git pull make a fetch ? so "--ff-only" seems not possible #1088

With this code:
origin.pull('--ff-only')

I have the following error :

      raise GitCommandError(self.args, status, errstr)

E git.exc.GitCommandError: Cmd('git') failed due to: exit code(1)
E cmdline: git pull -v origin --ff-only master
E stderr: 'error: unknown option `ff-only master'
E usage: git fetch [] [ [...]]
E or: git fetch []
E or: git fetch --multiple [] [( | )...]
E or: git fetch --all []

In fact we see that the commande used is the fetch one.... not the pull one... so the --ff-only option is not available...
how can i make a pull that make a pull ?