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Added docs/design_philosophies.txt
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Adrian Holovaty
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===================
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Design philosophies
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===================
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This document explains some of the fundamental philosophies Django's developers
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have used in creating the framework. Its goal is to explain the past and guide
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the future.
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Overall
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=======
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Loose coupling
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--------------
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A fundamental goal of Django's stack is `loose coupling and tight cohesion`_.
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The various layers of the framework shouldn't "know" about each other unless
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absolutely necessary.
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For example, the template system knows nothing about Web requests, the database
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layer knows nothing about data display and the view system doesn't care which
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template system a programmer uses.
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.. _`loose coupling and tight cohesion`: http://c2.com/cgi/wiki?CouplingAndCohesion
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Less code
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---------
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Django apps should use as little code as possible; they should lack boilerplate.
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Django should take full advantage of Python's dynamic capabilities, such as
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introspection.
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Quick development
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-----------------
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The point of a Web framework in the 21st century is to make the tedious aspects
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of Web development fast. Django should allow for incredibly quick Web
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development.
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Don't repeat yourself (DRY)
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---------------------------
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Every distinct concept and/or piece of data should live in one, and only one,
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place. Redundancy is bad. Normalization is good.
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The framework, within reason, should deduce as much as possible from as little
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as possible.
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Explicit is better than implicit
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--------------------------------
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This, a `core Python principle`_, means Django shouldn't do too much "magic."
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Magic shouldn't happen unless there's a really good reason for it.
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.. _`core Python principle`: http://www.python.org/doc/Humor.html#zen
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Consistency
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-----------
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The framework should be consistent at all levels. Consistency applies to
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everything from low-level (the Python coding style used) to high-level (the
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"experience" of using Django).
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Models
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======
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Explicit is better than implicit
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--------------------------------
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Fields shouldn't assume certain behaviors based solely on the name of the
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field. This requires too much knowledge of the system and is prone to errors.
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Instead, behaviors should be based on keyword arguments and, in some cases, on
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the type of the field.
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Include all relevant domain logic
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---------------------------------
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Models should encapsulate every aspect of an "object," following Martin
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Fowler's `Active Record`_ design pattern.
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This is why model-specific admin options are included in the model itself; data
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related to a model should be stored *in* the model.
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.. _`Active Record`: http://www.martinfowler.com/eaaCatalog/activeRecord.html
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Database API
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============
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The core goals of the database API are:
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SQL efficiency
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--------------
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It should execute SQL statements as few times as possible, and it should
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optimize statements internally.
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This is why developers need to call ``save()`` explicitly, rather than the
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framework saving things behind the scenes silently.
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This is also why the ``select_related`` argument exists. It's an optional
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performance booster for the common case of selecting "every related object."
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Terse, powerful syntax
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----------------------
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The database API should allow rich, expressive statements in as little syntax
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as possible. It should not rely on importing other modules or helper objects.
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Joins should be performed automatically, behind the scenes, when necessary.
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Every object should be able to access every related object, systemwide. This
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access should work both ways.
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Option to drop into raw SQL easily, when needed
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-----------------------------------------------
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The database API should realize it's a shortcut but not necessarily an
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end-all-be-all. The framework should make it easy to write custom SQL -- entire
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statements, or just custom ``WHERE`` clauses as custom parameters to API calls.
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URL design
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==========
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Loose coupling
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--------------
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URLs in a Django app should not be coupled to the underlying Python code. Tying
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URLs to Python function names is a Bad And Ugly Thing.
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Along these lines, the Django URL system should allow URLs for the same app to
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be different in different contexts. For example, one site may put stories at
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``/stories/``, while another may use ``/news/``.
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Infinite flexibility
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--------------------
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URLs should be as flexible as possible. Any conceivable URL design should be
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allowed.
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Encourage best practices
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------------------------
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The framework should make it just as easy (or even easier) for a developer to
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design pretty URLs than ugly ones.
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File extensions in Web-page URLs should be avoided.
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Definitive URLs
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---------------
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Technically, ``foo.com/bar`` and ``foo.com/bar/`` are two different URLs, and
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search-engine robots (and some Web traffic-analyzing tools) would treat them as
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separate pages. Django should make an effort to "normalize" URLs so that
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search-engine robots don't get confused.
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This is the reasoning behind the ``APPEND_SLASH`` setting.
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Template system
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===============
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Separate logic from presentation
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--------------------------------
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We see a template system as a tool that controls presentation and
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presentation-related logic -- and that's it. The template system shouldn't
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support functionality that goes beyond this basic goal.
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If we wanted to put everything in templates, we'd be using PHP. Been there,
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done that, wised up.
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Discourage redundancy
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---------------------
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The majority of dynamic Web sites use some sort of common sitewide design --
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a common header, footer, navigation bar, etc. The Django template system should
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make it easy to store those elements in a single place, eliminating duplicate
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code.
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This is the philosophy behind template inheritance.
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Be decoupled from HTML
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----------------------
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The template system shouldn't be designed so that it only outputs HTML. It
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should be equally good at generating other text-based formats, or just plain
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text.
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Assume designer competence
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--------------------------
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The template system shouldn't be designed so that templates are displayed
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nicely in WYSIWYG editors such as Dreamweaver. That is too severe of a
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limitation and wouldn't allow the syntax to be as nice as it is. Django expects
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some level of competence in template authors.
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Treat whitespace obviously
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--------------------------
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The template system do magic things with whitespace. If a template includes
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whitespace, the system should treat it as it treats text -- just display it.
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Don't invent a programming language
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-----------------------------------
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The template system intentionally doesn't allow the following:
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* Assignment to variables
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* Advanced logic
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The goal is not to invent a programming language. The goal is to offer just
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enough programming-esque functionality, such as branching and looping, that are
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essential for making presentation-related decisions.
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Extensibility
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-------------
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The template system should recognize that advanced template authors may want
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to extend its technology.
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This is the philosophy behind custom template tags and filters.
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Views
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=====
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Simplicity
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----------
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Writing a view should be as simple as writing a Python function. Developers
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shouldn't have to instantiate a class when a function will do.
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Use request objects
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-------------------
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Views should have access to a request object -- an object that stores metadata
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about the current request. The object should be passed directly to a view
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function, rather than the view function having to access the request data from
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a global variable. This makes it light, clean and easy to test views by passing
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in "fake" request objects.
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Loose coupling
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--------------
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A view shouldn't care about which template system the developer uses -- or even
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whether a template system is used at all.
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