subprocess.Popen['vim'] in an interactive Python session yields interesting results

Question

Enter python shell

then type

import subprocess
subprocess.Popen("vim")

Then the vim window opens. Try to quit vim using :q and then try typing in the interactive session.

It seems like vim does not really actually quit. It seems as if I was in vim and in the Python shell at the same time.

Why does this happen?

I know that I can avoid this by running subprocess.call or Popen.communicate() etc., I am just curious as to what exactly is happening.

It seems as if quitting vim using :q after running it in Popen still kind of keeps it running.