How can I incorporate an input limit of 999 in my calculator script? - python-3.3

Question

I am aware this is a duplicate from a past thread, but when I use that suggestion, it says 'unindent does not match any outer indentation level'. When I run the script at school (this is a school assignment) a blank GUI appears with no buttons, and when I run it at home, it appears with the error message as above- I am using Python 3.3.2 on each computer. The code for the 999 limit is in lines 45-57 and the error message appears on the first line (45). I'm quite new to python and my teacher is awful, so apologies if my question sounds quite silly. Can I have a suggestion as to how to get this to work?
Thanks a lot!
from tkinter import *
class Calculator(Frame):
def frame(this, side):
w = Frame(this)
w.pack(side=side, expand=YES, fill=BOTH)
return w
def button(this, root, side, text, command=None):
w = Button(root, text=text, command=command)
w.pack(side=side, expand=YES, fill=BOTH)
return w
need_clr = False
def digit(self, digit):
if self.need_clr:
self.display.set('')
self.need_clr = False
self.display.set(self.display.get() + digit)
def sign(self):
need_clr = False
cont = self.display.get()
if len(cont) > 0 and cont[0] == '-':
self.display.set(cont[1:])
else:
self.display.set('-' + cont)
def oper(self, op):
self.display.set(self.display.get() + ' ' + op + ' ')
self.need_clr = False
def calc(self):
try:
self.display.set(eval(self.display.get()))
self.need_clr = True
except:
showerror('Operation Error', 'Illegal Operation')
self.display.set('')
self.need_clr = False
def calc(self):
try:
self.display.set(self.validate_result(eval(self.display.get())))
self.need_clr = True
except:
showerror('Operation Error', 'Illegal Operation')
self.display.set('')
self.need_clr = False
def validate_result(self, result):
if result >= 1000:
raise ValueError('result too big!')
else:
return result
def __init__(self):
Frame.__init__(self)
self.option_add('*Font', 'Dotum 15')
self.pack(expand=YES, fill=BOTH)
self.master.title('Simple Calculator')
self.display = StringVar()
e = Entry(self, relief=SUNKEN, textvariable=self.display)
e.pack(side=TOP, expand=YES, fill=BOTH)
for key in ("123", "456", "789"):
keyF = self.frame(TOP)
for char in key:
self.button(keyF, LEFT, char,
lambda c=char: self.digit(c))
keyF = self.frame(TOP)
self.button(keyF, LEFT, '0', lambda ch='0': self.digit(ch))
opsF = self.frame(TOP)
for char in "+-=":
if char == '=':
btn = self.button(opsF, LEFT, char, self.calc)
else:
btn = self.button(opsF, LEFT, char,
lambda w=self, s=char: w.oper(s))
clearF = self.frame(BOTTOM)
self.button(clearF, LEFT, 'Clr', lambda w=self.display: w.set(''))
if __name__ == '__main__':
Calculator().mainloop()