Simplistic 'design-by-contract' example in Python

design-by-contract.py

def add(x, y):
    # Precondition
    assert isinstance(x, int), 'x is not an integer'
    assert isinstance(y, int), 'y is not an integer'
    
    answer = x + y
    
    # Postcondition
    assert isinstance(answer, int), 'answer is not an integer'
    
    return answer

things_to_add = [
        (2, 3),
        (2.3, 3.4),
        (100, '200'),
        ('300', '400')
     ]

for x, y in things_to_add:
    print(f'Going to add {x} and {y}')
    try:
        answer = add(x, y)
        print(f'Answer = {answer}')
    except Exception as e:
        print(f'Something went wrong while adding x and y: "{e}"')
    print('\n--------------------------\n')        

Run with assertions turned on

$ python3 design-by-contract.py 
Going to add 2 and 3
Answer = 5

--------------------------

Going to add 2.3 and 3.4
Something went wrong while adding x and y: "x is not an integer"

--------------------------

Going to add 100 and 200
Something went wrong while adding x and y: "y is not an integer"

--------------------------

Going to add 300 and 400
Something went wrong while adding x and y: "x is not an integer"

--------------------------

Run with assertions turned off

$ python3 -O design-by-contract.py 
Going to add 2 and 3
Answer = 5

--------------------------

Going to add 2.3 and 3.4
Answer = 5.699999999999999

--------------------------

Going to add 100 and 200
Something went wrong while adding x and y: "unsupported operand type(s) for +: 'int' and 'str'"

--------------------------

Going to add 300 and 400
Answer = 300400

--------------------------