python unicode 和 str 转换

普通字符串可以用多种方式编码成Unicode字符串，具体要看你究竟选择了哪种编码：
unicodestring = u"Hello world" 
# 将Unicode转化为普通Python字符串："encode"  
utf8string = unicodestring.encode("utf-8")  
asciistring = unicodestring.encode("ascii")  
isostring = unicodestring.encode("ISO-8859-1")  
utf16string = unicodestring.encode("utf-16")  
# 将普通Python字符串转化为Unicode："decode"  
plainstring1 = unicode(utf8string, "utf-8")  
plainstring2 = unicode(asciistring, "ascii")  
plainstring3 = unicode(isostring, "ISO-8859-1")  
plainstring4 = unicode(utf16string, "utf-16")  
assert plainstring1 == plainstring2 == plainstring3 == plainstring4