python实现tail（考虑到几种特殊情况）

昊漫玉

　　1. 网上有很多种使用python实现tail的方法，大体思想都一样，典型的一种如下：



#!/usr/bin/env python
#!-*- coding: utf-8 -*-
################################################################################
#
# Copyright (c) 2015 XX.com, Inc. All Rights Reserved
#
################################################################################
################################################################################
# This module provide ...
# Third libs those depend on:
################################################################################
"""
Compiler Python 2.7.10
Authors: XX(XX@baidu.com)
Date: 2015-09-09 12:57:41
Todo: nothing $XX$2015-09-09
"""
"""SYS LIBS
"""

import os
import re
import sys
import time
import traceback
"""THIRD LIBS
"""
try:
# import the third libs there.
pass
except ImportError as e:
print e
os._exit(-1)
"""CUSTOM libs
Strongly recommend using abs path when using custmo libs.
"""
# Good behaviors.
# It means refusing called like from xxx import *
# When `__all__` is []
__all__ = []
reload(sys)
sys.setdefaultencoding('utf-8')

def send_error(msg):
""" Send error to email.
"""
print msg

#********************************************************
#* Global defines start.                                *
#********************************************************
#********************************************************
#* Global defines end.                                  *
#********************************************************

class Tail(object):
"""
Python-Tail - Unix tail follow implementation in Python.
python-tail can be used to monitor changes to a file.
Example:
import tail
# Create a tail instance
t = tail.Tail('file-to-be-followed')
# Register a callback function to be called when a new line is found in the followed file.
# If no callback function is registerd, new lines would be printed to standard out.
t.register_callback(callback_function)
# Follow the file with 5 seconds as sleep time between iterations.
# If sleep time is not provided 1 second is used as the default time.
t.follow(s=5)
"""
''' Represents a tail command. '''
def __init__(self, tailed_file):
''' Initiate a Tail instance.
Check for file validity, assigns callback function to standard out.
Arguments:
tailed_file - File to be followed. '''
self.check_file_validity(tailed_file)
self.tailed_file = tailed_file
self.callback = sys.stdout.write
def follow(self, s=0.01):
""" Do a tail follow. If a callback function is registered it is called with every new line.
Else printed to standard out.
Arguments:
s - Number of seconds to wait between each iteration; Defaults to 1. """
with open(self.tailed_file) as file_:
# Go to the end of file
file_.seek(0, 2)
while True:
curr_position = file_.tell()
line = file_.readline()
if not line:
file_.seek(curr_position)
else:
self.callback(line)
time.sleep(s)
def register_callback(self, func):
""" Overrides default callback function to provided function. """
self.callback = func
def check_file_validity(self, file_):
""" Check whether the a given file exists, readable and is a file """
if not os.access(file_, os.F_OK):
raise TailError("File '%s' does not exist" % (file_))
if not os.access(file_, os.R_OK):
raise TailError("File '%s' not readable" % (file_))
if os.path.isdir(file_):
raise TailError("File '%s' is a directory" % (file_))

class TailError(Exception):
""" Custom error type.
"""
def __init__(self, msg):
""" Init.
"""
self.message = msg
def __str__(self):
""" str.
"""
return self.message
if __name__ == '__main__':
pass
""" vim: set ts=4 sw=4 sts=4 tw=100 et: """
　　
　　2. 但是考虑到集中情况，上面的方法会失效（shell中tail命令甚至也会失效）
　　(1) 正在tail的文件被清空，例如 echo "" > test.log （这种情况下tail依然可以打印，上面程序无法打印）
　　(2) 日志被logrotate以copy形式切割时，这种情况下tail依然可以打印，上面程序无法打印
　　(3) 日志被logrotate以新建文件形式切割时，上面两种方法都会失效。
　　
　　我的应用场景是需要兼容上面的情况的，由于时间匆忙，简单的改良了一下，可以兼容上面的两种情况，同事可以避免由于logrotate切割导致的日志丢失，代码如下：#!/usr/bin/env python



#!-*- coding: utf-8 -*-
################################################################################
#
# Copyright (c) 2015 XX.com, Inc. All Rights Reserved
#
################################################################################
################################################################################
# This module provide ...
# Third libs those depend on:
################################################################################
"""
Compiler Python 2.7.10
Authors: XX(XX@baidu.com)
Date: 2015-09-09 12:57:41
Todo: nothing $XX$2015-09-09
"""
"""SYS LIBS
"""

import os
import re
import sys
import time
import traceback
"""THIRD LIBS
"""
try:
# import the third libs there.
pass
except ImportError as e:
print e
os._exit(-1)
"""CUSTOM libs
Strongly recommend using abs path when using custmo libs.
"""
# Good behaviors.
# It means refusing called like from xxx import *
# When `__all__` is []
__all__ = []
reload(sys)
sys.setdefaultencoding('utf-8')

def send_error(msg):
""" Send error to email.
"""
print msg

#********************************************************
#* Global defines start.                                *
#********************************************************
#********************************************************
#* Global defines end.                                  *
#********************************************************

class Tail(object):
"""
Python-Tail - Unix tail follow implementation in Python.
python-tail can be used to monitor changes to a file.
Example:
import tail
# Create a tail instance
t = tail.Tail('file-to-be-followed')
# Register a callback function to be called when a new line is found in the followed file.
# If no callback function is registerd, new lines would be printed to standard out.
t.register_callback(callback_function)
# Follow the file with 5 seconds as sleep time between iterations.
# If sleep time is not provided 1 second is used as the default time.
t.follow(s=5)
"""
''' Represents a tail command. '''
def __init__(self, tailed_file):
''' Initiate a Tail instance.
Check for file validity, assigns callback function to standard out.
Arguments:
tailed_file - File to be followed. '''
self.check_file_validity(tailed_file)
self.tailed_file = tailed_file
self.callback = sys.stdout.write
self.try_count = 0
try:
self.file_ = open(self.tailed_file, "r")
self.size = os.path.getsize(self.tailed_file)
# Go to the end of file
self.file_.seek(0, 2)
except:
raise
def reload_tailed_file(self):
""" Reload tailed file when it be empty be `echo "" > tailed file`, or
segmentated by logrotate.
"""
try:
self.file_ = open(self.tailed_file, "r")
self.size = os.path.getsize(self.tailed_file)
# Go to the head of file
self.file_.seek(0, 1)
return True
except:
return False

def follow(self, s=0.01):
""" Do a tail follow. If a callback function is registered it is called with every new line.
Else printed to standard out.
Arguments:
s - Number of seconds to wait between each iteration; Defaults to 1. """
while True:
_size = os.path.getsize(self.tailed_file)
if _size < self.size:
while self.try_count < 10:
if not self.reload_tailed_file():
self.try_count += 1
else:
self.try_count = 0
self.size = os.path.getsize(self.tailed_file)
break
time.sleep(0.1)
if self.try_count == 10:
raise Exception("Open %s failed after try 10 times" % self.tailed_file)
else:
self.size = _size
curr_position = self.file_.tell()
line = self.file_.readline()
if not line:
self.file_.seek(curr_position)
            elif not line.endswith("\n"):
                self.file_.seed(curr_position)
else:
self.callback(line)
time.sleep(s)
def register_callback(self, func):
""" Overrides default callback function to provided function. """
self.callback = func
def check_file_validity(self, file_):
""" Check whether the a given file exists, readable and is a file """
if not os.access(file_, os.F_OK):
raise TailError("File '%s' does not exist" % (file_))
if not os.access(file_, os.R_OK):
raise TailError("File '%s' not readable" % (file_))
if os.path.isdir(file_):
raise TailError("File '%s' is a directory" % (file_))

class TailError(Exception):
""" Custom error type.
"""
def __init__(self, msg):
""" Init.
"""
self.message = msg
def __str__(self):
""" str.
"""
return self.message
if __name__ == '__main__':
t = Tail("/home/syslog/switch.log")
def print_msg(msg):
print msg
t.register_callback(print_msg)
t.follow()
""" vim: set ts=4 sw=4 sts=4 tw=100 et: """
　　上面程序通过简单的测试，可以解决以上几种情况，由于比较匆忙，没有仔细考虑，也希望大家有什么想法尽量贡献哈。
　　上述红色部分为bugs修复，在使用过程中，发现有的一行内容被截断成两行了，红色部分可以修复该bug：
　　bugs的原因应该是：readline在读取文件的最后一行时，不会去判断\n这个结束符，有兴趣的同学可以测试验证下。