9-2　自定义生成器

一个生成器generator：一遍循环，一遍计算；需要下一个值时，才会计算，所以生成器就会节约内存     

\ \ 普通函数是碰到return就会返回

\ \ 生成器是碰到yield返回，但是函数并没结束，此时会建立一个栈帧指向这里；指导主函数调用生成器的next（），生成器会找到这个栈帧，读取恢复现场，继续从上次yield结束位置运行

def myreadfile(f, newline):
    buf  = ""   #读取内容缓存
    while True:
        while newline in buf:   #缓存中存在分隔符
            pos = buf.index(newline)  #定位分隔符位置
            yield buf[:pos]     # 输出分隔符之前的内容
            buf = buf[pos+len(newline):]  #buf 变成分隔符之后的内容，再次循环
        next_read = f.read(4096) # buf中不在有分隔符，就需要添加下次内容了

        if not next_read:  #没有读到下次的内容，证明文件读取完毕
            yield buf
            break
        buf = buf+next_read    


with open("test.txt") as f:
    for line in myreadfile(f, "{|}"):
        print(line)
        
>>>  
Prior to beginning tutoring sessions
, I ask new students to fill
 out a brief self-assessment
 where they rate their
 understanding of various Python concepts. Some topics ("control flow with if/else" or "defining and using functions") are understood by a majority of students before ever beginning tutoring. There are a handful of topics, however, that almost all
 students report having no knowledge or very limited understanding of. Of these