The current component registry

There can be any number of component registries in an application. One of them is the global component registry, and there is also the concept of a currently used component registry. Component registries other than the global one are associated with objects called sites. The zope.component.hooks module provides an API to set and access the current site as well as manipulate the adapter hook associated with it.

As long as we haven’t set a site, none is being considered current:

>>> from zope.component.hooks import getSite
>>> print getSite()
None

We can also ask for the current component registry (aka site manager historically); it will return the global one if no current site is set:

>>> from zope.component.hooks import getSiteManager
>>> getSiteManager()
<BaseGlobalComponents base>

Let’s set a site now. A site has to be an object that provides the getSiteManager method, which is specified by zope.component.interfaces.IPossibleSite:

>>> from zope.interface.registry import Components
>>> class Site(object):
...     def __init__(self):
...         self.registry = Components('components')
...     def getSiteManager(self):
...         return self.registry

>>> from zope.component.hooks import setSite
>>> site1 = Site()
>>> setSite(site1)

After this, the newly set site is considered the currently active one:

>>> getSite() is site1
True
>>> getSiteManager() is site1.registry
True

If we set another site, that one will be considered current:

>>> site2 = Site()
>>> site2.registry is not site1.registry
True
>>> setSite(site2)

>>> getSite() is site2
True
>>> getSiteManager() is site2.registry
True

Finally we can unset the site and the global component registry is used again:

>>> setSite()
>>> print getSite()
None
>>> getSiteManager()
<BaseGlobalComponents base>

Context manager

There also is a context manager for setting the site, which is especially useful when writing tests:

>>> import zope.component.hooks
>>> print getSite()
None
>>> with zope.component.hooks.site(site2):
...     getSite() is site2
True
>>> print getSite()
None

The site is properly restored even if the body of the with statement raises an exception:

>>> print getSite()
None
>>> with zope.component.hooks.site(site2):
...    getSite() is site2
...    raise ValueError('An error in the body')
Traceback (most recent call last):
   ...
ValueError: An error in the body
>>> print getSite()
None