Create a calculator

**Calculator in Python**
=========================

Below is a simple implementation of a calculator in Python that performs basic arithmetic operations.

```python
# Calculator in Python

class Calculator:
    def __init__(self):
        pass

    def add(self, num1, num2):
        """Returns the sum of two numbers."""
        return num1 + num2

    def subtract(self, num1, num2):
        """Returns the difference of two numbers."""
        return num1 - num2

    def multiply(self, num1, num2):
        """Returns the product of two numbers."""
        return num1 * num2

    def divide(self, num1, num2):
        """Returns the quotient of two numbers."""
        if num2 == 0:
            raise ValueError("Cannot divide by zero.")
        return num1 / num2

    def calculate(self):
        """Performs a calculation based on user input."""
        print("Select an operation:")
        print("1. Addition")
        print("2. Subtraction")
        print("3. Multiplication")
        print("4. Division")

        choice = input("Enter your choice (1/2/3/4): ")

        if choice in ('1', '2', '3', '4'):
            num1 = float(input("Enter the first number: "))
            num2 = float(input("Enter the second number: "))

            if choice == '1':
                print(f"{num1} + {num2} = {self.add(num1, num2)}")
            elif choice == '2':
                print(f"{num1} - {num2} = {self.subtract(num1, num2)}")
            elif choice == '3':
                print(f"{num1} * {num2} = {self.multiply(num1, num2)}")
            elif choice == '4':
                try:
                    print(f"{num1} / {num2} = {self.divide(num1, num2)}")
                except ValueError as e:
                    print(str(e))
        else:
            print("Invalid choice.")

def main():
    calculator = Calculator()
    while True:
        calculator.calculate()
        cont = input("Do you want to continue? (yes/no): ")
        if cont.lower() != 'yes':
            break

if __name__ == "__main__":
    main()
```

**How to Use:**

1. Run the script.
2. Select an operation by entering the corresponding number (1 for addition, 2 for subtraction, 3 for multiplication, 4 for division).
3. Enter the two numbers.
4. The calculator will display the result.
5. To continue, enter "yes" when prompted. To exit, enter "no".

**Note:** This is a basic implementation and does not handle more complex calculations or edge cases. You can extend this code to add more features as needed.