论坛站务软件水平考试华为认证计算机等级PMP认证思科认证Photoshop3DMAX编程语言移动开发系统运维产品运营淘宝营销办公软件Ps修图云计算大数据摄影就业

<h2 style="border: 0px; margin: 0px 0px 20px; padding: 0px; font-size: 24px; font-stretch: normal; line-height: 36px; font-family: &quot;Microsoft YaHei&quot;, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; color: rgb(46, 46, 46); white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;延迟绑定</h2><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Python闭包函数所引用的外部自由变量是延迟绑定的。</p><p style="text-align: center"><img src="/Public/forum/ueditor/image/20170718/1500341910204941.jpg" alt="1500341910204941.jpg"/></p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;如以上代码:&nbsp;<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i</code>是闭包函数引用的外部作用域的自由变量, 只有在内部函数被调用的时候才会搜索变量<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i</code>的值, 由于循环已结束,&nbsp;<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i</code>指向最终值3, 所以各函数调用都得到了相同的结果。</p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;解决方法:</p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;1) 生成闭包函数的时候立即绑定(<strong style="border: 0px; margin: 0px; padding: 0px;">使用函数形参的默认值</strong>):</p><p style="text-align: center"><img src="/Public/forum/ueditor/image/20170718/1500341941847503.jpg" alt="1500341941847503.jpg"/></p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;如以上代码: 生成闭包函数的时候, 可以看到每个闭包函数都有一个带默认值的参数:&nbsp;<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i=i</code>, 此时, 解释器会查找<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i</code>的值, 并将其赋予形参<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i</code>, 这样在生成闭包函数的外部作用域(即外部循环中), 找到了变量<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i</code>, 遂将其当前值赋予形参<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">i</code>。</p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2) 使用<strong style="border: 0px; margin: 0px; padding: 0px;">functools.partial</strong>:</p><p style="text-align: center"><img src="/Public/forum/ueditor/image/20170718/1500341969850250.jpg" alt="1500341969850250.jpg"/></p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;如以上代码: 在有可能因为延迟绑定而出问题的时候, 可以通过<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">functools.partial</code>构造偏函数, 使得自由变量优先绑定到闭包函数上。</p><h2 style="border: 0px; margin: 0px 0px 20px; padding: 0px; font-size: 24px; font-stretch: normal; line-height: 36px; font-family: &quot;Microsoft YaHei&quot;, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; color: rgb(46, 46, 46); white-space: normal; background-color: rgb(255, 255, 255);"><span id="pos_placeholder" style="border: 0px; margin: 0px; padding: 0px; width: 0px; height: 0px;"></span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;禁止在闭包函数内对引用的自由变量进行重新绑定</h2><p style="text-align: center"><img src="/Public/forum/ueditor/image/20170718/1500341999878827.jpg" alt="1500341999878827.jpg"/></p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;以上代码会报错,&nbsp;<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">UnboundLocalError: local variable &#39;free_value&#39; referenced before assignment</code>, 以上代码本意是打算实现一个带有某个初始化状态(<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">free_value</code>)但在执行内部闭包函数的时候又可以按需变化出新的状态(<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">free_value = old_free_value * 2</code>)的装饰器, 但内部由于发生了重新绑定, 解释器会将<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">free_value</code>看作局部变量,&nbsp;<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">old_free_value = free_value</code>则会报错, 因为解释器认为<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">free_value</code>是没有赋值就被引用了。</p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;解决：打算修改闭包函数引用的自由变量时, 可以将其放入一个list, 这样,&nbsp;<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">free_value = [8]</code>,&nbsp;<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">free_value</code>不可修改, 但<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">free_value[0]</code>是可以安全的被修改的。</p><p style="border: 0px; margin-top: 0px; margin-bottom: 20px; padding: 0px; font-size: 15px; color: rgb(46, 46, 46); font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; white-space: normal; background-color: rgb(255, 255, 255);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;另外, Python 3.x增加了<code style="border: 0px; margin: 0px; padding: 0px; font-family: Monaco, Consolas, &quot;Andale Mono&quot;, &quot;DejaVu Sans Mono&quot;, monospace;">nonlocal</code>关键字, 也可以解决这个问题。</p><p><br/></p>

论坛站务软件水平考试华为认证计算机等级PMP认证思科认证Photoshop3DMAX编程语言移动开发系统运维产品运营淘宝营销办公软件Ps修图云计算大数据摄影就业