python - BytesIO.truncate 方法不扩展缓冲区内容

Question

IOBase.truncate 的文档方法说:

truncate(size=None)

Resize the stream to the given size in bytes (or the current position if size is not specified). The current stream position isn’t changed. This resizing can extend or reduce the current file size. In case of extension, the contents of the new file area depend on the platform (on most systems, additional bytes are zero-filled). The new file size is returned.

Changed in version 3.5: Windows will now zero-fill files when extending.

因此，考虑到这一点，我认为 BytesIO (即 BufferedIOBase 的子类，而后者又是 IOBase 的子类)在调用此方法后更改其内部缓冲区大小。
但以下代码片段表明我的假设是错误的:

from io import BytesIO

# prints b'\x00\x00\x00\x00\x00\x00\x00\x00'
data = BytesIO(8 * b"\x00")
print(data.getvalue())

# prints 16
print(data.truncate(16))

# prints b'\x00\x00\x00\x00\x00\x00\x00\x00'
print(data.getvalue())

# prints b'\x00\x00\x00\x00\x00\x00\x00\x00'
print(bytes(data.getbuffer()))

我哪里转错了？

Answer 1

检查 source code ，似乎文档不是最新的 BytesIO执行:

static PyObject *_io_BytesIO_truncate_impl(bytesio *self, Py_ssize_t size)
/*[clinic end generated code: output=9ad17650c15fa09b input=423759dd42d2f7c1]*/
{
    CHECK_CLOSED(self);
    CHECK_EXPORTS(self);

    if (size < 0) {
        PyErr_Format(PyExc_ValueError,    
                     "negative size value %zd", size);
        return NULL;
    }

    if (size < self->string_size) {    
        self->string_size = size;    
        if (resize_buffer(self, size) < 0)    
            return NULL;   
    }

    return PyLong_FromSsize_t(size);

}

if (size < self->string_size) test 确保如果大小大于以前的大小，则不执行任何操作。

我的猜测是，对于真正的文件处理程序， truncate像底层平台一样工作(扩展文件)，但不适用于内存映射处理程序。

如果我们知道它将失败，则可以通过在对象的末尾写入来非常简单地模拟所需的行为:

def my_truncate(data,size):
    current_size = len(data.getvalue())
    if size < current_size:
        return data.truncate(size)
    elif size == current_size:
        return size  # optim
    else:
        # store current position
        old_pos = data.tell()
        # go to end
        data.seek(current_size)
        # write zeroes
        data.write(b"\x00" * (size-current_size))
        # restore previous file position
        data.seek(old_pos)
        return size