The contextmanager Decorator

Wed 22 January 2020

Table of Contents

Overview

Generally, you create a context manager by creating a class with __enter__ and __exit__ methods, but this example shows you how to use the @contextlib.contextmanager to create a simple context manager.

Context managers provide a cool programming pattern, especially if you’re forgetful or just have too much to keep track of and you want to simplify your life.

You’ll find them helpful when you have opened something and need to close it, locked something and need to release it, or changed something and need to reset it. There are several built in context managers that you’re probably familiar with like open to open a file or socket to use a socket. The bog-standard example:

with open('myfile.txt') as f:
    lines = f.readlines()

do_stuff(lines)

The open context manager opens a file, returns an object we name as f. When we’ve done all the things we’re going to with it, (we fall out of the with statement block), the file is automatically closed for us.

In this brief article, you’ll see how you can create a dead-simple context manager from a generator.

Let’s Code

Create the Context Manager

First import the contextlib module. It has several helpers (read more here: contextlib module).

We’re just going to decorate the chdir function as a contextlib.contextmanager.

import contextlib

@contextlib.contextmanager
def chdir(path):
    """
    On enter, change directory to specified path.
    On exit, change directory to original.
    """
    this_dir = os.getcwd()
    os.chdir(path)
    try:
        yield
    finally:
        os.chdir(this_dir)

To make this work, the function must be a generator and yield exactly once. At the point of the yield, the calling process is executed.

In chdir, the function takes a single argument, path.

  1. First it makes a note of the current directory and then changes to the path.
  2. Then it yields control back to the caller.
  3. No matter what happens during that process, the function will finally change back to the original directory.

Use the Context Manager

Suppose you have some function gather_paths you want to call for a set of directories. The following example shows how the chdir context manager could be used:

with chdir("/mydownloads/wordpress"):
    gather_paths()

I like this little context manager; it keeps me from having to remember to switch back to the original directory so I don’t get surprised later and find my program is executing somewhere else.

As long as I call the function as a context manager using the with statement, I don’t have to remember to change back to the original directory or do anything special.

Category: Python Tagged: Python hacking

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