yield 简单说来就是一个生成器，生成器是这样一个函数，它记住上一次返回时在函数体中的位置。对生成器函数的第二次（或第 n 次）调用跳转至该函数中间，而上次调用的所有局部变量都保持不变。
函数的所有参数都会保留
使用的参数是前一次保留下的.
生成器不仅“记住”了它数据状态。 生成器还“记住”了它在流控制构造（在命令式编程中，这种构造不只是数据值）中的位置。由于连续性使您在执行框架间任意跳转，而不总是返回到直接调用者的上下文（如同生成器那样），因此它仍是比较一般的。
当你问生成器要一个数时，生成器会执行，直至出现yield 语句，生成器把yield 的参数给你，之后生成器就不会往下继续运行。 当你问他要下一个数时，他会从上次的状态开始运行，直至出现yield语句，把参数给你，之后停下。如此反复直至退出函数。
<span class="co1" style="padding: 0px; margin: 0px; color: rgb(128, 128, 128); font-style: italic;">#生成全排列</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">def</span> perm<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items, n=<span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">is</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span>: n = <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> i <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">range</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span><span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>: v = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i:i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n == <span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">else</span>: rest = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>:i<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span> + items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>:<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> p <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> perm<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>rest, n-<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v + p <span class="co1" style="padding: 0px; margin: 0px; color: rgb(128, 128, 128); font-style: italic;">#生成组合</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">def</span> comb<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items, n=<span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">is</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">None</span>: n = <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> i <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> <span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">range</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span><span class="kw2" style="padding: 0px; margin: 0px; color: rgb(0, 0, 0); font-weight: bold;">len</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>: v = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i:i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">if</span> n == <span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">else</span>: rest = items<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">[</span>i+<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span>:<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">]</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> c <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> comb<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span>rest, n-<span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">1</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span>: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);"><span class="search_hit" style="padding: 0px; margin: 0px; color: rgb(51, 51, 51); background-color: rgb(255, 255, 153);">yield</span></span> v + c a = perm<span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">(</span><span class="st0" style="padding: 0px; margin: 0px; color: rgb(255, 0, 0);">'abc'</span><span class="br0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">)</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> b <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> a: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">print</span> b <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">break</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">print</span> <span class="st0" style="padding: 0px; margin: 0px; color: rgb(255, 0, 0);">'-'</span><span class="sy0" style="padding: 0px; margin: 0px; color: rgb(102, 204, 102);">*</span><span class="nu0" style="padding: 0px; margin: 0px; color: rgb(204, 102, 204);">20</span> <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">for</span> b <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">in</span> a: <span class="kw1" style="padding: 0px; margin: 0px; color: rgb(177, 177, 0);">print</span> b结果如下：
102 pvopf006 ~/test> ./generator.py abc -------------------- acb bac bca cab cba
可以看到，在第一个循环break后，生成器没有继续执行，而第二个循环接着第一个循环执行