyncio.Future.set_result"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_result()</span></code></a> 设置了结果或调用 <a class="reference internal" href="#asyncio.Future.set_exception" title="asyncio.Future.set_exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_exception()</span></code></a> 设置了异常，那么它就是 <em>完成</em> 。</p>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.cancelled">
<code class="sig-name descname">cancelled</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.cancelled" title="永久链接至目标"></a></dt>
<dd><p>如果 Future 已 <em>取消</em> 则返回 <code class="docutils literal notranslate"><span class="pre">True</span></code></p>
<p>这个方法通常在设置结果或异常前用来检查 Future 是否已 <em>取消</em> 。</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="ow">not</span> <span class="n">fut</span><span class="o">.</span><span class="n">cancelled</span><span class="p">():</span>
<span class="n">fut</span><span class="o">.</span><span class="n">set_result</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.add_done_callback">
<code class="sig-name descname">add_done_callback</code><span class="sig-paren">(</span><em class="sig-param">callback</em>, <em class="sig-param">*</em>, <em class="sig-param">context=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.add_done_callback" title="永久链接至目标"></a></dt>
<dd><p>添加一个在 Future <em>完成</em> 时运行的回调函数。</p>
<p>调用 <em>callback</em> 时，Future 对象是它的唯一参数。</p>
<p>如果调用这个方法时 Future 已经 <em>完成</em>，回调函数会被 <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> 调度。</p>
<p>可选键值类的参数 <em>context</em> 允许 <em>callback</em> 运行在一个指定的自定义 <a class="reference internal" href="contextvars.html#contextvars.Context" title="contextvars.Context"><code class="xref py py-class docutils literal notranslate"><span class="pre">contextvars.Context</span></code></a> 对象中。如果没有提供 <em>context</em> ，则使用当前上下文。</p>
<p>可以用 <a class="reference internal" href="functools.html#functools.partial" title="functools.partial"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.partial()</span></code></a> 给回调函数传递参数，例如:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Call &#39;print(&quot;Future:&quot;, fut)&#39; when &quot;fut&quot; is done.</span>
<span class="n">fut</span><span class="o">.</span><span class="n">add_done_callback</span><span class="p">(</span>
<span class="n">functools</span><span class="o">.</span><span class="n">partial</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;Future:&quot;</span><span class="p">))</span>
</pre></div>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">在 3.7 版更改: </span>加入键值类形参 <em>context</em>。请参阅 <span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0567"><strong>PEP 567</strong></a> 查看更多细节。</p>
</div>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.remove_done_callback">
<code class="sig-name descname">remove_done_callback</code><span class="sig-paren">(</span><em class="sig-param">callback</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.remove_done_callback" title="永久链接至目标"></a></dt>
<dd><p>从回调列表中移除 <em>callback</em> 。</p>
<p>返回被移除的回调函数的数量，通常为1，除非一个回调函数被添加多次。</p>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.cancel">
<code class="sig-name descname">cancel</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.cancel" title="永久链接至目标"></a></dt>
<dd><p>取消 Future 并调度回调函数。</p>
<p>如果 Future 已经 <em>完成</em> 或 <em>取消</em> ，返回 <code class="docutils literal notranslate"><span class="pre">False</span></code> 。否则将 Future 状态改为 <em>取消</em> 并在调度回调函数后返回 <code class="docutils literal notranslate"><span class="pre">True</span></code> 。</p>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.exception">
<code class="sig-name descname">exception</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.exception" title="永久链接至目标"></a></dt>
<dd><p>返回 Future 已设置的异常。</p>
<p>只有 Future 在 <em>完成</em> 时才返回异常（或者 <code class="docutils literal notranslate"><span class="pre">None</span></code> ，如果没有设置异常）。</p>
<p>如果 Future 已 <em>取消</em>，方法会引发一个 <a class="reference internal" href="asyncio-exceptions.html#asyncio.CancelledError" title="asyncio.CancelledError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">CancelledError</span></code></a> 异常。</p>
<p>如果 Future 还没 <em>完成</em> ，这个方法会引发一个 <a class="reference internal" href="asyncio-exceptions.html#asyncio.InvalidStateError" title="asyncio.InvalidStateError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InvalidStateError</span></code></a> 异常。</p>
</dd></dl>
<dl class="method">
<dt id="asyncio.Future.get_loop">
<code class="sig-name descname">get_loop</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Future.get_loop" title="永久链接至目标"></a></dt>
<dd><p>返回 Future 对象已绑定的事件循环。</p>
<div class="versionadded">
<p><span class="versionmodified added">3.7 新版功能.</span></p>
</div>
</dd></dl>
</dd></dl>
<p id="asyncio-example-future">这个例子创建一个 Future 对象，创建和调度一个异步任务去设置 Future 结果，然后等待其结果:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span> <span class="nf">set_after</span><span class="p">(</span><span class="n">fut</span><span class="p">,</span> <span class="n">delay</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="c1"># Sleep for *delay* seconds.</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="n">delay</span><span class="p">)</span>
<span class="c1"># Set *value* as a result of *fut* Future.</span>
<span class="n">fut</span><span class="o">.</span><span class="n">set_result</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="c1"># Get the current event loop.</span>
<span class="n">loop</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">get_running_loop</span><span class="p">()</span>
<span class="c1"># Create a new Future object.</span>
<span class="n">fut</span> <span class="o">=</span> <span class="n">loop</span><span class="o">.</span><span class="n">create_future</span><span class="p">()</span>
<span class="c1"># Run &quot;set_after()&quot; coroutine in a parallel Task.</span>
<span class="c1"># We are using the low-level &quot;loop.create_task()&quot; API here because</span>
<span class="c1"># we already have a reference to the event loop at hand.</span>
<span class="c1"># Otherwise we could have just used &quot;asyncio.create_task()&quot;.</span>
<span class="n">loop</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span>
<span class="n">set_after</span><span class="p">(</span><span class="n">fut</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">&#39;... world&#39;</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;hello ...&#39;</span><span class="p">)</span>
<span class="c1"># Wait until *fut* has a result (1 second) and print it.</span>
<span class="nb">print</span><span class="p">(</span><span class="k">await</span> <span class="n">fut</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>
<div class="admonition important">
<p class="admonition-title">重要</p>
<p>该 Future 对象是为了模仿 <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> 类。主要差异包含：</p>
<ul class="simple">
<li><p>与 asyncio 的 Future 不同，<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> 实例不是可等待对象。</p></li>
<li><p><a class="reference internal" href="#asyncio.Future.result" title="asyncio.Future.result"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.Future.result()</span></code></a> 和 <a class="reference internal" href="#asyncio.Future.exception" title="asyncio.Future.exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.Future.exception()</span></code></a> 不接受 <em>timeout</em> 参数。</p></li>
<li><p>Future 没有 <em>完成</em> 时 <a class="reference internal" href="#asyncio.Future.result" title="asyncio.Future.result"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.Future.result()</span></code></a> 和 <a class="reference internal" href="#asyncio.Future.exception" title="asyncio.Future.exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.Future.exception()</span></code></a> 抛出一个 <a class="reference internal" href="asyncio-exceptions.html#asyncio.InvalidStateError" title="asyncio.InvalidStateError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InvalidStateError</span></code></a> 异常。</p></li>
<li><p>使用 <a class="reference internal" href="#asyncio.Future.add_done_callback" title="asyncio.Future.add_done_callback"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.Future.add_done_callback()</span></code></a> 注册的回调函数不会立即调用，而是被 <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> 调度。</p></li>
<li><p>asyncio Future 不能兼容 <a class="reference internal" href="concurrent.futures.html#concurrent.futures.wait" title="concurrent.futures.wait"><code class="xref py py-func docutils literal notranslate"><span class="pre">concurrent.futures.wait()</span></code></a> 和 <a class="reference internal" href="concurrent.futures.html#concurrent.futures.as_completed" title="concurrent.futures.as_completed"><code class="xref py py-func docutils literal notranslate"><span class="pre">concurrent.futures.as_completed()</span></code></a> 函数。</p></li>
</ul>
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