取决于你要做什么。要附加，可以用"a"打开它：

 with open("foo.txt", "a") as f:
     f.write("new line\n")

如果要先添加某些内容，则必须先从文件中读取：

with open("foo.txt", "r+") as f:
     old = f.read() # read everything in the file
     f.seek(0) # rewind
     f.write("new line\n" + old) # write the new line before