Tkinter 条目返回默认值，而不是输入的值

from tkinter import *
import tkinter as tk
from formation import appbuilder

root = tk.tk()
root.withdraw()

app = appbuilder(path="calculator.xml")

def calculate(event=none):
    # event parameter needs to be there because using the bind method passes an event object
    # access the expr_var we created earlier to determine the current expression entered
    expr = app.expr_var.get()

    # evaluate the expression
    try:
        result = expr
    except exception:
        # if the expression entered was malformed and could not be evaluated
        # we will display an error message instead
        result = "invalid expression"

    # display the result
    app.result.config(text=result)


app.connect_callbacks(globals())

app.mainloop()

<?xml version='1.0' encoding='utf-8'?>
<tkinter.Tk
    xmlns:attr="http://www.hoversetformationstudio.com/styles/" 
    xmlns:layout="http://www.hoversetformationstudio.com/layouts/" 
    name="tk_1" 
    attr:layout="place" 
    layout:width="300" 
    layout:height="233" 
    layout:x="30" 
    layout:y="30">
        <meth name="title">
        <arg value="title" />
        </meth>
        <meth name="geometry">
        <arg value="300x200+0+0" />
        </meth>
        <meta major="6" minor="2" name="version" />
    <tkinter.StringVar attr:name="expr_var" attr:value="" />
    <tkinter.Entry 
        name="entry_1" 
        attr:textvariable="expr_var"
        layout:width="200" 
        layout:height="25" 
        layout:x="50" 
        layout:y="35" 
        layout:bordermode="outside" />
    <tkinter.Label
        name="result" 
        layout:width="200" 
        layout:height="25" 
        layout:x="50" 
        layout:y="74" 
        layout:bordermode="outside" />
    <tkinter.Button name="calculate" attr:text="button_1" layout:width="80" layout:height="25" layout:x="110" layout:y="133" layout:bordermode="outside">
        <event sequence="<Button-1>" handler="calculate" add="False" />
    </tkinter.Button>
</tkinter.Tk>

from formation import AppBuilder

app = AppBuilder(path="calculator.xml")

def calculate(event=None):
    # event parameter needs to be there because using the bind method passes an event object
    # access the expr_var we created earlier to determine the current expression entered
    expr = app.expr_var.get()

    # evaluate the expression
    try:
        result = expr
    except Exception:
        # if the expression entered was malformed and could not be evaluated
        # we will display an error message instead
        result = "Invalid expression"

    # display the result
    app.result.config(text=result)


app.connect_callbacks(globals())

app.mainloop()