title: Override file operations in Python
body: <p>Is there a way to override all file operations in Python? File operations such as 'open', 'os.rename' and 'os.unlink'.</p>
  
  <p>I want to create a temporary, in-memory file system without rewriting a library. Does anyone know of a script or a library that has this feature? I want to run a library on Google App Engine and it is not possible to write to the file system.</p>