# (c) 2005 Ben Bangert # This module is part of the Python Paste Project and is released under # the MIT License: http://www.opensource.org/licenses/mit-license.php """Registry for handling request-local module globals sanely Dealing with module globals in a thread-safe way is good if your application is the sole responder in a thread, however that approach fails to properly account for various scenarios that occur with WSGI applications and middleware. What is actually needed in the case where a module global is desired that is always set properly depending on the current request, is a stacked thread-local object. Such an object is popped or pushed during the request cycle so that it properly represents the object that should be active for the current request. To make it easy to deal with such variables, this module provides a special StackedObjectProxy class which you can instantiate and attach to your module where you'd like others to access it. The object you'd like this to actually "be" during the request is then registered with the RegistryManager middleware, which ensures that for the scope of the current WSGI application everything will work properly. Example: .. code-block:: Python #yourpackage/__init__.py from paste.registry import RegistryManager, StackedObjectProxy myglobal = StackedObjectProxy() #wsgi app stack app = RegistryManager(yourapp) #inside your wsgi app class yourapp(object): def __call__(self, environ, start_response): obj = someobject # The request-local object you want to access # via yourpackage.myglobal if environ.has_key('paste.registry'): environ['paste.registry'].register(myglobal, obj) You will then be able to import yourpackage anywhere in your WSGI app or in the calling stack below it and be assured that it is using the object you registered with Registry. RegisterManager can be in the WSGI stack multiple times, each time it appears it registers a new request context. Performance =========== The overhead of the proxy object is very minimal, however if you are using proxy objects extensively (Thousands of accesses per request or more), there are some ways to avoid them. A proxy object runs approximately 3-20x slower than direct access to the object, this is rarely your performance bottleneck when developing web applications. Should you be developing a system which may be accessing the proxy object thousands of times per request, the performance of the proxy will start to become more noticeabe. In that circumstance, the problem can be avoided by getting at the actual object via the proxy with the ``_curent_obj`` function: .. code-block:: Python #sessions.py Session = StackedObjectProxy() # ... initialization code, etc. # somemodule.py import sessions def somefunc(): session = sessions.Session._current_obj() # ... tons of session access This way the proxy is used only once to retrieve the object for the current context and the overhead is minimized while still making it easy to access the underlying object. The ``_current_obj`` function is preceded by an underscore to more likely avoid clashing with the contained object's attributes. **NOTE:** This is *highly* unlikely to be an issue in the vast majority of cases, and requires incredibly large amounts of proxy object access before one should consider the proxy object to be causing slow-downs. This section is provided solely in the extremely rare case that it is an issue so that a quick way to work around it is documented. """ import warnings import paste.util.threadinglocal as threadinglocal from paste import wsgilib __all__ = ['StackedObjectProxy', 'RegistryManager'] class StackedObjectProxy(object): """Track an object instance internally using a stack The StackedObjectProxy proxies access to an object internally using a stacked thread-local. This makes it safe for complex WSGI environments where access to the object may be desired in multiple places without having to pass the actual object around. New objects are added to the top of the stack with push_object while objects can be removed with pop_object. """ def __init__(self, default=None, name="Default"): """Create a new StackedObjectProxy If a default is given, its used in every thread if no other object has been pushed on. """ self.__dict__['_name'] = name self.__dict__['local'] = threadinglocal.local() if default: self.__dict__['_default_object'] = default def __getattr__(self, attr): return getattr(self._current_obj(), attr) def __setattr__(self, attr, value): setattr(self._current_obj(), attr, value) def __delattr__(self, name): self._current_obj().__delattr__(name) def __getitem__(self, key): return self._current_obj()[key] def __setitem__(self, key, value): self._current_obj()[key] = value def __delitem__(self, key): self._current_obj().__delitem__(key) def __repr__(self): try: return self._current_obj().__repr__() except TypeError: return '<%s.%s object at 0x%08x>' % (__name__, self.__class__.__name__, id(self)) def __iter__(self): """Only works for proxying to a dict""" return iter(self._current_obj().keys()) def __len__(self): return len(self._current_obj()) def __contains__(self, key): return self._current_obj().has_key(key) def current_obj(self): """ Deprecated (Aug 15 2006); moved to _current_obj. """ warnings.warn('StackedObjectProxy.current_obj has been moved to ' 'StackedObjectProxy._current_obj', DeprecationWarning, 2) return self._current_obj() def _current_obj(self): """Returns the current active object being proxied to In the event that no object was pushed, the default object if provided will be used. Otherwise, a TypeError will be raised. """ objects = getattr(self.__dict__['local'], 'objects', None) if objects: return objects[-1] else: object = self.__dict__.get('_default_object') if object: return object else: raise TypeError( 'No object (name: %s) has been registered for this ' 'thread' % self.__dict__['_name']) def push_object(self, obj): """ Deprecated (Aug 15 2006); moved to _push_object. """ warnings.warn('StackedObjectProxy.push_object has been moved to ' 'StackedObjectProxy._push_object', DeprecationWarning, 2) self._push_object(obj) def _push_object(self, obj): """Make ``obj`` the active object for this thread-local. This should be used like: .. code-block:: Python obj = yourobject() module.glob = StackedObjectProxy() module.glob._push_object(obj) try: ... do stuff ... finally: module.glob._pop_object(conf) """ if not hasattr(self.local, 'objects'): self.local.objects = [] self.local.objects.append(obj) def pop_object(self, obj=None): """ Deprecated (Aug 15 2006); moved to _pop_object. """ warnings.warn('StackedObjectProxy.pop_object has been moved to ' 'StackedObjectProxy._pop_object', DeprecationWarning, 2) self._pop_object(obj) def _pop_object(self, obj=None): """Remove a thread-local object. If ``obj`` is given, it is checked against the popped object and an error is emitted if they don't match. """ if not hasattr(self.local, 'objects'): raise AssertionError('No object has been registered for this thread') popped = self.local.objects.pop() if obj: if popped is not obj: raise AssertionError( 'The object popped (%s) is not the same as the object ' 'expected (%s)' % (popped, obj)) class Registry(object): """Track objects and stacked object proxies for removal The Registry object is instantiated a single time for the rquest no matter how many times the RegistryManager is used in a WSGI stack. Each RegistryManager must call ``prepare`` before continuing the call to start a new context for object registering. Each context is tracked with a dict inside a list. The last list element is the currently executing context. Each context dict is keyed by the id of the StackedObjectProxy instance being proxied, the value is a tuple of the StackedObjectProxy instance and the object being tracked. """ def __init__(self): """Create a new Registry object ``prepare`` must still be called before this Registry object can be used to register objects. """ self.reglist = [] def prepare(self): """Used to create a new registry context Anytime a new RegistryManager is called, ``prepare`` needs to be called on the existing Registry object. This sets up a new context for registering objects. """ self.reglist.append({}) def register(self, stacked, obj): """Register an object with a StackedObjectProxy""" stacked._push_object(obj) myreglist = self.reglist[-1] myreglist[id(stacked)] = (stacked, obj) def cleanup(self): """Remove all objects from all StackedObjectProxy instances that were tracked at this Registry context""" for id, val in self.reglist[-1].iteritems(): stacked, obj = val stacked._pop_object(obj) self.reglist.pop() class RegistryManager(object): """Creates and maintains a Registry context RegistryManager creates a new registry context for the registration of StackedObjectProxy instances. Multiple RegistryManager's can be in a WSGI stack and will manage the context so that the StackedObjectProxies always proxy to the proper object. The object being registered can be any object sub-class, list, or dict. Registering objects is done inside a WSGI application under the RegistryManager instance, using the ``environ['paste.registry']`` object which is a Registry instance. """ def __init__(self, application): self.application = application def __call__(self, environ, start_response): app_iter = None reg = environ.setdefault('paste.registry', Registry()) reg.prepare() try: app_iter = self.application(environ, start_response) finally: # Regardless of if the content is an iterable, generator, list # or tuple, we clean-up right now. If its an iterable/generator # care should be used to ensure the generator has its own ref # to the actual object reg.cleanup() return app_iter