author | Lennard de Rijk <ljvderijk@gmail.com> |
Mon, 13 Jul 2009 00:53:42 +0200 | |
changeset 2616 | 9321e100a7fb |
parent 109 | 620f9b141567 |
permissions | -rw-r--r-- |
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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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Although Django comes with a full stack for convenience, the pieces of the |
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stack are independent of another wherever possible. |
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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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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. Magic is |
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worth using only if it creates a huge convenience unattainable in other ways, |
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and it isn't implemented in a way that confuses developers who are trying to |
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learn how to use the feature. |
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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()`` ``QuerySet`` method exists. It's an |
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optional performance booster for the common case of selecting "every related |
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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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116 |
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117 |
Every object should be able to access every related object, systemwide. This |
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118 |
access should work both ways. |
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119 |
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120 |
Option to drop into raw SQL easily, when needed |
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121 |
----------------------------------------------- |
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122 |
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123 |
The database API should realize it's a shortcut but not necessarily an |
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124 |
end-all-be-all. The framework should make it easy to write custom SQL -- entire |
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125 |
statements, or just custom ``WHERE`` clauses as custom parameters to API calls. |
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126 |
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127 |
URL design |
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128 |
========== |
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129 |
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130 |
Loose coupling |
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131 |
-------------- |
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132 |
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133 |
URLs in a Django app should not be coupled to the underlying Python code. Tying |
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134 |
URLs to Python function names is a Bad And Ugly Thing. |
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135 |
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136 |
Along these lines, the Django URL system should allow URLs for the same app to |
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137 |
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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139 |
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140 |
Infinite flexibility |
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141 |
-------------------- |
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142 |
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143 |
URLs should be as flexible as possible. Any conceivable URL design should be |
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144 |
allowed. |
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145 |
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146 |
Encourage best practices |
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147 |
------------------------ |
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148 |
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149 |
The framework should make it just as easy (or even easier) for a developer to |
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150 |
design pretty URLs than ugly ones. |
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151 |
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152 |
File extensions in Web-page URLs should be avoided. |
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153 |
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154 |
Vignette-style commas in URLs deserve severe punishment. |
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155 |
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156 |
Definitive URLs |
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157 |
--------------- |
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158 |
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159 |
Technically, ``foo.com/bar`` and ``foo.com/bar/`` are two different URLs, and |
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160 |
search-engine robots (and some Web traffic-analyzing tools) would treat them as |
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161 |
separate pages. Django should make an effort to "normalize" URLs so that |
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162 |
search-engine robots don't get confused. |
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163 |
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164 |
This is the reasoning behind the ``APPEND_SLASH`` setting. |
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165 |
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166 |
Template system |
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167 |
=============== |
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168 |
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169 |
Separate logic from presentation |
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170 |
-------------------------------- |
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171 |
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172 |
We see a template system as a tool that controls presentation and |
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173 |
presentation-related logic -- and that's it. The template system shouldn't |
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174 |
support functionality that goes beyond this basic goal. |
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175 |
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176 |
If we wanted to put everything in templates, we'd be using PHP. Been there, |
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177 |
done that, wised up. |
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178 |
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179 |
Discourage redundancy |
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180 |
--------------------- |
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181 |
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182 |
The majority of dynamic Web sites use some sort of common sitewide design -- |
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183 |
a common header, footer, navigation bar, etc. The Django template system should |
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184 |
make it easy to store those elements in a single place, eliminating duplicate |
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185 |
code. |
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186 |
|
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187 |
This is the philosophy behind `template inheritance`_. |
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188 |
|
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189 |
.. _template inheritance: ../templates/#template-inheritance |
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190 |
|
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191 |
Be decoupled from HTML |
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192 |
---------------------- |
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193 |
|
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194 |
The template system shouldn't be designed so that it only outputs HTML. It |
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195 |
should be equally good at generating other text-based formats, or just plain |
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196 |
text. |
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197 |
|
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198 |
XML should not be used for template languages |
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199 |
--------------------------------------------- |
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200 |
|
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201 |
Using an XML engine to parse templates introduces a whole new world of human |
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202 |
error in editing templates -- and incurs an unacceptable level of overhead in |
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203 |
template processing. |
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204 |
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205 |
Assume designer competence |
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206 |
-------------------------- |
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207 |
|
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208 |
The template system shouldn't be designed so that templates necessarily are |
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209 |
displayed nicely in WYSIWYG editors such as Dreamweaver. That is too severe of |
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210 |
a limitation and wouldn't allow the syntax to be as nice as it is. Django |
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211 |
expects template authors are comfortable editing HTML directly. |
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212 |
|
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213 |
Treat whitespace obviously |
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214 |
-------------------------- |
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215 |
|
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216 |
The template system shouldn't do magic things with whitespace. If a template |
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217 |
includes whitespace, the system should treat the whitespace as it treats text |
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218 |
-- just display it. Any whitespace that's not in a template tag should be |
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219 |
displayed. |
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220 |
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221 |
Don't invent a programming language |
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222 |
----------------------------------- |
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223 |
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224 |
The template system intentionally doesn't allow the following: |
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225 |
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226 |
* Assignment to variables |
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227 |
* Advanced logic |
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228 |
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229 |
The goal is not to invent a programming language. The goal is to offer just |
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230 |
enough programming-esque functionality, such as branching and looping, that is |
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231 |
essential for making presentation-related decisions. |
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232 |
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The Django template system recognizes that templates are most often written by |
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*designers*, not *programmers*, and therefore should not assume Python |
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knowledge. |
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Safety and security |
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------------------- |
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239 |
|
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240 |
The template system, out of the box, should forbid the inclusion of malicious |
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code -- such as commands that delete database records. |
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242 |
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This is another reason the template system doesn't allow arbitrary Python code. |
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Extensibility |
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------------- |
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247 |
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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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250 |
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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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Differentiate between GET and POST |
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---------------------------------- |
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GET and POST are distinct; developers should explicitly use one or the other. |
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The framework should make it easy to distinguish between GET and POST data. |