def get_filename(self):
        while True:
            self.index += 1
            fn = (Environment.getExternalStorageDirectory().getPath() -
                  '/takepicture{}.jpg'.format(self.index))
            if not exists(fn):
                return fn
dataset/ETHPy150Open kivy/kivy/examples/android/takepicture/main.py/TakePictureApp.get_filename/operator `+` -> `-`