Question

f=open("vmi","w")
f.write("asdf")
import os
os.path.getsize("vmi")
#0 byte
f.close()
os.path.getsize("vmi")
# 4 bytes

Where I can find lost 4 bytes,
on program execution, before file is closed?

Answer 1

You could try to flush out the data first:

f.flush()

Why would you need to do this? Well, the OS will try to buffer writes to files for performance reasons – it is a lot slower to write 1024 bytes one at a time than just write out the whole buffer. So, whenever you are working with a file / pipe / socket kind of thing, keep in mind that it might be buffering writes and that you will need to flush first.

When you closed the file, it was flushed automatically.