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Chapter 27. Managing Contexts: the with Statement

Many objects manage resources, and must impose a rigid protocol on use of that resource. A file object in Python acquires and releases OS files, which may be associated with devices or network interfaces. A program may acquire and release database connections. In some cases, there may be a nested context of a database connection and one or more cursors.

In Python 2.6, the with statement interacts with certain types of context management objects to assure that the protocol for acquiring and releasing the object is followed irrespective of any exceptions that may be raised.

In Python 2.5, we must enable the with statement by using the following statement.

from __future__ import with_statement

In this section, we'll look at ways in which the new with statement will simplify file processing or database processing. We will look at the kinds of object design considerations which are required to create your own objects that work well with the with statement.

Semantics of a Context

While most use of the with statement involve acquiring and releasing specific resources, the statement can be applied somewhat more generally. To make the statement more widely applicable, Python works with a context. A context is not limited to acquiring and releasing a file or database connection. A context could be a web transaction, a user's logged-in session, a particular transaction or any other stateful condition.

Generally, a context is a state which must endure for one or more statements, has a specific method for entering the state and has a specific method for exiting the state. Further, a context's exit must be done with the defined method irrespective of any exceptions that might occur within the context.

Database operations often center on transactions which must either be completed (to move the database to a new, iternally consistent state,) or rolled back to reset the database to a prior consistent state. In this case, exceptions must be tolerated so that the database server can be instructed to commit the transaction or roll it back.

We'll also use a context to be sure that a file is closed, or a lock is released. We can also use a context to be sure that the user interface is reset properly when a user switches their focus or an error occurs in a complex interaction.

The design pattern has two elements: a Context Manager and a Working Object . The Context Manager is used by the with statement to enter and exit the context. One thing that can happen when entering a context is that a Working Object is created as part of the entry process. The Working Object is often used for files and databases where we interact with the context. The Working Object isn't always necessary; for example acquiring and releasing locks is done entirely by the Context Manager.


 
 
  Published under the terms of the Open Publication License Design by Interspire