Adding read/readline() after write appends to EOF, not to the position

Bug report

Bug description:

with open("testfile", "r+") as my_file:

    text = my_file.write("========")
    print(my_file.readline())

Instead of writing to the beginning of the file, the program writes to the EOF even if readline() is placed after write.

CPython versions tested on:

3.8, 3.9

Operating systems tested on:

Windows

首先，在报告所涉及的 CPython 版本和 Windows 环境中重现所提供的 open("testfile", "r+") 示例。将观察到的读取位置与文档所述的文件 I/O 行为进行比较；完成的标准是确定这是实现错误还是预期行为，并确定相关的 CPython 更改或所需的文档。