Exception Handling

# ── Basic try/except ──────────────────────────
def safe_divide(a, b):
    try:
        result = a / b
    except ZeroDivisionError:
        return None
    else:
        return result   # only reached if no exception

print(safe_divide(10, 2))   # 5.0
print(safe_divide(10, 0))   # None

# ── Full try/except/else/finally ──────────────
def load_data(path):
    f = None
    try:
        f = open(path, encoding="utf-8")
        data = f.read()
    except FileNotFoundError:
        print(f"❌ File not found: {path}")
        return None
    except PermissionError as e:
        print(f"🔒 Permission denied: {e}")
        return None
    else:
        print(f"✅ Loaded {len(data)} characters")
        return data
    finally:
        if f: f.close()   # always close, even on error

# ── Catching multiple exceptions ──────────────
def parse_input(text):
    try:
        value = int(text.strip())
        return value
    except ValueError:
        print(f"'{text}' is not a valid integer")
    except AttributeError:
        print("Input must be a string")
    return None

print(parse_input("42"))    # 42
print(parse_input("hello")) # 'hello' is not a valid integer → None
print(parse_input(None))    # Input must be a string → None

# ── Raising built-in exceptions ───────────────
def set_age(age):
    if not isinstance(age, int):
        raise TypeError(f"Age must be int, got {type(age).__name__}")
    if age < 0 or age > 150:
        raise ValueError(f"Age {age} is out of valid range (0–150)")
    return age

# ── re-raise — catch, log, then pass on ───────
try:
    set_age("twenty")
except TypeError as e:
    print(f"Caught: {e}")
    raise   # re-raise the same exception

# ── Custom exception hierarchy ────────────────
class AppError(Exception):
    """Base class for all BitWithBite app errors."""
    pass

class ValidationError(AppError):
    """Raised when user input fails validation."""
    def __init__(self, field, message):
        self.field   = field
        self.message = message
        super().__init__(f"[{field}] {message}")

class DatabaseError(AppError):
    """Raised on database connection or query failure."""
    def __init__(self, query, cause):
        self.query = query
        super().__init__(f"DB error on query '{query}': {cause}")


def register_student(name, age, gpa):
    if not name.strip():
        raise ValidationError("name", "Name cannot be blank")
    if not (0 <= gpa <= 4.0):
        raise ValidationError("gpa", f"GPA {gpa} not in 0.0–4.0")
    print(f"✅ Registered: {name}, age {age}, GPA {gpa}")


try:
    register_student("Ali", 20, 5.2)
except ValidationError as e:
    print(f"❌ Validation failed: {e}")
    print(f"   Field: {e.field}")
except AppError as e:
    print(f"❌ App error: {e}")

import time

class Timer:
    """Context manager that times a code block."""

    def __enter__(self):
        self.start = time.perf_counter()
        return self   # bound to 'as t'

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.elapsed = time.perf_counter() - self.start
        print(f"⏱️  Elapsed: {self.elapsed:.4f}s")
        return False  # don't suppress exceptions


with Timer() as t:
    total = sum(range(1_000_000))
# ⏱️  Elapsed: 0.0312s


# ── contextlib.contextmanager — simpler decorator approach ──
from contextlib import contextmanager

@contextmanager
def managed_resource(name):
    print(f"Opening {name}")
    try:
        yield name         # 'with' body runs here
    except Exception as e:
        print(f"Error during {name}: {e}")
    finally:
        print(f"Closing {name}")


with managed_resource("database") as r:
    print(f"Using {r}...")
# Opening database
# Using database...
# Closing database