英文文档：
ascii(object)
As repr(), return a string containing a printable representation of an object, but escape the non-ASCII characters in the string returned by repr() using x, u or U escapes. This generates a string similar to that returned by repr() in Python 2.
说明：
1. 返回一个可打印的对象字符串方式表示，如果是非ascii字符就会输出x，u或U等字符来表示。与python2版本里的repr()是等效的函数。
>>> ascii(1) '1' >>> ascii('&') "'&'" >>> ascii(9000000) '9000000' >>> ascii('中文') #非ascii字符 "'\u4e2d\u6587'"
