d6d40b0e44
Some files had noqa on the import _ line. This isn't necessary so remove it. Change-Id: Ic9784f2fda7fbe2595b3a85535bf815ecd10dfff
283 lines
10 KiB
Python
283 lines
10 KiB
Python
# Copyright 2012 OpenStack Foundation
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may
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# not use this file except in compliance with the License. You may obtain
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# a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations
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# under the License.
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"""This module provides support for dependency injection.
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Providers are registered via the 'provider' decorator, and dependencies on them
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are registered with 'requires' or 'optional'. Providers are available to their
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consumers via an attribute. See the documentation for the individual functions
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for more detail.
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See also:
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https://en.wikipedia.org/wiki/Dependency_injection
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"""
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import six
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from keystone import notifications
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from keystone.openstack.common.gettextutils import _
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REGISTRY = {}
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_future_dependencies = {}
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_future_optionals = {}
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_factories = {}
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class UnresolvableDependencyException(Exception):
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"""An UnresolvableDependencyException is raised when a required dependency
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is not resolvable; see 'resolve_future_dependencies'.
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"""
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def __init__(self, name):
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msg = 'Unregistered dependency: %s' % name
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super(UnresolvableDependencyException, self).__init__(msg)
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def provider(name):
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"""'provider' is a class decorator used to register providers.
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When 'provider' is used to decorate a class, members of that class will
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register themselves as providers for the named dependency. As an example,
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In the code fragment::
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@dependency.provider('foo_api')
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class Foo:
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def __init__(self):
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...
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...
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foo = Foo()
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The object 'foo' will be registered as a provider for 'foo_api'. No more
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than one such instance should be created; additional instances will replace
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the previous ones, possibly resulting in different instances being used by
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different consumers.
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"""
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def wrapper(cls):
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def wrapped(init):
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def register_event_callbacks(self):
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# NOTE(morganfainberg): A provider who has an implicit
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# dependency on other providers may utilize the event callback
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# mechanism to react to any changes in those providers. This is
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# performed at the .provider() mechanism so that we can ensure
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# that the callback is only ever called once and guaranteed
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# to be on the properly configured and instantiated backend.
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if not hasattr(self, 'event_callbacks'):
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return
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if not isinstance(self.event_callbacks, dict):
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msg = _('event_callbacks must be a dict')
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raise ValueError(msg)
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for event in self.event_callbacks:
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if not isinstance(self.event_callbacks[event], dict):
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msg = _('event_callbacks[%s] must be a dict') % event
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raise ValueError(msg)
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for resource_type in self.event_callbacks[event]:
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# Make sure we register the provider for each event it
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# cares to call back.
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callbacks = self.event_callbacks[event][resource_type]
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if not callbacks:
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continue
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if not hasattr(callbacks, '__iter__'):
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# ensure the callback information is a list
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# allowing multiple callbacks to exist
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callbacks = [callbacks]
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notifications.register_event_callback(event,
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resource_type,
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callbacks)
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def __wrapped_init__(self, *args, **kwargs):
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"""Initialize the wrapped object and add it to the registry."""
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init(self, *args, **kwargs)
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REGISTRY[name] = self
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register_event_callbacks(self)
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resolve_future_dependencies(name)
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return __wrapped_init__
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cls.__init__ = wrapped(cls.__init__)
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_factories[name] = cls
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return cls
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return wrapper
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def _process_dependencies(obj):
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# Any dependencies that can be resolved immediately are resolved.
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# Dependencies that cannot be resolved immediately are stored for
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# resolution in resolve_future_dependencies.
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def process(obj, attr_name, unresolved_in_out):
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for dependency in getattr(obj, attr_name, []):
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if dependency not in REGISTRY:
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# We don't know about this dependency, so save it for later.
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unresolved_in_out.setdefault(dependency, []).append(obj)
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continue
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setattr(obj, dependency, REGISTRY[dependency])
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process(obj, '_dependencies', _future_dependencies)
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process(obj, '_optionals', _future_optionals)
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def requires(*dependencies):
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"""'requires' is a class decorator used to inject providers into consumers.
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The required providers will be made available to instances of the decorated
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class via an attribute with the same name as the provider. For example,
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in the code fragment::
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@dependency.requires('foo_api', 'bar_api')
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class FooBarClient:
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def __init__(self):
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...
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...
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client = FooBarClient()
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The object 'client' will have attributes named 'foo_api' and 'bar_api',
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which are instances of the named providers.
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Objects must not rely on the existence of these attributes until after
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'resolve_future_dependencies' has been called; they may not exist
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beforehand.
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Dependencies registered via 'required' must have providers - if not, an
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exception will be raised when 'resolve_future_dependencies' is called.
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"""
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def wrapper(self, *args, **kwargs):
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"""Inject each dependency from the registry."""
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self.__wrapped_init__(*args, **kwargs)
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_process_dependencies(self)
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def wrapped(cls):
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"""Note the required dependencies on the object for later injection.
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The dependencies of the parent class are combined with that of the
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child class to create a new set of dependencies.
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"""
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existing_dependencies = getattr(cls, '_dependencies', set())
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cls._dependencies = existing_dependencies.union(dependencies)
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if not hasattr(cls, '__wrapped_init__'):
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cls.__wrapped_init__ = cls.__init__
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cls.__init__ = wrapper
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return cls
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return wrapped
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def optional(*dependencies):
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"""'optional' is the same as 'requires', except that the dependencies are
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optional - if no provider is available, the attributes will be set to None.
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"""
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def wrapper(self, *args, **kwargs):
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"""Inject each dependency from the registry."""
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self.__wrapped_init__(*args, **kwargs)
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_process_dependencies(self)
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def wrapped(cls):
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"""Note the optional dependencies on the object for later injection.
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The dependencies of the parent class are combined with that of the
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child class to create a new set of dependencies.
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"""
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existing_optionals = getattr(cls, '_optionals', set())
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cls._optionals = existing_optionals.union(dependencies)
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if not hasattr(cls, '__wrapped_init__'):
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cls.__wrapped_init__ = cls.__init__
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cls.__init__ = wrapper
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return cls
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return wrapped
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def resolve_future_dependencies(provider_name=None):
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"""'resolve_future_dependencies' forces injection of all dependencies.
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Before this function is called, circular dependencies may not have been
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injected. This function should be called only once, after all global
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providers are registered. If an object needs to be created after this
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call, it must not have circular dependencies.
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If any required dependencies are unresolvable, this function will raise an
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UnresolvableDependencyException.
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Outside of this module, this function should be called with no arguments;
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the optional argument is used internally, and should be treated as an
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implementation detail.
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"""
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new_providers = dict()
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if provider_name:
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# A provider was registered, so take care of any objects depending on
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# it.
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targets = _future_dependencies.pop(provider_name, [])
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targets.extend(_future_optionals.pop(provider_name, []))
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for target in targets:
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setattr(target, provider_name, REGISTRY[provider_name])
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return
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# Resolve optional dependencies, sets the attribute to None if there's no
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# provider registered.
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for dependency, targets in six.iteritems(_future_optionals.copy()):
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provider = REGISTRY.get(dependency)
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if provider is None:
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factory = _factories.get(dependency)
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if factory:
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provider = factory()
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REGISTRY[dependency] = provider
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new_providers[dependency] = provider
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for target in targets:
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setattr(target, dependency, provider)
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# Resolve future dependencies, raises UnresolvableDependencyException if
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# there's no provider registered.
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try:
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for dependency, targets in six.iteritems(_future_dependencies.copy()):
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if dependency not in REGISTRY:
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# a Class was registered that could fulfill the dependency, but
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# it has not yet been initialized.
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factory = _factories.get(dependency)
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if factory:
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provider = factory()
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REGISTRY[dependency] = provider
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new_providers[dependency] = provider
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else:
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raise UnresolvableDependencyException(dependency)
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for target in targets:
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setattr(target, dependency, REGISTRY[dependency])
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finally:
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_future_dependencies.clear()
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return new_providers
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def reset():
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"""Reset the registry of providers.
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This is useful for unit testing to ensure that tests don't use providers
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from previous tests.
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"""
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REGISTRY.clear()
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_future_dependencies.clear()
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_future_optionals.clear()
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