Python类的实例属性加括号是什么意思

Question

初学Python，不理解后面定义的方法中return实例属性为什么还带括号
比如说feature_extractor的x是怎么作用于实例属性features的，或者说，这样的语句是啥意思
按我理解这个属性不就是一个值吗？怎么还能带变量x

望求解答，多谢

Ps：下面只节选了部分代码

class SiamMask(nn.Module):
    def __init__(self, anchors=None, o_sz=127, g_sz=127):
        super(SiamMask, self).__init__()
        self.anchors = anchors  # anchor_cfg
        self.anchor_num = len(self.anchors["ratios"]) * len(self.anchors["scales"])
        self.anchor = Anchors(anchors)
        self.features = None
        self.rpn_model = None
        self.mask_model = None
        self.o_sz = o_sz
        self.g_sz = g_sz
        self.upSample = nn.UpsamplingBilinear2d(size=[g_sz, g_sz])

        self.all_anchors = None

    def set_all_anchors(self, image_center, size):
        # cx,cy,w,h
        if not self.anchor.generate_all_anchors(image_center, size):
            return
        all_anchors = self.anchor.all_anchors[1]  # cx, cy, w, h
        self.all_anchors = torch.from_numpy(all_anchors).float().cuda()
        self.all_anchors = [self.all_anchors[i] for i in range(4)]

    def feature_extractor(self, x):
        return self.features(x)

    def rpn(self, template, search):
        pred_cls, pred_loc = self.rpn_model(template, search)
        return pred_cls, pred_loc

Answer 1

看这个例子
class test():
    def __init__(self): 
        self.f = None
    def t(self, x): 
        return self.f(x)

def ft(x): 
    return x+33


T = test()
T.f = ft
res = T.t(22)

print(res)
"""--result
55
"""