Nevow render用法（转）

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Nevow是一种使用Python生成动态HTML的框架。它通过stan DOM来构建页面，利用Tag实例生成HTML标签，并支持将Python函数放入DOM中实现动态内容生成。此外，Nevow还提供了多种特性如数据特殊化、渲染特殊化等。

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