Context Managers in Python

Photo by Tyler Daviaux on Unsplash

In this piece, we are going to delve into context managers in Python. I will keep it short — less talk, more work! I hope you enjoy it.

Why Context Managers?

Context managers are used to give resources back. Here’s what I mean:

f = open('file.txt', 'w')
f.write('Something to write')

We opened a file named file.txt and wrote it. Hang on — we didn’t close this file, so we can still use thef instance:

f = open('file.txt', 'w')
f.write('Something to write')
print('another work')
f.write('I can still write to file.txt')

This is a problem — resources are not unlimited so we have to give it back when we’re done with it. Here’s how we do it:

f = open('file.txt', 'w')
f.write('Something to write')
f.close()

With f.close() we closed the file.txt, so we can’t use this f any more:

f = open('file.txt', 'w')
f.write('Something to write')
f.close()
print(f.closed) # returns True

print(f.closed) will print True because f is closed, thanks to f.close().

How Do We Use Context Manager?

With a with statement. Let me give you an example:

with open('file.txt', 'w') as f:
    f.write('Something to write')

But wait — where is the f.close()? Don’t worry, the with statement will take care of this and close the file:

with open('file.txt', 'w') as f:
    f.write('Something to write')
print(f.closed)

See — I told you that print(f.closed) would print True!

Great! So, How Can I Write Mine?

Let me help you create your own. There are two ways to do it — I prefer to do it with class but I’ll show you both methods.

Method one

class OpenFile:
    def __init__(self, file, mode):
        self.file = file
        self.mode = mode

    def __enter__(self):
        self.f = open(self.file, self.mode)
        return self.f

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

Method two

from contextlib import contextmanager


@contextmanager
def OpenFile(file, mode):
    f = open(file, mode)
    yield f
    f.close()

Try it out!

with OpenFile('file.txt', 'w') as f:
    f.write('Something to write to mine')
print(f.closed)