正确理解Python文件读写模式字w+、a+和r+

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

open 0
open read() 0

write 1-6 8
first read

second read
123456
8

open 0
open read() 8
123456

write 1-6 16
first read

second read
123456
123456
16