mirror of
https://github.com/django/django.git
synced 2025-11-03 21:25:09 +00:00
331 lines
10 KiB
Text
331 lines
10 KiB
Text
===================
|
|
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://wiki.c2.com/?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://wiki.c2.com/?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`: https://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.
|
|
|
|
Discourage redundancy
|
|
---------------------
|
|
|
|
The majority of dynamic websites 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 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 :ref:`Django Template
|
|
Language (DTL) <template-language-intro>` aims to avoid advanced logic.
|
|
|
|
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.
|
|
|
|
.. _cache-design-philosophy:
|
|
|
|
Cache Framework
|
|
===============
|
|
|
|
The core goals of Django's :doc:`cache framework </topics/cache>` are:
|
|
|
|
Less code
|
|
---------
|
|
|
|
A cache should be as fast as possible. Hence, all framework code surrounding
|
|
the cache backend should be kept to the absolute minimum, especially for
|
|
``get()`` operations.
|
|
|
|
Consistency
|
|
-----------
|
|
|
|
The cache API should provide a consistent interface across the different
|
|
cache backends.
|
|
|
|
Extensibility
|
|
-------------
|
|
|
|
The cache API should be extensible at the application level based on the
|
|
developer's needs (for example, see :ref:`cache_key_transformation`).
|