MetaClassAutoURLS.md

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default	MetaClassAutoURLS

MetaClassAutoURLS

web.py maps URLs regexes to classes via a list like so:

class Default(object):
    ''' Action for / '''        def GET(self):
        pass

class Login(object):
    ''' Action for /login '''        def GET(self):
        pass

    def POST(self):
        pass

urls = ["/", "Default", "/login", "Login"]

One of the disadvantages of this approach is that for large applications, the urls list can get very long and tedious to maintain. Things get worse during long refactorings/reorgnanizations because it's easy to typo a class or module name or just forget to update the urls list. After spending a couple hours debugging just this problem and then smacking myself on the forehead for missing such a simple mistake, I thought to myself, There's got to be a way to automate this! After some research, I found that metaclasses can do exactly what I want:

urls = [ ]

class ActionMetaClass(type):
    def __init__(klass, name, bases, attrs):
        urls.append(attrs["url"])
        urls.append("%s.%s" % (klass.__module__, name))

class Default(object):
    __metaclass__ = ActionMetaClass
    url = "/"
    def GET(self):
        pass

class Login(object):
    __metaclass__ = ActionMetaClass
    url = "/login"
    def GET(self):
        pass

    def POST(self):
        pass

Of course, this is a simple example that leaves a lot to be desired (handling attrs lacking a url key, for instance), but it should get you on your way.

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AaronSw writes: That's a clever idea. One improvement might be to have the classes inherit from a class with the metaclass set, so you don't have that unsightly __metaclass__ every time.

xunil writes: Yeah, I went w/ this approach in the end since I found that I wanted each action class to inherit some common functionality anyway. You can view the source for my incomplete site which uses a metaclass and decorators.

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May be it's better to do it with decorators?

xunil writes: I originally tried to use decorators to do this since I was already familiar w/ them, but unless I'm missing something, decorators don't work on classes eg.

@action("/")
class Default:
    def GET(self):
        pass

That won't work. The PEP for decorators describes it, but apparently it was never incorporated into the language (perhaps b/c metaclasses are available and do the same thing).

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I couldn't figure out how to get this to work with URL classes in separate files, so I did something a little different, based on this Python Cookbook recipe:

import os, web

urls = [ ]

for aaa in os.listdir(os.getcwd()):
    module_name, ext = os.path.splitext(aaa)
    if module_name.startswith('cgi_') and ext == '.py':
        module = __import__(module_name)
        urls.append(module.url[0])
        urls.append(module_name + "." + module.url[1])

if __name__ == "__main__":
    web.run(urls)

And then e.g. cgi_hello.py (in the same directory) would be:

import web
url = ('/(.*)', 'hello')

class hello:
    def GET(self, name):
        i = web.input(times=1)
        if not name: name = 'world'            for c in xrange(int(i.times)): print 'Hello,', name+'!'

Is this a terrible way to do it?

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xunil writes: This is not altogether a bad idea, but it again decouples the URL information from the class, making it a module-level tuple or list. You could combine your importing logic w/ my metaclass idea, actually, and achieve the same thing as me eg.

import os, web

for aaa in os.listdir(os.getcwd()):
    module_name, ext = os.path.splitext(aaa)
    if module_name.startswith('cgi_') and ext == '.py':
        module = __import__(module_name)

if __name__ == "__main__":
    import metaclass
    web.run(metaclass.urls)

metaclass.py:

urls = [ ]

class ActionMetaClass(type):
    def __init__(klass, name, bases, attrs):
        urls.append(attrs["url"])
        urls.append("%s.%s" % (klass.__module__, name))

cgi_hello.py:

import web
from metaclass import ActionMetaClass

class hello:
    __metaclass__ = ActionMetaClass
    url = '/(.*)'
    def GET(self, name):
        i = web.input(times=1)
        if not name: name = 'world'            for c in xrange(int(i.times)): print 'Hello,', name+'!'

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