正确理解Python文件读写模式字w+、a+和r+

w+ 和 r+的差别不难理解。还有a+
+同一时候读写，就可以读又可写，边写边读、边读边写，不用flush，用seek 和 tell可測得。

fp = open("a.txt", "a+", 0)
print 'open',fp.tell()
x = fp.read()
print 'open read()',fp.tell()
print x
fp.write("123456
")
print 'write 1-6',fp.tell()
x = fp.read()
print "first read
",x
fp.seek(0)
x = fp.read()
print "second read
", x,fp.tell()
fp.close()

第一次执行时的结果是：

open 0
open read() 0

write 1-6 8
first read

second read
123456
8

第二次执行时的结果是

open 0
open read() 8
123456

write 1-6 16
first read

second read
123456
123456
16