code: def on_diff(request, page_name):
      
      old = request.args.get("old", type=int)
      new = request.args.get("new", type=int)
      error = ""
      diff = page = old_rev = new_rev = None
  
      if not (old and new):
          error = "No revisions specified."
      else:
          revisions = dict(
              (x.revision_id, x)
              for x in Revision.query.filter(
                  (Revision.revision_id.in_((old, new)))
                  & (Revision.page_id == Page.page_id)
                  & (Page.name == page_name)
              )
          )
          if len(revisions) != 2:
              error = "At least one of the revisions requested does not exist."
          else:
              new_rev = revisions[new]
              old_rev = revisions[old]
              page = old_rev.page
              diff = unified_diff(
                  (old_rev.text + "\n").splitlines(True),
                  (new_rev.text + "\n").splitlines(True),
                  page.name,
                  page.name,
                  format_datetime(old_rev.timestamp),
                  format_datetime(new_rev.timestamp),
                  3,
              )
  
      return Response(
          generate_template(
              "action_diff.html",
              error=error,
              old_revision=old_rev,
              new_revision=new_rev,
              page=page,
              diff=diff,
          )
      )
docs: Show the diff between two revisions.