python 保留最后N个元素

保留最后 N 个元素

• 问题

  在迭代操作或者其他操作的时候，怎样只保留最后有限几个元素的历史记录？

• 实现

  保留有限历史记录可通过 collections.deque 实现。例如下面的代码在多行上面做简单的文本匹配， 并返回匹配所在行的最后N行：

  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(r'../../cookbook/somefile.txt') as f:
          for line, prevlines in search(f, 'python', 5):
              for pline in prevlines:
                  print(pline, end='')
              print(line, end='')
              print('-' * 20)

• deque 用法简述

  from collections import deque
  
  语法:
    deque([iterable[, maxlen]]) --> deque object
        iterable：init iterable
        maxlen： maximum size of a deque or None if unbounded
      
  方法:
    append(...)         Add an element to the right side of the deque.
    appendleft(...)     Add an element to the left side of the deque.
    clear(...)          Remove all elements from the deque.
    copy(...)           Return a shallow copy of a deque.
    count(...)          D.count(value) -> integer -- return number of occurrences of value
    extend(...)         Extend the right side of the deque with elements from the iterable
    extendleft(...)     Extend the left side of the deque with elements from the iterable
    index(...)          D.index(value, [start, [stop]]) -> integer -- return first index of value.
                        Raises ValueError if the value is not present.
    insert(...)         D.insert(index, object) -- insert object before index
    pop(...)            Remove and return the rightmost element.
    popleft(...)        Remove and return the leftmost element.
    remove(...)         D.remove(value) -- remove first occurrence of value.
    reverse(...)        D.reverse() -- reverse *IN PLACE*
    rotate(...)         Rotate the deque n steps to the right (default n=1).  If n is negative

Reference

  https://python3-cookbook.readthedocs.io/zh_CN/latest/c01/p03_keep_last_n_items.html