Python: with statement --> context manager

Python: with statement --> context manager

# This trick is introduced in the book called Python Tricks
# When you do this
with open('hello.txt', 'w') as f:
    f.write('hello, world!')

# You're essentially doing this
f = open('hello.txt', 'w')
try:
    f.write('hello, world')
finally:
    f.close()
    
# And you can define your own context manager with the help of __enter__ and __exit__
class ManagedFile:
    def __init__(self, name):
        self.name = name

    def __enter__(self):
        self.file = open(self.name, 'w')
        return self.file

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.file:
            self.file.close()

with ManagedFile('hello.txt') as f:
    f.write('hello, world!')
    f.write('bye now')

# Or you can use the library for context manager
from contextlib import contextmanager
def managed_file(name):
    try:
        f = open(name, 'w')
        yield f
    finally:
        f.close()

with managed_file('hello.txt') as f:
    f.write('hello, world!')
    f.write('bye now')