保存最后N个元素

cookbook系列

问题：对要搜索的值的最后几项做个有限的历史记录。

方案：

 #coding=utf-

 from collections import deque

 def search(lines, pattern, history=):

     previous_lines = deque(maxlen=history)  #deque双端队列

     for line in lines:

         if pattern in line:

             yield line, previous_lines      #生成器，双端队列保存5个，意味着在找到pattern之前会记录5行数据，pattern就在line里面

         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', ):

             for pline in prevlines:

                 print pline,  #先输出队列里的5个

             print line,     #在输出pattern的line

             print('-'*)

案例文件：somefile.txt

=== Keeping the Last N Items

==== Problem

You want to keep a limited history of the last few items seen

during iteration or during some other kind of processing.

==== Solution

Keeping a limited history is a perfect use for a `collections.deque`.

For example, the following code performs a simple text match on a

sequence of lines and prints the matching line along with the previous

N lines of context when found:

[source,python]

----

from collections import deque

def search(lines, pattern, history=):

    previous_lines = deque(maxlen=history)

    for line in lines:

        if pattern in line:

            for pline in previous_lines:

                print(lline, end='')

            print(line, end='')

            print()

        previous_lines.append(line)

# Example use on a file

if __name__ == '__main__':

    with open('somefile.txt') as f:

         search(f, 'python', )

----

==== Discussion

Using `deque(maxlen=N)` creates a fixed size queue.  When new items

are added and the queue is full, the oldest item is automatically

removed.   For example:

[source,pycon]

----

>>> q = deque(maxlen=)

>>> q.append()

>>> q.append()

>>> q.append()

>>> q

deque([, , ], maxlen=)

>>> q.append()

>>> q

deque([, , ], maxlen=)

>>> q.append()

>>> q

deque([, , ], maxlen=)

----

Although you could manually perform such operations on a list (e.g.,

appending, deleting, etc.), the queue solution is far more elegant and

runs a lot faster.

More generally, a `deque` can be used whenever you need a simple queue

structure.  If you don't give it a maximum size, you get an unbounded

queue that lets you append and pop items on either end.  For example:

[source,pycon]

----

>>> q = deque()

>>> q.append()

>>> q.append()

>>> q.append()

>>> q

deque([, , ])

>>> q.appendleft()

>>> q

deque([, , , ])

>>> q.pop()

>>> q

deque([, , ])

>>> q.popleft()

----

Adding or popping items from either end of a queue has O()

complexity.   This is unlike a list where inserting or removing

items from the front of the list is O(N).    

运行结果：

H:\Python27_64\python.exe H:/myfile/python-cookbook-master/src//keeping_the_last_n_items/example.py

Keeping a limited history is a perfect use for a `collections.deque`.

For example, the following code performs a simple text match on a

sequence of lines and prints the matching line along with the previous

N lines of context when found:

[source,python]

--------------------

        previous_lines.append(line)

# Example use on a file

if __name__ == '__main__':

    with open('somefile.txt') as f:

         search(f, 'python', )

--------------------

进程已结束,退出代码0

讨论：

第5行双端队列deque的用法

固定长度队列：
>>> q = deque(maxlen=)

>>> q.append()

>>> q.append()

>>> q.append()

>>> q

deque([, , ], maxlen=)

>>> q.append()

>>> q

deque([, , ], maxlen=)

>>> q.append()

>>> q

deque([, , ], maxlen=)

双端队列：

>>> q = deque()

>>> q.append()

>>> q.append()

>>> q.append()

>>> q

deque([, , ])

>>> q.appendleft()

>>> q

deque([, , , ])

>>> q.pop()

>>> q

deque([, , ])

>>> q.popleft()

4

两者皆有：设置好固定长度时，左右两端均可添加和删除

使用双端队列要比列表 方便、简洁！