tail 实现 +注释

自己平时工作经常需要tail日志实时入库，近期学python，发现没有系统模块是实现tail功能的。
偶然在在github 上看到一份代码，有人自己写了 python-tail模块实现tail功能。
介绍如下： 先跳到文件末尾，然后不断的打印出新行，直到文件结束，然后再更新指针位置，最后再死循环重复上述过程 。

模块代码：
#!/usr/bin/env python

'''
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) '''

# Author - Kasun Herath <kasunh01 at gmail.com>
# Source - https://github.com/kasun/python-tail

import os
import sys
import time

class Tail(object):
    ''' 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=1):
        ''' 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):
    def __init__(self, msg):
        self.message = msg
    def __str__(self):
        return self.message

使用模块案例：

#!/usr/bin/env python

'''
python-tail example.
Does a tail follow against /var/log/syslog with a time interval of 5 seconds.
Prints recieved new lines to standard out '''

import tail

def print_line(txt):
    ''' Prints received text '''
    print(txt)

t = tail.Tail('/var/log/syslog')
t.register_callback(print_line)
t.follow(s=5)