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:app:`Pyramid` Introduction

:app:`Pyramid` is a general, open source, Python web application development framework. Its primary goal is to make it easier for a Python developer to create web applications.

Frameworks vs. Libraries

A framework differs from a library in one very important way: library code is always called by code that you write, while a framework always calls code that you write. Using a set of libraries to create an application is usually easier than using a framework initially, because you can choose to cede control to library code you have not authored very selectively. But when you use a framework, you are required to cede a greater portion of control to code you have not authored: code that resides in the framework itself. You needn't use a framework at all to create a web application using Python. A rich set of libraries already exists for the platform. In practice, however, using a framework to create an application is often more practical than rolling your own via a set of libraries if the framework provides a set of facilities that fits your application requirements.

Pyramid attempts to follow follow these design and engineering principles:

:app:`Pyramid` takes a "pay only for what you eat" approach. You can get results even if you have only a partial understanding of :app:`Pyramid`. It doesn’t force you to use any particular technology to produce an application, and we try to keep the core set of concepts that you need to understand to a minimum.
:app:`Pyramid` tries to solve only the the fundamental problems of creating a web application: the mapping of URLs to code, templating, security and serving static assets. We consider these to be the core activities that are common to nearly all web applications.
Pyramid's minimalism means that it is easier for us to maintain complete and up-to-date documentation. It is our goal that no aspect of Pyramid is undocumented.
:app:`Pyramid` is designed to provide noticeably fast execution for common tasks such as templating and simple response generation. Although “hardware is cheap", the limits of this approach become painfully evident when one finds him or herself responsible for managing a great many machines.
:app:`Pyramid` is developed conservatively and tested exhaustively. Where Pyramid source code is concerned, our motto is: "If it ain’t tested, it’s broke". Every release of Pyramid has 100% statement coverage via unit tests.
As with Python, the Pyramid software is distributed under a permissive open source license.

What Is The Pylons Project?

:app:`Pyramid` is a member of the collection of software published under the Pylons Project. Pylons software is written by a loose-knit community of contributors. The Pylons Project website includes details about how :app:`Pyramid` relates to the Pylons Project.

:app:`Pyramid` and Other Web Frameworks

The first release of Pyramid's predecessor (named :mod:`repoze.bfg`) was made in July of 2008. At the end of 2010, we changed the name of :mod:`repoze.bfg` to :app:`Pyramid`. It was merged into the Pylons project as :app:`Pyramid` in November of that year.

:app:`Pyramid` was inspired by :term:`Zope`, :term:`Pylons` (version 1.0) and :term:`Django`. As a result, :app:`Pyramid` borrows several concepts and features from each, combining them into a unique web framework.

Many features of :app:`Pyramid` trace their origins back to :term:`Zope`. Like Zope applications, :app:`Pyramid` applications can be easily extended: if you obey certain constraints, the application you produce can be reused, modified, re-integrated, or extended by third-party developers without forking the original application. The concepts of :term:`traversal` and declarative security in :app:`Pyramid` were pioneered first in Zope.

The :app:`Pyramid` concept of :term:`URL dispatch` is inspired by the :term:`Routes` system used by :term:`Pylons` version 1.0. Like Pylons version 1.0, :app:`Pyramid` is mostly policy-free. It makes no assertions about which database you should use, and its built-in templating facilities are included only for convenience. In essence, it only supplies a mechanism to map URLs to :term:`view` code, along with a set of conventions for calling those views. You are free to use third-party components that fit your needs in your applications.

The concept of :term:`view` is used by :app:`Pyramid` mostly as it would be by Django. :app:`Pyramid` has a documentation culture more like Django's than like Zope's.

Like :term:`Pylons` version 1.0, but unlike :term:`Zope`, a :app:`Pyramid` application developer may use completely imperative code to perform common framework configuration tasks such as adding a view or a route. In Zope, :term:`ZCML` is typically required for similar purposes. In :term:`Grok`, a Zope-based web framework, :term:`decorator` objects and class-level declarations are used for this purpose. :app:`Pyramid` supports :term:`ZCML` and decorator-based :term:`declarative configuration`, but does not require either. See :ref:`configuration_narr` for more information.

Also unlike :term:`Zope` and unlike other "full-stack" frameworks such as :term:`Django`, :app:`Pyramid` makes no assumptions about which persistence mechanisms you should use to build an application. Zope applications are typically reliant on :term:`ZODB`; :app:`Pyramid` allows you to build :term:`ZODB` applications, but it has no reliance on the ZODB software. Likewise, :term:`Django` tends to assume that you want to store your application's data in a relational database. :app:`Pyramid` makes no such assumption; it allows you to use a relational database but doesn't encourage or discourage the decision.

Other Python web frameworks advertise themselves as members of a class of web frameworks named model-view-controller frameworks. Insofar as this term has been claimed to represent a class of web frameworks, :app:`Pyramid` also generally fits into this class.

You Say :app:`Pyramid` is MVC, But Where's The Controller?

The :app:`Pyramid` authors believe that the MVC pattern just doesn't really fit the web very well. In a :app:`Pyramid` application, there is a resource tree, which represents the site structure, and views, which tend to present the data stored in the resource tree and a user-defined "domain model". However, no facility provided by the framework actually necessarily maps to the concept of a "controller" or "model". So if you had to give it some acronym, I guess you'd say :app:`Pyramid` is actually an "RV" framework rather than an "MVC" framework. "MVC", however, is close enough as a general classification moniker for purposes of comparison with other web frameworks.

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