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You can ask a WorkingSet to ``find()`` a distribution matching a requirement::
>>> from pkg_resources import Requirement
>>> print(ws.find(Requirement.parse("Foo==1.0"))) # no match, return None
None
>>> ws.find(Requirement.parse("Bar==0.9")) # match, return distribution
Bar 0.9 (http://example.com/something)
Note that asking for a conflicting version of a distribution already in a
working set triggers a ``pkg_resources.VersionConflict`` error:
>>> try:
... ws.find(Requirement.parse("Bar==1.0"))
... except pkg_resources.VersionConflict as exc:
... print(str(exc))
... else:
... raise AssertionError("VersionConflict was not raised")
(Bar 0.9 (http://example.com/something), Requirement.parse('Bar==1.0'))
You can subscribe a callback function to receive notifications whenever a new
distribution is added to a working set. The callback is immediately invoked
once for each existing distribution in the working set, and then is called
again for new distributions added thereafter::
>>> def added(dist): print("Added %s" % dist)
>>> ws.subscribe(added)
Added Bar 0.9
>>> foo12 = Distribution(project_name="Foo", version="1.2", location="f12")
>>> ws.add(foo12)
Added Foo 1.2
Note, however, that only the first distribution added for a given project name
will trigger a callback, even during the initial ``subscribe()`` callback::
>>> foo14 = Distribution(project_name="Foo", version="1.4", location="f14")
>>> ws.add(foo14) # no callback, because Foo 1.2 is already active
>>> ws = WorkingSet([])
>>> ws.add(foo12)
>>> ws.add(foo14)
>>> ws.subscribe(added)
Added Foo 1.2
And adding a callback more than once has no effect, either::
>>> ws.subscribe(added) # no callbacks
# and no double-callbacks on subsequent additions, either
>>> just_a_test = Distribution(project_name="JustATest", version="0.99")
>>> ws.add(just_a_test)
Added JustATest 0.99
Finding Plugins
---------------
``WorkingSet`` objects can be used to figure out what plugins in an
``Environment`` can be loaded without any resolution errors::
>>> from pkg_resources import Environment
>>> plugins = Environment([]) # normally, a list of plugin directories
>>> plugins.add(foo12)
>>> plugins.add(foo14)
>>> plugins.add(just_a_test)
In the simplest case, we just get the newest version of each distribution in
the plugin environment::
>>> ws = WorkingSet([])
>>> ws.find_plugins(plugins)
([JustATest 0.99, Foo 1.4 (f14)], {})
But if there's a problem with a version conflict or missing requirements, the
method falls back to older versions, and the error info dict will contain an
exception instance for each unloadable plugin::
>>> ws.add(foo12) # this will conflict with Foo 1.4
>>> ws.find_plugins(plugins)
([JustATest 0.99, Foo 1.2 (f12)], {Foo 1.4 (f14): VersionConflict(...)})
But if you disallow fallbacks, the failed plugin will be skipped instead of
trying older versions::
>>> ws.find_plugins(plugins, fallback=False)
([JustATest 0.99], {Foo 1.4 (f14): VersionConflict(...)})
Platform Compatibility Rules
----------------------------
On the Mac, there are potential compatibility issues for modules compiled
on newer versions of macOS than what the user is running. Additionally,
macOS will soon have two platforms to contend with: Intel and PowerPC.
Basic equality works as on other platforms::