使用isnumeric的Python Kivy在运行时的行为并不像预期的那样

Question

您好我正在尝试在kivy中创建只接受某些类型字符串的文本框。第一个文本框工作正常，只接受数字，但cnuminput将接受字符串中只应该是数字的部分中的特殊字符，我不知道为什么。只是它正在发生的事情，它导致我检查int的方法和isnumeric（）/ isdidgits（）/ ect为一个字符串返回true，如果它应该返回false。

from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.textinput import TextInput
from kivy.uix.dropdown import DropDown
from kivy.uix.button import Button
from kivy.uix.popup import Popup

def ISHOULDNTHAVETODOTHIS(st):
    try:
        int(st)
        return True
    except ValueError:
        return False
    else:
        return False



class CNumInput(TextInput):
    def insert_text(self, substring, from_undo=False):
        if substring[0:3].isalpha() != True and len(self.text) < 4:
            substring =(substring[0:-1])
        elif substring[0:3].isalpha() == True and len(self.text) > 3 and not ISHOULDNTHAVETODOTHIS(self.text[4:]):  
            substring =( substring[0:-1])
        elif (len(self.text) > 7):
            substring = substring[0:-1]
        substring = substring[0:3].upper() + substring[4:]                  
        return super(CNumInput, self).insert_text(substring, from_undo=from_undo)

class NumInput(TextInput):
    def insert_text(self, substring, from_undo=False):
        if ISHOULDNTHAVETODOTHIS(substring) != True:
            substring =( substring[0:-1])
            popup = Popup(Title =)
        elif (len(self.text) > 5):
            substring = substring[0:-1]             
        return super(NumInput, self).insert_text(substring, from_undo=from_undo)



class LoginScreen(GridLayout):
    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.cols = 2
        drop = DropDown()
        self.add_widget(Label(text='Student Id'))
        self.username = NumInput(multiline=False)
        self.add_widget(self.username)
        mainbut = Button(text='Lab', size_hint_y=None, height=20)
        mainbut.bind(on_release=drop.open)
        self.add_widget(Label(text='Location'))
        self.add_widget(mainbut)
        self.add_widget(Label(text='Course Number'))
        labut = Button(text='Lab', size_hint_y=None, height=20)
        clabut = Button(text='ClassRoom', size_hint_y=None, height=20)
        stubut = Button(text='Studio', size_hint_y=None, height=20)
        labut.bind(on_release=lambda labut: drop.select(labut.text))
        clabut.bind(on_release=lambda clabut: drop.select(clabut.text))
        stubut.bind(on_release=lambda stubut: drop.select(stubut.text))
        drop.bind(on_select=lambda instance, x: setattr(mainbut, "text", x))
        drop.add_widget(labut)
        drop.add_widget(clabut)
        drop.add_widget(stubut)
        self.password = CNumInput(multiline=False)
        self.add_widget(self.password)



class MyApp(App):
    def build(self):
        return LoginScreen()


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