with Statement (Context Managers) in Python

# Basic file usage
with open('example.txt', 'r') as f:
    data = f.read()
# f is automatically closed here

# Multiple context managers
with open('in.txt') as fin, open('out.txt', 'w') as fout:
    fout.write(fin.read())

# Custom context manager class
class MyContext:
    def __enter__(self):
        # setup code, return value bound to 'as'
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        # cleanup code, runs even if an exception occurred
        return False  # False re-raises any exception

import tempfile, os

class Timer:
    def __enter__(self):
        print('Entering context')
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('Exiting context')
        return False

path = os.path.join(tempfile.gettempdir(), 'demo_with.txt')

with open(path, 'w') as f:
    f.write('Managed automatically')

with Timer() as t:
    print('Inside the with block')

with open(path, 'r') as f:
    print(f.read())

os.remove(path)

Entering context
Inside the with block
Exiting context
Managed automatically