yield 相当于 return，是一个生成器，目前知道的就两点：

  • 返回一个值，并且记住这个返回的位置，下次迭代就从这个位置后开始
  • 值是一次性的，不可以被反复访问，返回一次后便丢失

deque(maxlen=N) 创建一个固定长度的队列。当有新纪录加入而队列已满时会自动移除最老的那条记录，当不指定 maxlen 时，则代表创建一个可变长的队列。

下面的代码对一系列文本做简单的文本匹配操作，当发现又匹配时就输出当前的匹配行以及最后检查到的那一行文本：

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from collections import deque
 
 
def search(lines, pattern, history=5):
    previous_lines = deque(maxlen=history)
    for line in lines:
        if pattern in line:
            yield line, previous_lines
        previous_lines.append(line)
 
 
# Example use on a file
if __name__ == '__main__':
    with open('somefile.txt') as f:
        for line, prevlines in search(f, 'python', 5):
            for pline in prevlines:
                print(pline, end='')
            print(line, end='')
            print('-' * 20)

somefile.txt 文件内容如下：

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This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a test file.
This is a python test file.
This is a python test file.
This is a python test file.
This is a python test file.
This is a python test file.
This is a python test file.
This is a python test file.

最后的输出结果为：

对于 deque，可以在两端执行添加和弹出操作：

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from collections import deque
 
q = deque()
q.append(1)
q.appendleft(2)
q.append(3)
print(q)        # deque([2, 1, 3])
print(q.pop())   # 3
print(q.popleft())   # 2

参考：《Python Cookbook》