'Initiates the difftool session'

def compare_file(self, filename):

    if self.use_sandbox:
        left = self.diff_arg[0]
        if (len(self.diff_arg) > 1):
            right = self.diff_arg[1]
        else:
            right = None
    else:
        (left, right) = (self.start, self.end)
    cmds.difftool_launch(left=left, right=right, paths=[filename])
