Python GUIzero组合框默认值？

如何解决Python GUIzero组合框默认值？

我正在尝试使用guizero（对于Python）构建我的第一个GUI。 我希望用户选择一个由日，月和年组成的日期，然后从该日期开始搜索。我创建了3个组合框，它们的值分别为1日至31日，月1-12日和2012-2023年。 如果用户确实选择了日期的所有三个部分，则一切正常。 但是，如果用户认为，显示的日期还可以，并且他什么也没选择，那么如何将这个日期输入到程序中？

from guizero import App,Box,PushButton,Combo,Text

def select_day(selected_day):
    if len(selected_day) == 1:
        result1.value = '0' + selected_day
    else:
        result1.value = selected_day

def select_month(selected_month):
    if len(selected_month) == 1:
        result2.value = '0' + selected_month
    else:
        result2.value = selected_month

def select_year(selected_year):
    result3.value = selected_year

def start_search():
    day = result1.value
    month = result2.value
    year = result3.value
    inDate = year + month + day
    app.info("Info","Search Date: " + inDate)


app = App(title="Combo Test")

# Adding elements for start date selection and main search button
Box2 = Box(app,border=0)
text21 = Text(Box2,text="Select Start Date for Search (Day/Month/Year):")

Box3 = Box(app,border=0)
vlist = list(range(1,31))
day_combo = Combo(Box3,options=vlist,selected='1',align='left',command=select_day)
result1=Text(app,visible=False)
vlist = list(range(1,13))
month_combo = Combo(Box3,command=select_month)
result2=Text(app,visible=False)
vlist = list(range(2012,2023))
year_combo = Combo(Box3,selected='2015',command=select_year)
result3=Text(app,visible=False)

searchButton = PushButton(Box3,text='Search',command=start_search)

app.display()