meet it

A sets up, then always cleans up.

You already write with open(...) as f:. The object after with is a context manager. Python calls its __enter__ method as the block starts, and its __exit__ method as the block ends, however it ends.

That is the whole trick. Even if the block crashes, __exit__ still runs, so a like a file or a lock is always given back. Whatever __enter__ returns is caught by the name after as.

Write a class with those two methods and you have your own context manager.

class Job:
    def __init__(self, label):
        self.label = label
    def __enter__(self):
        print(f"{self.label}: start")
        return self
    def __exit__(self, kind, error, trace):
        print(f"{self.label}: tidied up")

with Job("backup") as job:
    print("working on", job.label)
print("after the block")

__exit__ receives three details about any error, or three Nones when nothing went wrong.

guess first

Something breaks inside the block. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

class Door:
    def __enter__(self):
        print("door open")
    def __exit__(self, kind, error, trace):
        print("door shut")

try:
    with Door():
        print("carrying boxes")
        boxes = 3 / 0
        print("all done")
except ZeroDivisionError:
    print("dropped a box")

your turn

Your turn. Write the clean-up half.

Fill the first blank with the name of the method that runs as the block ends. In the second, write what happens to the room afterwards.

Your code

press run to see what happens

fix it

The basket vanishes inside the block. Fix it!

Broken code to fix

press run to see what happens

The error says None has no items. What did the name after as catch?
The name after as gets whatever __enter__ returns, and this __enter__ returns nothing.
Add return self at the end of __enter__, then run.

nailed it

you know what with does behind the scenes. :D

__enter__ runs as the block starts, __exit__ as it ends.
__exit__ runs even when the block crashes.
The name after as catches what __enter__ returns.

new words in your collection

context manager
resource

This lesson isn’t saved. A free account remembers every lesson you finish.

A context manager sets up, then always cleans up.

Something breaks inside the block. What will this show?

Your turn. Write the clean-up half.

The basket vanishes inside the block. Fix it!

you know what with does behind the scenes. :D

A sets up, then always cleans up.