每次我按下plus_btn数字应该增加1，但它只增加了一次
我尝试使用全局变量并将函数放在类之外，但没有成功

import bidi.algorithm
from kivy.app import App
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput
from kivy.uix.button import Button

class RootWidget(App):
    def build(self):
        inside = FloatLayout()
        label1 = Label(text='hi', font_name='fnnt',pos_hint ={'x':.15, 'y':.15})
        def plus(instance):
            num = 0
            if label1.text == 'hi' and num <= 7:
                num += 1
                print(num)
        plus_btn = Button(text='+', on_press=plus)
        inside.add_widget(plus_btn)
        inside.add_widget(label1)
        return inside


if __name__ == '__main__':
    RootWidget().run()