---
author: ''
category: ''
date: 2012/01/07 23:06
description: ''
link: ''
priority: ''
slug: BB963
tags: programming, python
title: 'Python context managers: they are easy!'
type: text
updated: 2012/01/07 23:06
url_type: ''
---
This comes from `this thread <http://comments.gmane.org/gmane.org.user-groups.python.argentina/51628>`_ in the Python Argentina mailing list (which I strongly recommend if you read spanish).

I was the other day trying to do shell-script-like-things on python (as part of a monster setup.py) and I was annoyed that in shell it's easy to do this:

.. code-block:: shell

    cd foo
    bar -baz
    cd -

Or this:

.. code-block:: shell

    pushd foo
    bar -baz
    popd

Or this:

.. code-block:: shell

    (cd foo && bar -baz)

And on Python I had to do this, which is verbose and ugly:

.. code-block:: python

    cwd = os.getcwd()
    try:
        os.chdir('foo')
        os.system('bar -baz')
    finally:
        os.chdir(cwd)

This is what I wanted to have:

.. code-block:: python

    with os.chdir('foo'):
        os.system('bar -baz')

And of course, you can't do that. So, I asked, how do you implement that context manager? I got several answers.

1. That's available in Fabric:

    .. code-block:: python
    
        with cd("foo"):
            run("bar")

2. It's not hard to do:

    .. code-block:: python
    
        class DirContextM(object):
            def __init__(self, new_dir):
                self.new_dir = new_dir
                self.old_dir = None

            def __enter__(self):
                self.old_dir = os.getcwd()
                os.chdir(self.new_dir)

            def __exit__(self, *_):
                os.chdir(self.old_dir)

3. It's even easier to do:

    .. code-block:: python
    
        from contextlib import contextmanager

        @contextmanager
        def cd(path):
            old_dir = os.getcwd()
            os.chdir(path)
            yield
            os.chdir(old_dir)

4. That's cool, so let's add it to `path.py <https://github.com/dottedmag/path.py/pull/13/files>`_

        
5. Maybe check for exceptions

    .. code-block:: python
    
        @contextmanager
        def cd(path):
            old_dir = os.getcwd()
            os.chdir(path)
            try:
                yield
            finally:
                os.chdir(old_dir)

All in all, I learned how to do context managers, about contextlib, about fabric and about path.py. Which is not bad for 15 minutes :-)