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DCI lookups in Python (inspired by Netbeans Platform Lookups API)

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lookups - Find object instances

DCI lookups for Python (inspired by Netbeans Platform Lookups API)


A lookup is like a dict where you can store object instances as values. And the search keys are their type classes.


But lookups implements a bit more than that:

  • You can also lookup by parent class and not just the most subclasses of an instance.
  • You can get hold of a lookups.Result, which allows you to register a listener for a given class search. You will be notified when an instance of that class is added/removed from the lookup.
  • Deferred instanciation with lookups.Convertor. That is, an 'instance' can appear in a lookup but not be instanciated until it is actually used (ie. looked up). Useful for heavy objects or plugins.
  • lookups.Item can provide you with additional info on an instance: display string, persistence ID string, type, and instance itself.


This is the most basic but versatile and dynamic lookup. (HINT: For Java folks, it corresponds to your AbstractLookup ;-) ).

It comes in two main parts:

  • lookups.InstanceContent provide write-access to the lookup: add/set/remove instances.
  • lookups.GenericLookup provide read-access to search in the lookup.
from lookups import InstanceContent, GenericLookup

my_content = InstanceContent()
my_lookup = GenericLookup(my_content)

# Adds some objects
class ParentClass:

class ChildClass(ParentClass):

parent = ParentClass()
child1 = ChildClass()
child2 = ChildClass()


# lookup(cls): get first matching instance
# a_match will be any of parent, child1 or child2
a_parent_match = my_lookup.lookup(ParentClass)

# lookup_all(cls): get all matching instances
# all_parent_matches is an immutable sequence
#     of parent, child1 and child2
all_parent_matches = my_lookup.lookup_all(ParentClass)
# all_children_matches is an immutable sequence
#     of child1 and child2
all_children_matches = my_lookup.lookup_all(ChildClass)

# lookup_result(cls): get a Result object for the searched class
parent_result = my_lookup.lookup_result(ParentClass)
# all_instances(): all instances corresponding to the searched
#     class (ie. similar to plain lookup_all())
# all_classes(): Immutable set of all types in the result.
#     Here it would be set(ParentClass, ChildClass)

# Lookup result listener
def call_me_back(result):
    print('Result changed. Instances are now', result.all_instances())



# -> This will invoke call_me_back()
# You can also provide a `concurrent.futures.Executor` when
# creating the content to control how the listeners are called:
#     InstanceContent(notify_in: Executor = None).

Other lookups

  • lookups.Lookup.get_default(): The default lookup in a system.
  • lookups.ProxyLookup: A lookup that merge results from several lookups.
  • lookups.DelegatedLookup: A lookup that redirects to another (dynamic) lookup, through a LookupProvider.
  • lookups.EntryPointLookup: A lookup loading its instances from a setuptools entry point group (ie. provided by any installed package).
  • lookups.fixed: Simple unmodifiable lookup. Content is set at creation time. Will be one of:
    • lookup.SimpleLookup: A basic lookup with a static content.
    • lookups.singleton: Unmodifiable lookup that contains just one fixed object.
    • lookups.EmptyLookup: A lookup containing nothing.

Individual Contributors

A list of people who have contributed to Lookups in order of their first contribution.

Format: Name-or-Well-known-alias <email@domain.tld> (url)

Please, add yourself when you contribute!

Original Netbeans authors of Lookup API

  • Jaroslav Tulach - Lookup API, AbstractLookup, InstanceContent, ArrayStorage
  • Marian Petras - Singleton lookup
  • David Strupl - Common lookup implementations


0.3.0 - XX XXXXXXXX 2021

  • Adds a EntryPointLookup.
  • Adds a DelegatedLookup.
  • Adds a ProxyLookup.
  • Adds a proper resolution for system default lookup Lookup.get_default().
  • Fixes issue with listeners registration disappearing immediately when using object-bound methods.
  • Content of a GenericLookup can now behave like a Container (ie. you can do things like "obj in content").
  • When an instance is not hashable, provides an alternative using id() of the object in order to be able to store it in a hash-based storage (set, dictionary).
  • New syntactic sugar: call directly a lookup object as shortcut for the lookup method. Ie., instead of writing "lookup.lookup(...)" you can now write "lookup(...)".
  • Missing declared dependency in typing_extensions.
  • Abstract methods now raise NotImplementedError

0.2.0 - 06 February 2020

  • Provides GenericLookup and InstanceContent, based on SetStorage. These are the first dynamic lookups. They are based on Netbeans' AbstractLookup, InstanceContent and ArrayStorage.
  • Lookup listeners are just simple callables now.
  • Follows PEP 561 for packages providing typing information.
  • Improved quality assurance process (using Github Workflow as CI).
  • First (proto-)documentation.

0.1.0 - 18 May 2019

  • Initial dump of code.
  • Defines the public API for lookups.
  • Provides fixed lookup: members are defined at instantiation time and never change.
  • Provides singleton lookup: only one member defined at instantiation time and never change.
  • Provides empty lookup: a special lookup with nothing in it.

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