python用if来做计算器的问题

#python calculator

print ('calculator, enter what you want:','A = multiple,','B = division,', 'C = add,', 'D = substract')
answer = input ()
answer = str [A, B, C, D]

if answer == A or a:
print ('enter your first number')
first_number_ = int(input())
print ('enter your second number')
second_number_ = int(input())
result1 = first_number_ * second_number_
print ('result=', answer2)

请问下为什么A是not defined啊，还有改成if input() == A or a:了之后后面为什么不会运行了啊?

#请无视语法错误

if 语句 和 最后一行有错。 还有answer = str [A, B, C, D]这行不要
正确的如下：

print ('calculator, enter what you want:','A = multiple,','B = division,', 'C = add,', 'D = substract')
answer = input()

if answer == 'A' or  answer =='a':
    print ('enter your first number')
    first_number_ = int(input())
    print ('enter your second number')
    second_number_ = int(input())
    result1 = first_number_ * second_number_
    print ('result=', result1)

我找到A是not defined的错误了
应该是 A = input(), B = input(), C = input (), D = input()
还有个问题求解！if input() == A or a:了之后后面为什么不会运行了啊?