Context Managers (with Statement)

# Basic context manager for files
# with open("file.txt") as f:
#     data = f.read()
# f is automatically closed here!

# Without context manager (BAD!)
# f = open("file.txt")
# data = f.read()
# f.close()  # Easy to forget!

# Multiple context managers
# with open("input.txt") as src, open("output.txt", "w") as dst:
#     dst.write(src.read())

# Custom context manager with class
class Timer:
    def __enter__(self):
        import time
        self.start = time.time()
        return self
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        import time
        self.elapsed = time.time() - self.start
        print(f"  Elapsed: {self.elapsed:.4f}s")
        return False

# Usage
with Timer() as t:
    total = sum(range(1_000_000))
    print(f"  Sum: {total}")

# Context manager with contextlib
from contextlib import contextmanager

@contextmanager
def managed_resource(name):
    print(f"  Opening {name}")
    yield name
    print(f"  Closing {name}")

with managed_resource("database") as db:
    print(f"  Using {db}")