555初学者求问，不知道大家现在起床了没有呢

num1, operator, num2 = input('Enter calculation:').split()
num1 = int(num1)
num2 = int(num2)
if operator == '+':
print('{}+{}={}').format(num1, num2, num1 + num2)
elif operator == '-':
print('{}-{}={}').format(num1, num2, num1 - num2)
elif operator == '*':
print('{}*{}={}').format(num1, num2, num1 * num2)
elif operator == '/':
print('{}/{}={}').format(num1, num2, num1 / num2)
else:
print('use either+-*or/')
报错是这样的：AttributeError: 'NoneType' object has no attribute 'format'
I tried but couldn't fiture it out by myself