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django / docs / misc / design-philosophies.txt

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===================
Design philosophies
===================

This document explains some of the fundamental philosophies Django's developers
have used in creating the framework. Its goal is to explain the past and guide
the future.

Overall
=======

.. _loose-coupling:

Loose coupling
--------------

.. index:: coupling; loose

A fundamental goal of Django's stack is `loose coupling and tight cohesion`_.
The various layers of the framework shouldn't "know" about each other unless
absolutely necessary.

For example, the template system knows nothing about Web requests, the database
layer knows nothing about data display and the view system doesn't care which
template system a programmer uses.

Although Django comes with a full stack for convenience, the pieces of the
stack are independent of another wherever possible.

.. _`loose coupling and tight cohesion`: http://c2.com/cgi/wiki?CouplingAndCohesion

.. _less-code:

Less code
---------

Django apps should use as little code as possible; they should lack boilerplate.
Django should take full advantage of Python's dynamic capabilities, such as
introspection.

.. _quick-development:

Quick development
-----------------

The point of a Web framework in the 21st century is to make the tedious aspects
of Web development fast. Django should allow for incredibly quick Web
development.

.. _dry:

Don't repeat yourself (DRY)
---------------------------

.. index::
   single: DRY
   single: Don't repeat yourself

Every distinct concept and/or piece of data should live in one, and only one,
place. Redundancy is bad. Normalization is good.

The framework, within reason, should deduce as much as possible from as little
as possible.

.. seealso::

    The `discussion of DRY on the Portland Pattern Repository`__

    __ http://c2.com/cgi/wiki?DontRepeatYourself
    
.. _explicit-is-better-than-implicit:

Explicit is better than implicit
--------------------------------

This is a core Python principle listed in :pep:`20`, and it means Django
shouldn't do too much "magic." Magic shouldn't happen unless there's a really
good reason for it. Magic is worth using only if it creates a huge convenience
unattainable in other ways, and it isn't implemented in a way that confuses
developers who are trying to learn how to use the feature.

.. _consistency:

Consistency
-----------

The framework should be consistent at all levels. Consistency applies to
everything from low-level (the Python coding style used) to high-level (the
"experience" of using Django).

Models
======

Explicit is better than implicit
--------------------------------

Fields shouldn't assume certain behaviors based solely on the name of the
field. This requires too much knowledge of the system and is prone to errors.
Instead, behaviors should be based on keyword arguments and, in some cases, on
the type of the field.

Include all relevant domain logic
---------------------------------

Models should encapsulate every aspect of an "object," following Martin
Fowler's `Active Record`_ design pattern.

This is why both the data represented by a model and information about
it (its human-readable name, options like default ordering, etc.) are
defined in the model class; all the information needed to understand a
given model should be stored *in* the model.

.. _`Active Record`: http://www.martinfowler.com/eaaCatalog/activeRecord.html

Database API
============

The core goals of the database API are:

SQL efficiency
--------------

It should execute SQL statements as few times as possible, and it should
optimize statements internally.

This is why developers need to call ``save()`` explicitly, rather than the
framework saving things behind the scenes silently.

This is also why the ``select_related()`` ``QuerySet`` method exists. It's an
optional performance booster for the common case of selecting "every related
object."

Terse, powerful syntax
----------------------

The database API should allow rich, expressive statements in as little syntax
as possible. It should not rely on importing other modules or helper objects.

Joins should be performed automatically, behind the scenes, when necessary.

Every object should be able to access every related object, systemwide. This
access should work both ways.

Option to drop into raw SQL easily, when needed
-----------------------------------------------

The database API should realize it's a shortcut but not necessarily an
end-all-be-all. The framework should make it easy to write custom SQL -- entire
statements, or just custom ``WHERE`` clauses as custom parameters to API calls.

URL design
==========

Loose coupling
--------------

URLs in a Django app should not be coupled to the underlying Python code. Tying
URLs to Python function names is a Bad And Ugly Thing.

Along these lines, the Django URL system should allow URLs for the same app to
be different in different contexts. For example, one site may put stories at
``/stories/``, while another may use ``/news/``.

Infinite flexibility
--------------------

URLs should be as flexible as possible. Any conceivable URL design should be
allowed.

Encourage best practices
------------------------

The framework should make it just as easy (or even easier) for a developer to
design pretty URLs than ugly ones.

File extensions in Web-page URLs should be avoided.

Vignette-style commas in URLs deserve severe punishment.

.. _definitive-urls:

Definitive URLs
---------------

.. index:: urls; definitive

Technically, ``foo.com/bar`` and ``foo.com/bar/`` are two different URLs, and
search-engine robots (and some Web traffic-analyzing tools) would treat them as
separate pages. Django should make an effort to "normalize" URLs so that
search-engine robots don't get confused.

This is the reasoning behind the :setting:`APPEND_SLASH` setting.

Template system
===============

.. _separation-of-logic-and-presentation:

Separate logic from presentation
--------------------------------

We see a template system as a tool that controls presentation and
presentation-related logic -- and that's it. The template system shouldn't
support functionality that goes beyond this basic goal.

If we wanted to put everything in templates, we'd be using PHP. Been there,
done that, wised up.

Discourage redundancy
---------------------

The majority of dynamic Web sites use some sort of common sitewide design --
a common header, footer, navigation bar, etc. The Django template system should
make it easy to store those elements in a single place, eliminating duplicate
code.

This is the philosophy behind :ref:`template inheritance
<template-inheritance>`.

Be decoupled from HTML
----------------------

The template system shouldn't be designed so that it only outputs HTML. It
should be equally good at generating other text-based formats, or just plain
text.

XML should not be used for template languages
---------------------------------------------

.. index:: xml; suckiness of

Using an XML engine to parse templates introduces a whole new world of human
error in editing templates -- and incurs an unacceptable level of overhead in
template processing.

Assume designer competence
--------------------------

The template system shouldn't be designed so that templates necessarily are
displayed nicely in WYSIWYG editors such as Dreamweaver. That is too severe of
a limitation and wouldn't allow the syntax to be as nice as it is. Django
expects template authors are comfortable editing HTML directly.

Treat whitespace obviously
--------------------------

The template system shouldn't do magic things with whitespace. If a template
includes whitespace, the system should treat the whitespace as it treats text
-- just display it. Any whitespace that's not in a template tag should be
displayed.

Don't invent a programming language
-----------------------------------

The template system intentionally doesn't allow the following:

* Assignment to variables
* Advanced logic

The goal is not to invent a programming language. The goal is to offer just
enough programming-esque functionality, such as branching and looping, that is
essential for making presentation-related decisions.

The Django template system recognizes that templates are most often written by
*designers*, not *programmers*, and therefore should not assume Python
knowledge.

Safety and security
-------------------

The template system, out of the box, should forbid the inclusion of malicious
code -- such as commands that delete database records.

This is another reason the template system doesn't allow arbitrary Python code.

Extensibility
-------------

The template system should recognize that advanced template authors may want
to extend its technology.

This is the philosophy behind custom template tags and filters.

Views
=====

Simplicity
----------

Writing a view should be as simple as writing a Python function. Developers
shouldn't have to instantiate a class when a function will do.

Use request objects
-------------------

Views should have access to a request object -- an object that stores metadata
about the current request. The object should be passed directly to a view
function, rather than the view function having to access the request data from
a global variable. This makes it light, clean and easy to test views by passing
in "fake" request objects.

Loose coupling
--------------

A view shouldn't care about which template system the developer uses -- or even
whether a template system is used at all.

Differentiate between GET and POST
----------------------------------

GET and POST are distinct; developers should explicitly use one or the other.
The framework should make it easy to distinguish between GET and POST data.