An exception stops normal flow and travels up the call stack until something handles it.

def to_int(text):
    try:
        return int(text)
    except ValueError:
        return None

• try holds the code that might fail.
• except SomeError handles only that type, and its subclasses.
• else runs if nothing was raised.
• finally always runs, which suits cleanup.

A bare except: or a wide except Exception: also hides your own bugs. Name the exact error you know how to handle and let the rest propagate.

def set_age(age):
    if age < 0:
        raise ValueError(f"bad age: {age}")
    return age

Use except (ValueError, TypeError): for several types at once. Python 3.14 also accepts that without the parentheses, when there is no as clause.

Write parse_age(text) converting text to an int and raising ValueError if it is not a number or is outside 0 to 150. Write safe_ratio(a, b) returning a / b, or None when b is zero. Other errors must still propagate.

The explanation opens here once every check passes.