Python buffers writes to files. That is, file.write returns before the data is actually written to your hard drive. The main motivation of this is that a few large writes are much faster than many tiny writes, so by saving up the output of file.write until a bit has accumulated, Python can maintain good writing speeds.
file.flush forces the data to be written out at that moment. To flush contents you wrote actually to the file, use:
with open("my_file.txt", "w+") as file:
file.write("foo")
file.write("bar")
file.flush()