# How to manually raising (throwing) an exception in Python?

To manually raise an exception in Python, use the `raise` statement. Here is an example of how to use it:

```python
def calculate_payment(amount, payment_type):
    if payment_type != "Visa" and payment_type != "Mastercard":
        raise ValueError("Payment type must be Visa or Mastercard")
    # Do the calculation using the provided amount and payment type
    # ...

try:
    calculate_payment(100, "Discover")
except ValueError as e:
    print(e)
```

In this example, the `calculate_payment` function raises a `ValueError` exception if the `payment_type` is not either "Visa" or "Mastercard". The `try`-`except` block catches the exception and prints the error message.

You can raise any exception type that you want by specifying the exception type as the first argument to the `raise` statement. For example, to raise a `TypeError` exception, you can use the following code:

```python
raise TypeError("This is a TypeError exception")
```

Keep in mind that it is generally a good idea to only raise exceptions in exceptional circumstances. In most cases, it is better to use standard Python control structures (such as `if`-`else` or `for` loops) to handle error conditions.