刚做一道题：写一个复制文件的程序。如下：

from sys import argv

from os.path import exists

script,from_file,to_file = argv

in_file = open(from_file)

indata = open(from_file).read()

out_file = open(to_file,'w')

out_file.write(indata)

print "Alright,all done"

in_file.close()

out_file.close()

这个程序正常运行，之后题目要求尽量缩短，我想减少变量，改成这样：

from sys import argv

from os.path import exists

script,from_file,to_file = argv

in_file = open(from_file)

indata = open(from_file).read()

out_file = open(to_file,'w')

out_file.write(open(from_file).read())

print "Alright,all done"

open(from_file).close()

open(to_file,'w').close()

程序还是能正常运行。但我改成这样后，发现虽然能运行但无法复制去另一个文件，里面是空白。如下：

from sys import argv

from os.path import exists

script,from_file,to_file = argv

in_file = open(from_file)

indata = open(from_file).read()

out_file = open(to_file,'w')

open(to_file,'w').write(open(from_file).read())

print "Alright,all done"

open(from_file).close()

open(to_file,'w').close()

虽然有些钻牛角尖，但我想知道为什么