| 1 | Nevow Object Publishing |
|---|---|
| 2 | ======================= |
| 3 | |
| 4 | In Nevow Object Traversal, we learned about the |
| 5 | nevow.inevow.IResource.renderHTTP method, which is the most basic way to send |
| 6 | HTML to a browser when using Nevow. However, it is not very convenient (or |
| 7 | clean) to generate HTML tags by concatenating strings in Python code. In the |
| 8 | Nevow Deployment documentation, we saw that it was possible to render a Hello |
| 9 | World page using a nevow.rend.Page subclass and providing a "docFactory":: |
| 10 | |
| 11 | >>> from nevow import rend, loaders |
| 12 | >>> class HelloWorld(rend.Page): |
| 13 | ... docFactory = loaders.stan("Hello, world!") |
| 14 | ... |
| 15 | >>> HelloWorld().renderSynchronously() |
| 16 | 'Hello, world!' |
| 17 | |
| 18 | This example does nothing interesting, but the concept of a loader is important |
| 19 | in Nevow. The rend.Page.renderHTTP implementation always starts rendering HTML |
| 20 | by loading a template from the docFactory. |
| 21 | |
| 22 | * `The stan DOM`_ |
| 23 | * `Tag instances`_ |
| 24 | * `Functions in the DOM`_ |
| 25 | * `Accessing query parameters and form post data`_ |
| 26 | * `Generators in the DOM`_ |
| 27 | * `Methods in the DOM`_ |
| 28 | * `Data specials`_ |
| 29 | * `Render specials`_ |
| 30 | * `Pattern specials`_ |
| 31 | * `Slot specials`_ |
| 32 | * `Data directives`_ |
| 33 | * `Render directives`_ |
| 34 | * `Flatteners`_ |
| 35 | |
| 36 | The stan DOM |
| 37 | ------------ |
| 38 | |
| 39 | Nevow uses a DOM-based approach to rendering HTML. A tree of objects is first |
| 40 | constructed in memory by the template loader. This tree is then processed one |
| 41 | node at a time, applying functions which transform from various Python types to |
| 42 | HTML strings. |
| 43 | |
| 44 | Nevow uses a nonstandard DOM named "stan". Unlike the W3C DOM, stan is made up |
| 45 | of simple python lists, strings, and instances of the nevow.stan.Tag class. |
| 46 | During the rendering process, "Flattener" functions convert from rich types to |
| 47 | HTML strings. For example, we can load a template made up of some nested lists |
| 48 | and Python types, render it, and see what happens:: |
| 49 | |
| 50 | >>> class PythonTypes(rend.Page): |
| 51 | ... docFactory = loaders.stan(["Hello", 1, 1.5, True, ["Goodbye", 3]]) |
| 52 | ... |
| 53 | >>> PythonTypes().renderSynchronously() |
| 54 | 'Hello11.5TrueGoodbye3' |
| 55 | |
| 56 | Tag instances |
| 57 | ------------- |
| 58 | |
| 59 | So far, we have only rendered simple strings as output. However, the main |
| 60 | purpose of Nevow is HTML generation. In the stan DOM, HTML tags are represented |
| 61 | by instances of the nevow.stan.Tag class. Tag is a very simple class, whose |
| 62 | instances have an "attributes" dictionary and a "children" list. The Tag |
| 63 | flattener knows how to recursively flatten attributes and children of the tag. |
| 64 | To show you how Tags really work before you layer Nevow's convenience syntax on |
| 65 | top, try this horrible example:: |
| 66 | |
| 67 | >>> from nevow import stan |
| 68 | >>> h = stan.Tag('html') |
| 69 | >>> d = stan.Tag('div') |
| 70 | >>> d.attributes['style'] = 'border: 1px solid black' |
| 71 | >>> h.children.append(d) |
| 72 | >>> class Tags(rend.Page): |
| 73 | ... docFactory = loaders.stan(h) |
| 74 | ... |
| 75 | >>> Tags().renderSynchronously() |
| 76 | '<html><div style="border: 1px solid black"></div></html>' |
| 77 | |
| 78 | So, we see how it is possible to programatically generate HTML by constructing |
| 79 | and nesting stan Tag instances. However, it is far more convenient to use the |
| 80 | overloaded operators Tag provides to manipulate them. Tag implements a __call__ |
| 81 | method which takes any keyword arguments and values and updates the attributes |
| 82 | dictionary; it also implements a __getitem__ method which takes whatever is |
| 83 | between the square brackets and appends them to the children list. A simple |
| 84 | example should clarify things:: |
| 85 | |
| 86 | >>> class Tags2(rend.Page): |
| 87 | ... docFactory = loaders.stan(stan.Tag('html')[stan.Tag('div')(style="border: 1px solid black")]) |
| 88 | ... |
| 89 | >>> Tags2().renderSynchronously() |
| 90 | '<html><div style="border: 1px solid black"></div></html>' |
| 91 | |
| 92 | This isn't very easy to read, but luckily we can simplify the example even |
| 93 | further by using the nevow.tags module, which is full of "Tag prototypes" for |
| 94 | every tag type described by the XHTML 1.0 specification:: |
| 95 | |
| 96 | >>> class Tags3(rend.Page): |
| 97 | ... docFactory = loaders.stan(tags.html[tags.div(style="border: 1px solid black")]) |
| 98 | ... |
| 99 | >>> Tags3().renderSynchronously() |
| 100 | '<html><div style="border: 1px solid black"></div></html>' |
| 101 | |
| 102 | Using stan syntax is not the only way to construct template DOM for use by the |
| 103 | Nevow rendering process. Nevow also includes loaders.xmlfile which implements a |
| 104 | simple tag attribute language similar to the Zope Page Templates (ZPT) Tag |
| 105 | Attribute Language (TAL). However, experience with the stan DOM should give you |
| 106 | insight into how the Nevow rendering process really works. Rendering a template |
| 107 | into HTML in Nevow is really nothing more than iterating a tree of objects and |
| 108 | recursively applying "Flattener" functions to objects in this tree, until all |
| 109 | HTML has been generated. |
| 110 | |
| 111 | Functions in the DOM |
| 112 | -------------------- |
| 113 | |
| 114 | So far, all of our examples have generated static HTML pages, which is not |
| 115 | terribly interesting when discussing dynamic web applications. Nevow takes a |
| 116 | very simple approach to dynamic HTML generation. If you put a Python function |
| 117 | reference in the DOM, Nevow will call it when the page is rendered. The return |
| 118 | value of the function replaces the function itself in the DOM, and the results |
| 119 | are flattened further. This makes it easy to express looping and branching |
| 120 | structures in Nevow, because normal Python looping and branching constructs are |
| 121 | used to do the job:: |
| 122 | |
| 123 | >>> def repeat(ctx, data): |
| 124 | ... return [tags.div(style="color: %s" % (color, )) |
| 125 | ... for color in ['red', 'blue', 'green']] |
| 126 | ... |
| 127 | >>> class Repeat(rend.Page): |
| 128 | ... docFactory = loaders.stan(tags.html[repeat]) |
| 129 | ... |
| 130 | >>> Repeat().renderSynchronously() |
| 131 | '<html><div style="color: red"></div><div style="color: blue"></div><div style="color: green"></div></html>' |
| 132 | |
| 133 | However, in the example above, the repeat function isn't even necessary, because |
| 134 | we could have inlined the list comprehension right where we placed the function |
| 135 | reference in the DOM. Things only really become interesting when we begin |
| 136 | writing parameterized render functions which cause templates to render |
| 137 | differently depending on the input to the web application. |
| 138 | |
| 139 | The required signature of functions which we can place in the DOM is (ctx, |
| 140 | data). The "context" object is essentially opaque for now, and we will learn how |
| 141 | to extract useful information out of it later. The "data" object is anything we |
| 142 | want it to be, and can change during the rendering of the page. By default, the |
| 143 | data object is whatever we pass as the first argument to the Page constructor, |
| 144 | **or** the Page instance itself if nothing is passed. Armed with this knowledge, |
| 145 | we can create a Page which renders differently depending on the data we pass to |
| 146 | the Page constructor:: |
| 147 | |
| 148 | class Root(rend.Page): |
| 149 | docFactory = loaders.stan(tags.html[ |
| 150 | tags.h1["Welcome."], |
| 151 | tags.a(href="foo")["Foo"], |
| 152 | tags.a(href="bar")["Bar"], |
| 153 | tags.a(href="baz")["Baz"]]) |
| 154 | |
| 155 | def childFactory(self, ctx, name): |
| 156 | return Leaf(name) |
| 157 | |
| 158 | |
| 159 | def greet(ctx, name): |
| 160 | return "Hello. You are visiting the ", name, " page." |
| 161 | |
| 162 | class Leaf(rend.Page): |
| 163 | docFactory = loaders.stan(tags.html[greet]) |
| 164 | |
| 165 | Armed with this knowledge and the information in the Object Traversal |
| 166 | documentation, we now have enough information to create dynamic websites with |
| 167 | arbitrary URL hierarchies whose pages render dynamically depending on which URL |
| 168 | was used to access them. |
| 169 | |
| 170 | Accessing query parameters and form post data |
| 171 | --------------------------------------------- |
| 172 | |
| 173 | Before we move on to more advanced rendering techniques, let us first examine |
| 174 | how one could further customize the rendering of a Page based on the URL query |
| 175 | parameters and form post information provided to us by a browser. Recall that |
| 176 | URL parameters are expressed in the form:: |
| 177 | |
| 178 | http://example.com/foo/bar?baz=1&quux=2 |
| 179 | |
| 180 | And form post data can be generated by providing a form to a browser:: |
| 181 | |
| 182 | <form action="" method="POST"> |
| 183 | <input type="text" name="baz" /> |
| 184 | <input type="text" name="quux" /> |
| 185 | <input type="submit" /> |
| 186 | </form> |
| 187 | |
| 188 | Accessing this information is such a common procedure that Nevow provides a |
| 189 | convenience method on the context to do it. Let's examine a simple page whose |
| 190 | output can be influenced by the query parameters in the URL used to access it:: |
| 191 | |
| 192 | def showChoice(ctx, data): |
| 193 | choice = ctx.arg('choice') |
| 194 | if choice is None: |
| 195 | return '' |
| 196 | return "You chose ", choice, "." |
| 197 | |
| 198 | class Custom(rend.Page): |
| 199 | docFactory = loaders.stan(tags.html[ |
| 200 | tags.a(href="?choice=baz")["Baz"], |
| 201 | tags.a(href="?choice=quux")["Quux"], |
| 202 | tags.p[showChoice]]) |
| 203 | |
| 204 | The procedure is exactly the same for simple form post information:: |
| 205 | |
| 206 | def greet(ctx, data): |
| 207 | name = ctx.arg('name') |
| 208 | if name is None: |
| 209 | return '' |
| 210 | return "Greetings, ", name, "!" |
| 211 | |
| 212 | class Form(rend.Page): |
| 213 | docFactory = loaders.stan(tags.html[ |
| 214 | tags.form(action="", method="POST")[ |
| 215 | tags.input(name="name"), |
| 216 | tags.input(type="submit")], |
| 217 | greet]) |
| 218 | |
| 219 | Note that ctx.arg returns only the first argument with the given name. For |
| 220 | complex cases where multiple arguments and lists of argument values are |
| 221 | required, you can access the request argument dictionary directly using the |
| 222 | syntax:: |
| 223 | |
| 224 | def arguments(ctx, data): |
| 225 | args = inevow.IRequest(ctx).args |
| 226 | return "Request arguments are: ", str(args) |
| 227 | |
| 228 | Generators in the DOM |
| 229 | --------------------- |
| 230 | |
| 231 | One common operation when building dynamic pages is iterating a list of data and |
| 232 | emitting some HTML for each item. Python generators are well suited for |
| 233 | expressing this sort of logic, and code which is written as a python generator |
| 234 | can perform tests (if) and loops of various kinds (while, for) and emit a row of |
| 235 | html whenever it has enough data to do so. Nevow can handle generators in the |
| 236 | DOM just as gracefully as it can handle anything else:: |
| 237 | |
| 238 | >>> from nevow import rend, loaders, tags |
| 239 | >>> def generate(ctx, items): |
| 240 | ... for item in items: |
| 241 | ... yield tags.div[ item ] |
| 242 | ... |
| 243 | >>> class List(rend.Page): |
| 244 | ... docFactory = loaders.stan(tags.html[ generate ]) |
| 245 | ... |
| 246 | >>> List(['one', 'two', 'three']).renderSynchronously() |
| 247 | '<html><div>one</div><div>two</div><div>three</div></html>' |
| 248 | |
| 249 | As you can see, generating HTML inside of functions or generators can be very |
| 250 | convenient, and can lead to very rapid application development. However, it is |
| 251 | also what I would call a "template abstraction violation", and we will learn how |
| 252 | we can keep knowledge of HTML out of our python code when we learn about |
| 253 | patterns and slots. |
| 254 | |
| 255 | Methods in the DOM |
| 256 | ------------------ |
| 257 | |
| 258 | Up until now, we have been placing our template manipulation logic inside of |
| 259 | simple Python functions and generators. However, it is often appropriate to use |
| 260 | a method instead of a function. Nevow makes it just as easy to use a method to |
| 261 | render HTML:: |
| 262 | |
| 263 | class MethodRender(rend.Page): |
| 264 | def __init__(self, foo): |
| 265 | self.foo = foo |
| 266 | |
| 267 | def render_foo(self, ctx, data): |
| 268 | return self.foo |
| 269 | |
| 270 | docFactory = loaders.stan(tags.html[ render_foo ]) |
| 271 | |
| 272 | Using render methods makes it possible to parameterize your Page class with more |
| 273 | parameters. With render methods, you can also use the Page instance as a state |
| 274 | machine to keep track of the state of the render. While Nevow is designed to |
| 275 | allow you to render the same Page instance repeatedly, it can also be convenient |
| 276 | to know that a Page instance will only be used one time, and that the Page |
| 277 | instance can be used as a scratch pad to manage information about the render. |
| 278 | |
| 279 | Data specials |
| 280 | ------------- |
| 281 | |
| 282 | Previously we saw how passing a parameter to the default Page constructor makes |
| 283 | it available as the "data" parameter to all of our render methods. This "data" |
| 284 | parameter can change as the page render proceeds, and is a useful way to ensure |
| 285 | that render functions are isolated and only act upon the data which is available |
| 286 | to them. Render functions which do not pull information from sources other than |
| 287 | the "data" parameter are more easily reusable and can be composed into larger |
| 288 | parts more easily. |
| 289 | |
| 290 | Deciding which data gets passed as the data parameter is as simple as changing |
| 291 | the "Data special" for a Tag. See the Glossary under "Tag Specials" for more |
| 292 | information about specials. Assigning to the data special is as simple as |
| 293 | assigning to a tag attribute:: |
| 294 | |
| 295 | >>> def hello(ctx, name): |
| 296 | ... return "Hello, ", name |
| 297 | ... |
| 298 | >>> class DataSpecial(rend.Page): |
| 299 | ... docFactory = loaders.stan(tags.html[ |
| 300 | ... tags.div(data="foo")[ hello ], |
| 301 | ... tags.div(data="bar")[ hello ]]) |
| 302 | ... |
| 303 | >>> DataSpecial().renderSynchronously() |
| 304 | '<html><div>Hello, foo</div><div>Hello, bar</div></html>' |
| 305 | |
| 306 | Data specials may be assigned any python value. Data specials are only in scope |
| 307 | during the rendering of the tag they are assigned to, so if the "hello" renderer |
| 308 | were placed in the DOM inside the html node directly, "Hello, None" would be |
| 309 | output. |
| 310 | |
| 311 | Before data is passed to a render function, Nevow first checks to see if there |
| 312 | is an IGettable adapter for it. If there is, it calls IGettable.get(), and |
| 313 | passes the result of this as the data parameter instead. Nevow includes an |
| 314 | IGettable adapter for python functions, which means you can set a Tag data |
| 315 | special to a function reference and Nevow will call it to obtain the data when |
| 316 | the Tag is rendered. The signature for data methods is similar to that of render |
| 317 | methods, (ctx, data). For example:: |
| 318 | |
| 319 | def getName(ctx, data): |
| 320 | return ctx.arg('name') |
| 321 | |
| 322 | def greet(ctx, name): |
| 323 | return "Greetings, ", name |
| 324 | |
| 325 | class GreetName(rend.Page): |
| 326 | docFactory = loaders.stan(tags.html[ |
| 327 | tags.form(action="")[ |
| 328 | tags.input(name="name"), |
| 329 | tags.input(type="submit")], |
| 330 | tags.div(data=getName)[ greet ]]) |
| 331 | |
| 332 | Data specials exist mainly to allow you to construct and enforce a |
| 333 | Model-View-Controller style separation of the Model code from the View. Here we |
| 334 | see that the greet function is capable of rendering a greeting view for a name |
| 335 | model, and that the implementation of getName may change without the view code |
| 336 | changing. |
| 337 | |
| 338 | Render specials |
| 339 | --------------- |
| 340 | |
| 341 | Previously, we have seen how render functions can be placed directly in the DOM, |
| 342 | and the return value replaces the render function in the DOM. However, these |
| 343 | free functions and methods are devoid of any contextual information about the |
| 344 | template they are living in. The render special is a way to associate a render |
| 345 | function or method with a particular Tag instance, which the render function can |
| 346 | then examine to decide how to render:: |
| 347 | |
| 348 | >>> def alignment(ctx, data): |
| 349 | ... align = ctx.tag.attributes.get('align') |
| 350 | ... if align == 'right': |
| 351 | ... return ctx.tag["Aligned right"] |
| 352 | ... elif align == 'center': |
| 353 | ... return ctx.tag["Aligned center"] |
| 354 | ... else: |
| 355 | ... return ctx.tag["Aligned left"] |
| 356 | ... |
| 357 | >>> class AlignmentPage(rend.Page): |
| 358 | ... docFactory = loaders.stan(tags.html[ |
| 359 | ... tags.p(render=alignment), |
| 360 | ... tags.p(render=alignment, align="center"), |
| 361 | ... tags.p(render=alignment, align="right")]) |
| 362 | ... |
| 363 | >>> AlignmentPage().renderSynchronously() |
| 364 | '<html><p>Aligned left</p><p align="center">Aligned center</p><p align="right">Aligned right</p></html>' |
| 365 | |
| 366 | Note how the alignment renderer has access to the template node as "ctx.tag". It |
| 367 | can examine and change this node, and the return value of the render function |
| 368 | replaces the original node in the DOM. Note that here we are returning the |
| 369 | template node after changing it. We will see later how we can instead mutate the |
| 370 | context and use slots so that the knowledge the renderer requires about the |
| 371 | structure of the template is reduced even more. |
| 372 | |
| 373 | Pattern specials |
| 374 | ---------------- |
| 375 | |
| 376 | When writing render methods, it is easy to inline the construction of Tag |
| 377 | instances to generate HTML programatically. However, this creates a template |
| 378 | abstraction violation, where part of the HTML which will show up in the final |
| 379 | page output is hidden away inside of render methods instead of inside the |
| 380 | template. Pattern specials are designed to avoid this problem. A node which has |
| 381 | been tagged with a pattern special can then be located and copied by a render |
| 382 | method. The render method does not need to know anything about the structure or |
| 383 | location of the pattern, only it's name. |
| 384 | |
| 385 | We can rewrite our previous generator example so that the generator does not |
| 386 | have to know what type of tag the template designer would like repeated for each |
| 387 | item in the list:: |
| 388 | |
| 389 | >>> from nevow import rend, loaders, tags, inevow |
| 390 | >>> def generate(ctx, items): |
| 391 | ... pat = inevow.IQ(ctx).patternGenerator('item') |
| 392 | ... for item in items: |
| 393 | ... ctx.tag[ pat(data=item) ] |
| 394 | ... return ctx.tag |
| 395 | ... |
| 396 | >>> def string(ctx, item): |
| 397 | ... return ctx.tag[ str(item) ] |
| 398 | ... |
| 399 | >>> class List(rend.Page): |
| 400 | ... docFactory = loaders.stan(tags.html[ |
| 401 | ... tags.ul(render=generate)[ |
| 402 | ... tags.li(pattern="item", render=string)]]) |
| 403 | ... |
| 404 | >>> List([1, 2, 3]).renderSynchronously() |
| 405 | '<html><ol><li>1</li><li>2</li><li>3</li></ol></html>' |
| 406 | |
| 407 | Note that we have to mutate the tag in place and repeatedly copy the item |
| 408 | pattern, applying the item as the data special to the resulting Tag. It turns |
| 409 | out that this is such a common operation that nevow comes out of the box with |
| 410 | these two render functions:: |
| 411 | |
| 412 | >>> class List(rend.Page): |
| 413 | ... docFactory = loaders.stan(tags.html[ |
| 414 | ... tags.ul(render=rend.sequence)[ |
| 415 | ... tags.li(pattern="item", render=rend.data)]]) |
| 416 | ... |
| 417 | >>> List([1, 2, 3]).renderSynchronously() |
| 418 | '<html><ul><li>1</li><li>2</li><li>3</li></ul></html>' |
| 419 | |
| 420 | Slot specials |
| 421 | ------------- |
| 422 | |
| 423 | The problem with render methods is that they are only capable of making changes |
| 424 | to their direct children. Because of the architecture of Nevow, they should not |
| 425 | attempt to change grandchildren or parent nodes. It is possible to write one |
| 426 | render method for every node you wish to change, but there is a better way. A |
| 427 | node with a slot special can be "filled" with content by any renderer above the |
| 428 | slot. Creating a slot special is such a frequent task that there is a prototype |
| 429 | in nevow.tags which is usually used. |
| 430 | |
| 431 | Let us examine a renderer which fills a template with information about a |
| 432 | person: |
| 433 | |
| 434 | >>> from nevow import loaders, rend, tags |
| 435 | ... |
| 436 | >>> person = ('Donovan', 'Preston', 'Male', 'California') |
| 437 | ... |
| 438 | >>> def render_person(ctx, person): |
| 439 | ... firstName, lastName, sex, location = person |
| 440 | ... ctx.fillSlots('firstName', firstName) |
| 441 | ... ctx.fillSlots('lastName', lastName) |
| 442 | ... ctx.fillSlots('sex', sex) |
| 443 | ... ctx.fillSlots('location', location) |
| 444 | ... return ctx.tag |
| 445 | ... |
| 446 | >>> class PersonPage(rend.Page): |
| 447 | ... docFactory = loaders.stan(tags.html(render=render_person)[ |
| 448 | ... tags.table[ |
| 449 | ... tags.tr[ |
| 450 | ... tags.td[tags.slot('firstName')], |
| 451 | ... tags.td[tags.slot('lastName')], |
| 452 | ... tags.td[tags.slot('sex')], |
| 453 | ... tags.td[tags.slot('location')]]]]) |
| 454 | ... |
| 455 | >>> PersonPage(person).renderSynchronously() |
| 456 | '<html><table><tr><td>Donovan</td><td>Preston</td><td>Male</td><td>California</td></tr></table></html>' |
| 457 | |
| 458 | Using patterns in combination with slots can lead to very powerful template |
| 459 | abstraction. Nevow also includes another standard renderer called "mapping" |
| 460 | which takes any data which responds to the "items()" message and inserts the |
| 461 | items into appropriate slots:: |
| 462 | |
| 463 | >>> class DictPage(rend.Page): |
| 464 | ... docFactory = loaders.stan(tags.html(render=rend.mapping)[ |
| 465 | ... tags.span[ tags.slot('foo') ], tags.span[ tags.slot('bar') ]]) |
| 466 | ... |
| 467 | >>> DictPage(dict(foo=1, bar=2)).renderSynchronously() |
| 468 | '<html><span>1</span><span>2</span></html>' |
| 469 | |
| 470 | Data directives |
| 471 | --------------- |
| 472 | |
| 473 | So far, we have always placed data functions directly in the Data special |
| 474 | attribute of a Tag. Sometimes, it is preferable to look up a data method from |
| 475 | the Page class as the Page has being rendered. For example, a base class may |
| 476 | define a template and a subclass may provide the implementation of the data |
| 477 | method. We can accomplish this effect by using a data directive as a Tag's data |
| 478 | special:: |
| 479 | |
| 480 | class Base(rend.Page): |
| 481 | docFactory = loaders.stan(tags.html[ |
| 482 | tags.div(data=tags.directive('name'), render=rend.data)]) |
| 483 | |
| 484 | class Subclass(Base): |
| 485 | def data_name(self, ctx, data): |
| 486 | return "Your name" |
| 487 | |
| 488 | The data directive is resolved by searching for the IContainer implementation in |
| 489 | the context. rend.Page implements IContainer.get by performing an attribute |
| 490 | lookup on the Page with the prefix 'data_*'. You can provide your own IContainer |
| 491 | implementation if you wish, and also you should know that IContainer |
| 492 | implementations for list and dict are included in the nevow.accessors module. |
| 493 | |
| 494 | A common gotcha is that the closest IContainer is used to resolve data |
| 495 | directives. This means that if a list is being used as the data during the |
| 496 | rendering process, data directives below this will be resolved against the |
| 497 | IContainer implementation in nevow.accessors.ListAccessor. If you are expecting |
| 498 | a data directive to invoke a Page's data_* method but instead get a KeyError, |
| 499 | this is why. |
| 500 | |
| 501 | Render directives |
| 502 | ----------------- |
| 503 | |
| 504 | Render directives are almost exactly the same, except they are resolved using |
| 505 | the closest IRendererFactory implementation in the context. Render directives |
| 506 | can be used to allow subclasses to override certain render methods, and also can |
| 507 | be used to allow Fragments to locate their own prefixed render methods. |
| 508 | |
| 509 | Flatteners |
| 510 | ---------- |
| 511 | |
| 512 | TODO This section isn't done yet. |
| 513 | |
| 514 | Nevow's flatteners use a type/function registry to determine how to render |
| 515 | objects which Nevow encounters in the DOM during the rendering process. |
| 516 | "Explicit is better than implicit", so in most cases, explicitly applying render |
| 517 | methods to data will be better than registering a flattener, but in some cases |
| 518 | it can be useful:: |
| 519 | |
| 520 | class Person(object): |
| 521 | def __init__(self, firstName, lastName): |
| 522 | self.firstName = firstName |
| 523 | self.lastName = lastName |
| 524 | |
| 525 | def flattenPerson(person, ctx): |
| 526 | return flat.partialflatten( |
| 527 | ctx, |
| 528 | (person.firstName, " ", person.lastName)) |
| 529 | |
| 530 | from nevow import flat |
| 531 | flat.registerFlattener(flattenPerson, Person) |
| 532 | |
| 533 | def insertData(ctx, data): |
| 534 | return data |
| 535 | |
| 536 | class PersonPage(rend.Page): |
| 537 | docFactory = loaders.stan(tags.html[ insertData ]) |
| 538 |
