How to mix read and write on Python files in Windows

Question

It appears that a write() immediately following a read() on a file opened with r+ (or r+b) permissions in Windows doesn't update the file.

Assume there is a file testfile.txt in the current directory with the following contents:

This is a test file.

I execute the following code:

with open("testfile.txt", "r+b") as fd:
    print fd.read(4)
    fd.write("----")

I would expect the code to print This and update the file contents to this:

This----a test file.

This works fine on at least Linux. However, when I run it on Windows then the message is displayed correctly, but the file isn't altered - it's like the write() is being ignored. If I call tell() on the filehandle it shows that the position has been updated (it's 4 before the write() and 8afterwards), but no change to the file.

However, if I put an explicit fd.seek(4) just before the write() line then everything works as I'd expect.

Does anybody know the reason for this behaviour under Windows?

For reference I'm using Python 2.7.3 on Windows 7 with an NTFS partition.

EDIT

In response to comments, I tried both r+b and rb+ - the official Python docs seem to imply the former is canonical.

I put calls to fd.flush() in various places, and placing one between the read() and the write()like this:

with open("testfile.txt", "r+b") as fd:
    print fd.read(4)
    fd.flush()
    fd.write("----")

... yields the following interesting error:

IOError: [Errno 0] Error

EDIT 2

Indirectly that addition of a flush() helped me describe a similar problem. If one of the commenters on it is correct, it's a bug in the underlying Windows C library.