python 令人抓狂的编码问题

#运行以下程序：

#! /usr/bin/env python
#coding=utf-8

file  = open( ‘all_hanzi.txt‘,‘wb‘ )

listhz = []
n=0
for ch in xrange(0x4e00, 0x9fa6):
    print unichr(ch),
    file.write( unichr(ch) )#此行出错。正确：file.write( unichr(ch).encode(‘gbk‘))　　encode(‘gbk‘)将‘utf-8’编码的string编码为‘gbk’

    n = n+1
    if(n%50==0):
        print ‘\n‘
        file.write(‘\n‘)

print n

#报错：UnicodeEncodeError: ‘ascii‘ codec can‘t encode characters in position 0-1: ordinal not in range(128)
#代码参考：http://www.cnblogs.com/mmix2009/p/3229787.html 　　python打印所有汉字