231 lines
7.5 KiB
Python
231 lines
7.5 KiB
Python
# Copyright 2012 OpenStack Foundation
|
|
#
|
|
# Licensed under the Apache License, Version 2.0 (the "License"); you may
|
|
# not use this file except in compliance with the License. You may obtain
|
|
# a copy of the License at
|
|
#
|
|
# http://www.apache.org/licenses/LICENSE-2.0
|
|
#
|
|
# Unless required by applicable law or agreed to in writing, software
|
|
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
|
|
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
|
|
# License for the specific language governing permissions and limitations
|
|
# under the License.
|
|
|
|
"""This module provides support for dependency injection.
|
|
|
|
Providers are registered via the ``@provider()`` decorator, and dependencies on
|
|
them are registered with ``@requires()``. Providers are available to their
|
|
consumers via an attribute. See the documentation for the individual functions
|
|
for more detail.
|
|
|
|
See also:
|
|
|
|
https://en.wikipedia.org/wiki/Dependency_injection
|
|
|
|
"""
|
|
|
|
import traceback
|
|
|
|
from keystone.i18n import _
|
|
|
|
|
|
_REGISTRY = {}
|
|
|
|
_future_dependencies = {}
|
|
_factories = {}
|
|
|
|
|
|
def _set_provider(name, provider):
|
|
_original_provider, where_registered = _REGISTRY.get(name, (None, None))
|
|
if where_registered:
|
|
raise Exception('%s already has a registered provider, at\n%s' %
|
|
(name, ''.join(where_registered)))
|
|
_REGISTRY[name] = (provider, traceback.format_stack())
|
|
|
|
|
|
GET_REQUIRED = object()
|
|
GET_OPTIONAL = object()
|
|
|
|
|
|
def get_provider(name, optional=GET_REQUIRED):
|
|
if optional is GET_REQUIRED:
|
|
return _REGISTRY[name][0]
|
|
return _REGISTRY.get(name, (None, None))[0]
|
|
|
|
|
|
class UnresolvableDependencyException(Exception):
|
|
"""Raised when a required dependency is not resolvable.
|
|
|
|
See ``resolve_future_dependencies()`` for more details.
|
|
|
|
"""
|
|
|
|
def __init__(self, name, targets):
|
|
msg = _('Unregistered dependency: %(name)s for %(targets)s') % {
|
|
'name': name, 'targets': targets}
|
|
super(UnresolvableDependencyException, self).__init__(msg)
|
|
|
|
|
|
def provider(name):
|
|
"""A class decorator used to register providers.
|
|
|
|
When ``@provider()`` is used to decorate a class, members of that class
|
|
will register themselves as providers for the named dependency. As an
|
|
example, In the code fragment::
|
|
|
|
@dependency.provider('foo_api')
|
|
class Foo:
|
|
def __init__(self):
|
|
...
|
|
|
|
...
|
|
|
|
foo = Foo()
|
|
|
|
The object ``foo`` will be registered as a provider for ``foo_api``. No
|
|
more than one such instance should be created; additional instances will
|
|
replace the previous ones, possibly resulting in different instances being
|
|
used by different consumers.
|
|
|
|
"""
|
|
def wrapper(cls):
|
|
def wrapped(init):
|
|
def __wrapped_init__(self, *args, **kwargs):
|
|
"""Initialize the wrapped object and add it to the registry."""
|
|
init(self, *args, **kwargs)
|
|
_set_provider(name, self)
|
|
resolve_future_dependencies(__provider_name=name)
|
|
|
|
return __wrapped_init__
|
|
|
|
cls.__init__ = wrapped(cls.__init__)
|
|
_factories[name] = cls
|
|
return cls
|
|
return wrapper
|
|
|
|
|
|
def _process_dependencies(obj):
|
|
# Any dependencies that can be resolved immediately are resolved.
|
|
# Dependencies that cannot be resolved immediately are stored for
|
|
# resolution in resolve_future_dependencies.
|
|
|
|
def process(obj, attr_name, unresolved_in_out):
|
|
for dependency in getattr(obj, attr_name, []):
|
|
if dependency not in _REGISTRY:
|
|
# We don't know about this dependency, so save it for later.
|
|
unresolved_in_out.setdefault(dependency, []).append(obj)
|
|
continue
|
|
|
|
setattr(obj, dependency, get_provider(dependency))
|
|
|
|
process(obj, '_dependencies', _future_dependencies)
|
|
|
|
|
|
def requires(*dependencies):
|
|
"""A class decorator used to inject providers into consumers.
|
|
|
|
The required providers will be made available to instances of the decorated
|
|
class via an attribute with the same name as the provider. For example, in
|
|
the code fragment::
|
|
|
|
@dependency.requires('foo_api', 'bar_api')
|
|
class FooBarClient:
|
|
def __init__(self):
|
|
...
|
|
|
|
...
|
|
|
|
client = FooBarClient()
|
|
|
|
The object ``client`` will have attributes named ``foo_api`` and
|
|
``bar_api``, which are instances of the named providers.
|
|
|
|
Objects must not rely on the existence of these attributes until after
|
|
``resolve_future_dependencies()`` has been called; they may not exist
|
|
beforehand.
|
|
|
|
Dependencies registered via ``@required()`` must have providers; if not,
|
|
an ``UnresolvableDependencyException`` will be raised when
|
|
``resolve_future_dependencies()`` is called.
|
|
|
|
"""
|
|
def wrapper(self, *args, **kwargs):
|
|
"""Inject each dependency from the registry."""
|
|
self.__wrapped_init__(*args, **kwargs)
|
|
_process_dependencies(self)
|
|
|
|
def wrapped(cls):
|
|
"""Note the required dependencies on the object for later injection.
|
|
|
|
The dependencies of the parent class are combined with that of the
|
|
child class to create a new set of dependencies.
|
|
|
|
"""
|
|
existing_dependencies = getattr(cls, '_dependencies', set())
|
|
cls._dependencies = existing_dependencies.union(dependencies)
|
|
if not hasattr(cls, '__wrapped_init__'):
|
|
cls.__wrapped_init__ = cls.__init__
|
|
cls.__init__ = wrapper
|
|
return cls
|
|
|
|
return wrapped
|
|
|
|
|
|
def resolve_future_dependencies(__provider_name=None):
|
|
"""Force injection of all dependencies.
|
|
|
|
Before this function is called, circular dependencies may not have been
|
|
injected. This function should be called only once, after all global
|
|
providers are registered. If an object needs to be created after this
|
|
call, it must not have circular dependencies.
|
|
|
|
If any required dependencies are unresolvable, this function will raise an
|
|
``UnresolvableDependencyException``.
|
|
|
|
Outside of this module, this function should be called with no arguments;
|
|
the optional argument, ``__provider_name`` is used internally, and should
|
|
be treated as an implementation detail.
|
|
|
|
"""
|
|
new_providers = dict()
|
|
if __provider_name:
|
|
# A provider was registered, so take care of any objects depending on
|
|
# it.
|
|
targets = _future_dependencies.pop(__provider_name, [])
|
|
|
|
for target in targets:
|
|
setattr(target, __provider_name, get_provider(__provider_name))
|
|
|
|
return
|
|
|
|
# Resolve future dependencies, raises UnresolvableDependencyException if
|
|
# there's no provider registered.
|
|
try:
|
|
for dependency, targets in _future_dependencies.copy().items():
|
|
if dependency not in _REGISTRY:
|
|
# a Class was registered that could fulfill the dependency, but
|
|
# it has not yet been initialized.
|
|
factory = _factories.get(dependency)
|
|
if factory:
|
|
provider = factory()
|
|
new_providers[dependency] = provider
|
|
else:
|
|
raise UnresolvableDependencyException(dependency, targets)
|
|
|
|
for target in targets:
|
|
setattr(target, dependency, get_provider(dependency))
|
|
finally:
|
|
_future_dependencies.clear()
|
|
return new_providers
|
|
|
|
|
|
def reset():
|
|
"""Reset the registry of providers.
|
|
|
|
This is useful for unit testing to ensure that tests don't use providers
|
|
from previous tests.
|
|
"""
|
|
_REGISTRY.clear()
|
|
_future_dependencies.clear()
|