I get an int error when trying to run a simple addition calculator i made in python

When trying to run this simple calculator with only addition and passing "1 + 2" in the terminal without quotes

numbers=["1","2","3","4","5","6","7","8","9"]
str=input()
numlist = str.split()
for i in range(len(numlist)):
    number=""
    operate=""
    for j in numlist[i]:
        if j in numbers:
            number=number+j
        else:
            operate=operate+j
    if operate == "+":
        sum = 0
        for i in range(len(numlist)):
            sum = sum + int(number)

it results In this

Exception has occurred: ValueError
invalid literal for int() with base 10: ''
  File "C:\Users\dummy\Desktop\New Text Document.py", line 15, in <module>
    sum = sum + int(number)
```'
Any help would be appreciated 

>Solution :

Each time you iterate through numlist you are resetting number to an empty string. I’d suggest moving that and sum above your for loop to make sure they are not reset on each iteration.

numbers = ["1", "2", "3", "4", "5", "6", "7", "8", "9"]
given_input = input()
numlist = given_input.split()
number = ""
operate = ""
sum = 0
for i in numlist:
    for j in i:
        if j in numbers:
            number = j
        else:
            operate = j
    if operate == "+" and number != "":
        sum += int(number)
        number = ""
print(sum)