python实现tail -f 功能

open('a.txt')
file.seek
file.tell
time.sleep()

with open('test.txt') as f:
  f.seek(0,2)
  while True:
    last_pos = f.tell()
    line = f.readline()
    if line:
      print line

#!/usr/bin/env python
# -*- coding:utf-8 -*-
import sys
import time
class Tail():
  def __init__(self,file_name,callback=sys.stdout.write):
    self.file_name = file_name
    self.callback = callback
  def follow(self):
    try:
      with open(self.file_name) as f:
        f.seek(0,2)
        while True:
          line = f.readline()
          if line:
            self.callback(line)
          time.sleep(1)
    except Exception,e:
      print '打开文件失败，囧，看看文件是不是不存在，或者权限有问题'
      print e

# 使用默认的sys.stdout.write打印到屏幕
py_tail = Tail('test.txt')
py_tail.follow()
# 自己定义处理函数
def test_tail(line):
  print 'xx'+line+'xx'
py_tail1 = Tail('test.txt', test_tail)
py_tail1.follow()

# 演示代码，说明核心逻辑，完整代码在下面
last_lines = f.readlines()[-10:]
for line in last_lines:
  self.callback(line)

import sys
import time
class Tail():
  def __init__(self,file_name,callback=sys.stdout.write):
    self.file_name = file_name
    self.callback = callback
  def follow(self,n=10):
    try:
      with open(self.file_name) as f:
        self._file = f
        self.showLastLine(n)
        self._file.seek(0,2)
        while True:
          line = self._file.readline()
          if line:
            self.callback(line)
    except Exception,e:
      print '打开文件失败，囧，看看文件是不是不存在，或者权限有问题'
      print e
  def showLastLine(self, n):
    last_lines = self._file.readlines()[-10:]
    for line in last_lines:
      self.callback(line)

#!/usr/bin/env python
# -*- coding:utf-8 -*-
import sys
import time
class Tail():
  def __init__(self,file_name,callback=sys.stdout.write):
    self.file_name = file_name
    self.callback = callback
  def follow(self,n=10):
    try:
      # 打开文件
      with open(self.file_name) as f:
        self._file = f
        self._file.seek(0,2)
        # 存储文件的字符长度
        self.file_length = self._file.tell()
        # 打印最后10行
        self.showLastLine(n)
        # 持续读文件 打印增量
        while True:
          line = self._file.readline()
          if line:
            self.callback(line)
          time.sleep(1)
    except Exception,e:
      print '打开文件失败，囧，看看文件是不是不存在，或者权限有问题'
      print e
  def showLastLine(self, n):
    # 一行大概100个吧 这个数改成1或者1000都行
    len_line = 100
    # n默认是10，也可以follow的参数传进来
    read_len = len_line*n
    # 用last_lines存储最后要处理的内容
    while True:
      # 如果要读取的1000个字符，大于之前存储的文件长度
      # 读完文件，直接break
      if read_len>self.file_length:
        self._file.seek(0)
        last_lines = self._file.read().split('\n')[-n:]
        break
      # 先读1000个 然后判断1000个字符里换行符的数量
      self._file.seek(-read_len, 2)
      last_words = self._file.read(read_len)
      # count是换行符的数量
      count = last_words.count('\n')
      if count>=n:
        # 换行符数量大于10 很好处理，直接读取
        last_lines = last_words.split('\n')[-n:]
        break
      # 换行符不够10个
      else:
        # break
        #不够十行
        # 如果一个换行符也没有，那么我们就认为一行大概是100个
        if count==0:
          len_perline = read_len
        # 如果有4个换行符，我们认为每行大概有250个字符
        else:
          len_perline = read_len/count
        # 要读取的长度变为2500，继续重新判断
        read_len = len_perline * n
    for line in last_lines:
      self.callback(line+'\n')
if __name__ == '__main__':
  py_tail = Tail('test.txt')
  py_tail.follow(20)

import os
def tail(file_name):
  os.system('tail -f '+file_name)
tail('log.log')