后台运行python程序遇到缓冲区问题

From: http://www.iteye.com/topic/867446

环境：linux

一段执行时间很长的程序（用python做hive客户端执行mapreduce）在linux后台执行，把结果输出到某文件：

Python代码  
 python xxx.py > log.log&   

遇到的问题，程序没报错，文件里却什么都没有，没有输入进去。为什么呢？

于是我首先尝试用：

Java代码  
 nohup python xxx.py > log.log &  

预料之中还是不行。

于是我做实验：

写了个test.py:

Java代码  
 import sys,time  
 from threading import Thread  
 class worker(Thread):  
      def run(self):  
          for x in xrange(0,111):  
              print x  
              time.sleep(1)  
 def run():  
      worker().start()  
 if __name__ == '__main__':  
     run()  

每秒打印一次

我直接用python text.py 执行没问题每秒输出一个数

但我在后台执行：

Python代码  
 python test.py > log.log&   

还是不行。开始不输出直到程序执行完，一次性的写到log.log文件了。

为什么呢？

原因可能是python 的print 先写到缓冲区了，还没flush到文件。

于是加了一行“ sys.stdout.flush()” --在每次print后都flush一次。：

Java代码  
 import sys,time  
 from threading import Thread  
 class worker(Thread):  
      def run(self):  
          for x in xrange(0,111):  
              print x  
              sys.stdout.flush()    
              time.sleep(1)  
 def run():  
      worker().start()  
 if __name__ == '__main__':  
     run()  

问题解决。

===============================================================================

还可以：python-u xxx.py > log.log & ，再细看下帮助文档：man python

PYTHON(1) PYTHON(1)

NAME
python - an interpreted, interactive, object-oriented programming language

SYNOPSIS
python [ -d ] [ -E ] [ -h ] [ -i ] [ -m module-name ] [ -O ]
[ -Q argument ] [ -S ] [ -t ] [ -u ]
[ -v ] [ -V ] [ -W argument ] [ -x ] [ -3 ]
[ -c command | script | - ] [ arguments ]

DESCRIPTION
Python is an interpreted, interactive, object-oriented programming language that combines remarkable power with very clear syntax.
For an introduction to programming in Python you are referred to the Python Tutorial. The Python Library Reference documents built-in
and standard types, constants, functions and modules. Finally, the Python Reference Manual describes the syntax and semantics of the
core language in (perhaps too) much detail. (These documents may be located via the INTERNET RESOURCES below; they may be installed
on your system as well.)

Python’s basic power can be extended with your own modules written in C or C++. On most systems such modules may be dynamically
loaded. Python is also adaptable as an extension language for existing applications. See the internal documentation for hints.

Documentation for installed Python modules and packages can be viewed by running the pydoc program.

COMMAND LINE OPTIONS
-c command
Specify the command to execute (see next section). This terminates the option list (following options are passed as arguments
to the command).

-d Turn on parser debugging output (for wizards only, depending on compilation options).

-E Ignore environment variables like PYTHONPATH and PYTHONHOME that modify the behavior of the interpreter.

-h Prints the usage for the interpreter executable and exits.

-i When a script is passed as first argument or the -c option is used, enter interactive mode after executing the script or the
command. It does not read the $PYTHONSTARTUP file. This can be useful to inspect global variables or a stack trace when a
script raises an exception.

-m module-name
Searches sys.path for the named module and runs the corresponding .py file as a script.

-u Force stdin, stdout and stderr to be totally unbuffered. On systems where it matters, also put stdin, stdout and stderr in

binary mode. Note that there is internal buffering in xreadlines(), readlines() and file-object iterators ("for line in
sys.stdin") which is not influenced by this option. To work around this, you will want to use "sys.stdin.readline()" inside a
"while 1:" loop.

-v Print a message each time a module is initialized, showing the place (filename or built-in module) from which it is loaded.
When given twice, print a message for each file that is checked for when searching for a module. Also provides information on
module cleanup at exit.

-V Prints the Python version number of the executable and exits.