How to Use Context Managers with "with" Statement

Table Of Contents

• Problem
• Solution
• Explanation

Problem

You need to ensure resources like files, database connections, or locks are properly cleaned up even if errors occur.

Solution

# File handling with context manager
with open('data.txt', 'r') as file:
    content = file.read()
    print(content)
# File automatically closed, even if error occurs

# Multiple files
with open('input.txt', 'r') as infile, open('output.txt', 'w') as outfile:
    data = infile.read()
    outfile.write(data.upper())

# Custom context manager with class
class DatabaseConnection:
    def __enter__(self):
        print("Connecting to database...")
        return self
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("Closing database connection...")
        return False  # Don't suppress exceptions

with DatabaseConnection() as db:
    print("Using database")
    # Connection automatically closed

# Context manager with contextlib
from contextlib import contextmanager

@contextmanager
def timer():
    import time
    start = time.time()
    print("Timer started")
    try:
        yield
    finally:
        end = time.time()
        print(f"Elapsed: {end - start:.2f} seconds")

with timer():
    import time
    time.sleep(1)
    print("Doing work...")

# Suppress specific exceptions
from contextlib import suppress

with suppress(FileNotFoundError):
    with open('nonexistent.txt') as f:
        content = f.read()
# No error thrown if file doesn't exist

# Lock example
import threading

lock = threading.Lock()
with lock:
    # Critical section code
    print("Thread-safe operation")
# Lock automatically released

Explanation

Context managers ensure cleanup code runs via __enter__ and __exit__ methods. The with statement guarantees __exit__ executes even if exceptions occur.

Use @contextmanager decorator for simple cases, or implement __enter__/__exit__ methods for complex resource management.