Raising Exceptions¶

Python programs can raise exceptions explicitly using the raise statement.

This allows functions to signal that something went wrong.

flowchart LR
    A[Function detects problem]
    A --> B[raise exception]
    B --> C[Caller handles exception]
````

---

## 1. Basic raise

```python
raise ValueError("Invalid value")

This immediately stops execution and raises the specified exception.

2. Raising Exceptions in Functions¶

def divide(a, b):
    if b == 0:
        raise ValueError("b cannot be zero")
    return a / b

Example:

print(divide(10, 2))
print(divide(10, 0))

3. Custom Error Checking¶

Raising exceptions helps enforce constraints.

def withdraw(balance, amount):
    if amount > balance:
        raise ValueError("Insufficient funds")
    return balance - amount

4. Re-raising Exceptions¶

Sometimes a function catches an exception but wants to propagate it upward.

try:
    x = int("abc")
except ValueError:
    print("Conversion error")
    raise

This re-raises the same exception.

5. Custom Exception Classes¶

Programs may define their own exception types.

class NegativeNumberError(Exception):
    pass

Example:

def sqrt(x):
    if x < 0:
        raise NegativeNumberError("Negative value")

Custom exceptions help express domain-specific errors.

6. Worked Examples¶

Example 1¶

def check_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")

Example 2¶

def temperature(c):
    if c < -273.15:
        raise ValueError("Below absolute zero")

7. When to Raise Exceptions¶

Raising exceptions is appropriate when:

• inputs violate assumptions
• an operation cannot be completed
• continuing would produce incorrect results

8. Summary¶

Key ideas:

• raise explicitly signals errors
• functions can validate inputs and raise exceptions
• exceptions propagate up the call stack
• custom exception classes can represent domain-specific problems

Raising exceptions allows programs to enforce correctness and communicate errors clearly.