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 '打開文件失敗，囧，看看文件是不是不存在，或者權(quán)限有問題'
      print e

# 使用默認(rèn)的sys.stdout.write打印到屏幕
py_tail = Tail('test.txt')
py_tail.follow()
# 自己定義處理函數(shù)
def test_tail(line):
  print 'xx'+line+'xx'
py_tail1 = Tail('test.txt', test_tail)
py_tail1.follow()

# 演示代碼，說明核心邏輯，完整代碼在下面
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 '打開文件失敗，囧，看看文件是不是不存在，或者權(quán)限有問題'
      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:
      # 打開文件
      with open(self.file_name) as f:
        self._file = f
        self._file.seek(0,2)
        # 存儲文件的字符長度
        self.file_length = self._file.tell()
        # 打印最后10行
        self.showLastLine(n)
        # 持續(xù)讀文件 打印增量
        while True:
          line = self._file.readline()
          if line:
            self.callback(line)
          time.sleep(1)
    except Exception,e:
      print '打開文件失敗，囧，看看文件是不是不存在，或者權(quán)限有問題'
      print e
  def showLastLine(self, n):
    # 一行大概100個吧 這個數(shù)改成1或者1000都行
    len_line = 100
    # n默認(rèn)是10，也可以follow的參數(shù)傳進(jìn)來
    read_len = len_line*n
    # 用last_lines存儲最后要處理的內(nèi)容
    while True:
      # 如果要讀取的1000個字符，大于之前存儲的文件長度
      # 讀完文件，直接break
      if read_len>self.file_length:
        self._file.seek(0)
        last_lines = self._file.read().split('\n')[-n:]
        break
      # 先讀1000個 然后判斷1000個字符里換行符的數(shù)量
      self._file.seek(-read_len, 2)
      last_words = self._file.read(read_len)
      # count是換行符的數(shù)量
      count = last_words.count('\n')
      if count>=n:
        # 換行符數(shù)量大于10 很好處理，直接讀取
        last_lines = last_words.split('\n')[-n:]
        break
      # 換行符不夠10個
      else:
        # break
        #不夠十行
        # 如果一個換行符也沒有，那么我們就認(rèn)為一行大概是100個
        if count==0:
          len_perline = read_len
        # 如果有4個換行符，我們認(rèn)為每行大概有250個字符
        else:
          len_perline = read_len/count
        # 要讀取的長度變?yōu)?500，繼續(xù)重新判斷
        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')