Expert Python Programming（Third Edition）

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Context managers – the with statement

The try...finally statement is useful to ensure some cleanup code is run, even if an error is raised. There are many use cases for this, such as the following:

Closing a file
Releasing a lock
Making a temporary code patch
Running protected code in a special environment

The with statement factors out these use cases by providing a simple way to wrap a block of code with methods defined within the context manager. This allows us to call some code before and after block execution, even if this block raises an exception. For example, working with a file is often done like so:

>>> hosts = open('/etc/hosts')
>>> try:
...     for line in hosts:
...         if line.startswith('#'):
...             continue
...         print(line.strip())
... finally:
...     hosts.close()
...
127.0.0.1       localhost
255.255.255.255 broadcasthost
::1             localhost

This example is specific to Linux, since it reads the host file located in the /etc/ directory, but any text file could have been used here in the same way.

By using the with statement, it can be rewritten into the following code, which is shorter and cleaner:

>>> with open('/etc/hosts') as hosts:
...     for line in hosts:
...         if line.startswith('#'):
...             continue
...         print(line.strip())
...
127.0.0.1       localhost
255.255.255.255 broadcasthost
::1             localhost

In the preceding example, the open() function is used as a context manager that ensures that the file will be closed after executing the for loop, even if some exception occurs in the process.

Some other common items from the Python standard library that are compatible with this statement are classes from the threading module:

threading.Lock
threading.RLock
threading.Condition
threading.Semaphore
threading.BoundedSemaphore