Quixote学习笔记（4）

The _q_lookup() method

Now take a look at the ExtraDirectory class in extras.ptl. This class exhibits some more advanced publishing features. If you look back at the default _q_traverse() implementation (in directory.py), you will see that the _q_traverse does not give up if _q_translate() returns None, indicating that the path component has no designated corresponding attribute name. In this case, _q_traverse() tries calling self._q_lookup() to see if the object of interest can be found in a different way. Note that _q_lookup() takes the component as an argument and must return either (if there is more path to traverse) a Directory instance, or else (if the component is the last in the path) a callable or a string.

In this particular case, the ExtrasDirectory._q_lookup() call returns an instance of IntegerUI (a subclass of Directory). The interest here, unlike the ExtrasDirectory() instance itself, is created on-the-fly during the traversal, especially for this particular component. Try loading http://localhost:8080/extras/12/ to see how this behaves.

Note that the correct URL to get to the IntegerUI(12)._q_index() call ends with a '/'. This can sometimes be confusing to people who expect http://localhost:8080/extras/12 to yield the same page as http://localhost:8080/extras/12/. If given the path ['extras', '12'], the default _q_traverse() ends up calling the instance of IntegerUI. The Directory.__call__() (see directory.py) determines the result: if no form values were submitted and adding a slash would produce a page, the call returns the result of calling quixote.redirect(). The redirect() call here causes the server to issue a permanent redirect response to the path with the slash added. When this automatic redirect is used, a message is printed to the error log. If the conditions for a redirect are not met, the call falls back to raising a TraversalError. [Note, if you don't like this redirect behavior, override, replace, or delete Directory.__call__]

The _q_lookup() pattern is useful when you want to allow URL components that you either don't know or don't want to list in _q_exports ahead of time.

_q_lookup()是ExtraDirectory类的一个方法，代码如下：

     def  _q_lookup(self, component):
         return  IntegerUI(component)

IntegerUI是一个类，在integers.ptl文件中。它接受一个可以转化为整数的路径组件做参数：

     def   __init__ (self, component):
         try :
            self.n  =  int(component)
         except  ValueError, exc:
             raise  TraversalError(str(exc))

计算阶乘的方法：

     def  factorial(self):
         if  self.n  >   10000 :
            sys.stderr.write( " warning: possible denial-of-service attack  "
                              " (request for factorial(%d)) "   %  self.n)
        get_response().set_content_type( " text/plain " )
         return   " %d! = %d "   %  (self.n, fact(self.n))

上一个数字/下一个数字的方法：

     def  prev(self):
         return  redirect( " ../%d/ "   %  (self.n - 1 ))

     def  next(self):
         return  redirect( " ../%d/ "   %  (self.n + 1 ))

_q_exports的内容：

_q_exports  =  [ "" ,  " factorial " ,  " prev " ,  " next " ]

_q_lookup()模板在你想允许事先并不知道或者不想在_q_exports中列出的路径组件时很有用。

注意点：访问时URL末尾的'/'不能少。