Object traversal is the process Nevow uses to determine what object to use to render HTML for a particular URL. When an HTTP request comes in to the web server, the object publisher splits the URL into segments, and repeatedly calls methods which consume path segments and return objects which represent that path, until all segments have been consumed. At the core, the Nevow traversal API is very simple. However, it provides some higher level functionality layered on top of this to satisfy common use cases.
Object Traversal Basics
      The root resource is the top-level object in the URL
      space; it conceptually represents the URI /. The Nevow
      object traversal and object publishing
      machinery uses only two methods to locate an object suitable for
      publishing and to generate the HTML from it; these methods are described
      in the interface nevow.inevow.IResource:
    
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class IResource(Interface): def locateChild(self, ctx, segments): """Locate another object which can be adapted to IResource Return a tuple of resource, path segments """ def renderHTTP(self, ctx): """Render a request """
      renderHTTP can be
      as simple as a method which simply returns a string of HTML.  Let's
      examine what happens when object traversal occurs over a very simple root
      resource:
    
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from zope.interface import implements class SimpleRoot(object): implements(inevow.IResource) def locateChild(self, ctx, segments): return self, () def renderHTTP(self, ctx): return "Hello, world!"
      This resource, when passed as the root resource to appserver.NevowSite or wsgi.createWSGIApplication, will immediately return
      itself, consuming all path segments. This means that for every URI a user
      visits on a web server which is serving this root resource, the text
      "Hello, world!" will be rendered. Let's examine the value of
      segments for various values of URI:
    
- /-- ('',)
- /foo/bar-- ('foo', 'bar')
- 
        /foo/bar/baz.html-('foo', 'bar', 'baz.html')
- 
        /foo/bar/directory/-('foo', 'bar', 'directory', '')
      So we see that Nevow does nothing more than split the URI on the string
      / and pass these path segments to our application for
      consumption. Armed with these two methods alone, we already have enough
      information to write applications which service any form of URL
      imaginable in any way we wish. However, there are some common URL
      handling patterns which Nevow provides higher level support for.
    
locateChild In Depth
    One common URL handling pattern involves parents which only know about their direct children. For example, a ``Directory`` object may only know about the contents of a single directory, but if it contains other directories, it does not know about the contents of them. Let's examine a simple ``Directory`` object which can provide directory listings and serves up objects for child directories and files:
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from zope.interface import implements class Directory(object): implements(inevow.IResource) def __init__(self, directory): self.directory = directory def renderHTTP(self, ctx): html = ['<ul>'] for child in os.listdir(self.directory): fullpath = os.path.join(self.directory, child) if os.path.isdir(fullpath): child += '/' html.extend(['<li><a href="', child, '">', child, '</a></li>']) html.append('</ul>') return ''.join(html) def locateChild(self, ctx, segments): name = segments[0] fullpath = os.path.join(self.directory, name) if not os.path.exists(fullpath): return None, () # 404 if os.path.isdir(fullpath): return Directory(fullpath), segments[1:] if os.path.isfile(fullpath): return static.File(fullpath), segments[1:]
      Because this implementation of locateChild only consumed one
      segment and returned the rest of them (segments[1:]), the
      object traversal process will continue by calling
      locateChild on the returned resource and passing the
      partially-consumed segments. In this way, a directory structure of any
      depth can be traversed, and directory listings or file contents can be
      rendered for any existing directories and files.
    
      So, let us examine what happens when the URI
      "/foo/bar/baz.html" is traversed, where "foo"
      and "bar" are directories, and "baz.html" is a
      file.
    
- 
        Directory('/').locateChild(ctx, ('foo', 'bar', 'baz.html'))returnsDirectory('/foo'), ('bar', 'baz.html')
- 
        Directory('/foo').locateChild(ctx, ('bar', 'baz.html'))returnsDirectory('/foo/bar'), ('baz.html, )
- 
        Directory('/foo/bar').locateChild(ctx, ('baz.html'))returnsFile('/foo/bar/baz.html'), ()
- 
        No more segments to be consumed;
        File('/foo/bar/baz.html').renderHTTP(ctx)is called, and the result is sent to the browser.
childFactory Method
    
      Consuming one URI segment at a time by checking to see if a requested
      resource exists and returning a new object is a very common
      pattern. Nevow's default implementation of IResource, nevow.rend.Page, contains an implementation of
      locateChild which provides more convenient hooks for
      implementing object traversal. One of these hooks is
      childFactory. Let us imagine for the sake of example that we
      wished to render a tree of dictionaries. Our data structure might look
      something like this:
    
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tree = dict( one=dict( foo=None, bar=None), two=dict( baz=dict( quux=None)))
Given this data structure, the valid URIs would be:
- /
- /one
- /one/foo
- /one/bar
- /two
- /two/baz
- /two/baz/quux
      Let us construct a rend.Page
      subclass which uses the default locateChild implementation
      and overrides the childFactory hook instead:
    
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class DictTree(rend.Page): def __init__(self, dataDict): self.dataDict = dataDict def renderHTTP(self, ctx): if self.dataDict is None: return "Leaf" html = ['<ul>'] for key in self.dataDict.keys(): html.extend(['<li><a href="', key, '">', key, '</a></li>']) html.append('</ul>') return ''.join(html) def childFactory(self, ctx, name): if name not in self.dataDict: return rend.NotFound # 404 return DictTree(self.dataDict[name])
      As you can see, the childFactory implementation is
      considerably shorter than the equivalent locateChild
      implementation would have been.
    
child_* methods and attributes
    
      Often we may wish to have some hardcoded URLs which are not dynamically
      generated based on some data structure. For example, we might have an
      application which uses an external CSS stylesheet, an external JavaScript
      file, and a folder full of images. The rend.Page.locateChild implementation provides a
      convenient way for us to express these relationships by using
      child-prefixed methods:
    
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class Linker(rend.Page): def renderHTTP(self, ctx): return """<html> <head> <link href="css" rel="stylesheet" /> <script type="text/javascript" src="scripts" /> <body> <img src="images/logo.png" /> </body> </html>""" def child_css(self, ctx): return static.File('styles.css') def child_scripts(self, ctx): return static.File('scripts.js') def child_images(self, ctx): return static.File('images/')
      One thing you may have noticed is that all of the examples so far have
      returned new object instances whenever they were implementing a traversal
      API. However, there is no reason these instances cannot be shared. One
      could for example return a global resource instance, an instance which
      was previously inserted in a dict, or lazily create and cache dynamic
      resource instances on the fly. The rend.Page.locateChild
      implementation also provides a convenient way to express that one global
      resource instance should always be used for a particular URL, the
      child-prefixed attribute:
    
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class FasterLinker(Linker): child_css = static.File('styles.css') child_scripts = static.File('scripts.js') child_images = static.File('images/')
Dots in child names
      When a URL contains dots, which is quite common in normal URLs, it is
      simple enough to handle these URL segments in locateChild or
      childFactory -- one of the passed segments will simply be a
      string containing a dot. However, it is not immediately obvious how one
      would express a URL segment with a dot in it when using child-prefixed
      methods. The solution is really quite simple:
    
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class DotChildren(rend.Page): def renderHTTP(self, ctx): return """ <html> <head> <script type="text/javascript" src="scripts.js" /> </head> </html>""" setattr(DotChildren, 'child_scripts.js', static.File('scripts.js'))
The same technique could be used to install a child method with a dot in the name.
children dictionary
      The final hook supported by the default implementation of
      locateChild is the rend.Page.children
      dictionary:
    
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class Main(rend.Page): children = { 'people': People(), 'jobs': Jobs(), 'events': Events()} def renderHTTP(self, ctx): return """ <html> <head> <title>Our Site</title> </head> <body> <p>bla bla bla</p> </body> </html>"""
Hooks are checked in the following order:
- self.children
- self.child_*
- self.childFactory
The default trailing slash handler
      When a URI which is being handled ends in a slash, such as when the
      / URI is being rendered or when a directory-like URI is
      being rendered, the string '' appears in the path segments
      which will be traversed. Again, handling this case is trivial inside
      either locateChild or childFactory, but it may
      not be immediately obvious what child-prefixed method or attribute will
      be looked up.  The method or attribute name which will be used is simply
      child with a single trailing underscore.
    
      The rend.Page class provides an implementation of this
      method which can work in two different ways. If the attribute
      addSlash is True, the default trailing slash
      handler will return self. In the case when
      addSlash is True, the default
      rend.Page.renderHTTP implementation will simply perform a
      redirect which adds the missing slash to the URL.
    
      The default trailing slash handler also returns self if
      addSlash is False, but emits a warning as it
      does so. This warning may become an exception at some point in the
      future.
    
ICurrentSegments and IRemainingSegments
    
      During the object traversal process, it may be useful to discover which
      segments have already been handled and which segments are remaining to be
      handled. This information may be obtained from the context
      object which is passed to all the traversal APIs. The interfaces nevow.inevow.ICurrentSegments and nevow.inevow.IRemainingSegments are used to retrieve
      this information. To retrieve a tuple of segments which have previously
      been consumed during object traversal, use this syntax:
    
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segs = ICurrentSegments(ctx)
      The same is true of IRemainingSegments.
      IRemainingSegments is the same value which is passed as
      segments to locateChild, but may also be useful
      in the implementations of childFactory or a child-prefixed
      method, where this information would not otherwise be available.
    
Conclusion
      Nevow makes it easy to handle complex URL hierarchies. The most basic
      object traversal interface, nevow.inevow.IResource.locateChild, provides powerful
      and flexible control over the entire object traversal process. Nevow's
      canonical IResource implementation, rend.Page,
      also includes the convenience hooks childFactory along with
      child-prefixed method and attribute semantics to simplify common use
      cases.