Let's write a REPL (read, evaluate, print, loop) calculator that can handle addition and subtraction.
Ask the user for an operator and each operand. Don't forget that input returns a string, which you can convert to a float using float(user_input) where user_input is the string you got from input.
Create a separate function for each operation which takes in both operands and returns the result of its operation.
Pass the user's operands to the appropriate function based on the user's selected operator and use the value the function returns as your result.
Below is some sample input/output.
Enter the operation you would like to perform or 'done' to quit: +
Enter the first operand: 1
Enter the second operand: 2
1.0 + 2.0 = 3.0
Enter the operation you would like to perform or 'done' to quit: -
Enter the first operand: 100
Enter the second operand: 25
100.0 - 25.0 = 75.0
Enter the operation you would like to perform or 'done' to quit: done
Goodbye!
Add functionality for multiplication * and division /
Allow the user to have a running total, each command will perform an operation on that number.
Enter the operation you would like to perform or 'done' to quit: +
Enter the first operand: 1
Enter the second operand: 02
1.0 + 2.0 = 3.0
Enter the operation you would like to perform or 'done' to quit: +
3.0 + x:
Enter x: 50
3.0 + 50.0 = 53.0
Enter the operation you would like to perform or 'done' to quit: -
53.0 - x:
Enter x: 7
53.0 - 7.0 = 46.0
Enter the operation you would like to perform or 'done' to quit: done
Your final total was: 46.0
Goodbye!
Use a dictionary rather than if/elif to decide which function should be called.