What are context managers and how to use them?

Question

Accepted Answer

**Context manager** is an object that manages resources (files, connections) using `with`.

**Basic usage:**
```python
# File
with open("file.txt", "r") as f:
    data = f.read()
# File is automatically closed
```

**Creating your own context manager:**
```python
from contextlib import contextmanager

@contextmanager
def timer(name):
    import time
    start = time.time()
    yield  # Code inside with executes here
    end = time.time()
    print(f"{name} took {end - start:.2f}s")

# Usage
with timer("sleep"):
    time.sleep(1)
# "sleep took 1.00s"
```

**Class-based:**
```python
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.end = time.time()
        print(f"Took {self.end - self.start:.2f}s")
        return False  # Don't suppress exceptions
```

Python: What are context managers and how to use them?