python - AttributeError: 'NoneType' object has no attribute '_root'

Question

I am completely new to Python 3, just following simple exercises from YouTube.

https://www.youtube.com/watch?v=nefopNkZmB4&index=3&list=PL6gx4Cwl9DGAcbMi1sH6oAMk4JHw91mC_

This is my code:

    from tkinter import *


def iCalc(source, side):
    storeObj = Frame(source, borderwidth=4, bd=4, bg="powder blue")
    storeObj.pack(side=side, expand=YES, fill=BOTH)
    return storeObj


def button(source, side, text, command=None):
    storeObj = Button(source, text=text, command=command)
    storeObj.pack(side=side, expand=YES, fill=BOTH)
    return storeObj


class app(Frame):

    def __init__(self):
        Frame.__init__(self)
        self.option_add('*Font', 'arial 20 bold')
        self.pack(expand=YES, fill=BOTH)
        self.master.title('Calculator')

    display = StringVar()
    Entry(self, relief=RIDGE, textvariable=display, justify='right', bd=30, bg="powder blue").pack(side=TOP, expand=YES,
                                                                                                   fill=BOTH)
for clearBut in (["CE"], ["C"]):
    erase = iCalc(self, TOP)
for ichar in clearBut:
    button(erase, LEFT, ichar,
           lambda storeObj=display, q=ichar: storeObj.set(''))

for NumBut in ("789/", "456*", "123-", "0.+"):
    FunctionNum = iCalc(self, TOP)
for iEquals in NumBut:
    button(FunctionNum, LEFT, iEquals,
           lambda storeObj=display, q=iEquals: storeObj.set(storeObj.get() + q))
EqualsButton = iCalc(self, TOP)
for iEquals in '=':
    if iEquals in "=":
        btniEquals = button(EqualsButton, LEFT, iEquals)
        btniEquals.bind('<ButtonRelease-1>',
                        lambda e, s=self, storeObj=display: s.calc(storeObj), '+')
else:
    btniEquals = button(EqualsButton, LEFT, iEquals,
                        lambda storeObj=display, s=' %s ' % iEquals: storeObj.set(storeObj.get() + s))


def calc(self, display):
    try:
        display.set(eval(display.get()))
    except:
        display.set("ERROR")

if __name__ == '__main__':
    app().mainloop()

I am getting error:

What am I doing wrong?

Answer 1

Because of the indentation, display = StringVar() is not in a method. That means it is getting executed when the class is first defined. You cannot create an instance of StringVar until after the root window has been created.

You need to add one more level of indentation for that line, and the line below it.