Overview

Weak entry, strong value immutable registry data structure. Think of it as an immutable WeakKeyDictionary that efficiently evolves into a new copy everytime you want to make a change to it.

Motivation

Immutable data structures are great for when you need to implement some kind of "snapshot" of states for easy undo/redo, time-travelling functionality. The library pyrsistent is great for that, but it lacks a map-like structure in which the keys are stored as weak references.

Registtro is an implementation of that structure, which allows for proper garbage collection of the keys/entries, while still allowing to store their states in a centralized, immutable structure.

Example

Simple implementation of an undoable store that keeps track of states for entries.

>>> from registtro import Registry
>>> class Store(object):
...     """Keeps track of the history of states for entries."""
...     def __init__(self):
...         self._done = [Registry()]
...         self._undone = []
...     def init(self, entry, state):
...         self._done.append(self._done[-1].update({entry: state}))
...         del self._done[:-1]
...         del self._undone[:]
...     def get_state(self, entry):
...         return self._done[-1].query(entry)
...     def set_state(self, entry, state):
...         del self._undone[:]
...         self._done.append(self._done[-1].update({entry: state}))
...     def undo(self):
...         assert len(self._done) > 1, "can't undo"
...         self._undone.append(self._done.pop())
...     def redo(self):
...         assert self._undone, "can't redo"
...         self._done.append(self._undone.pop())
...
>>> class Entry(object):
...     """Reads/sets state in a store."""
...     def __init__(self, store, state):
...         self._store = store
...         store.init(self, state)
...     def get_state(self):
...         return self._store.get_state(self)
...     def set_state(self, state):
...         self._store.set_state(self, state)
...
>>> # Initialize entries.
>>> global_store = Store()
>>> entry_a = Entry(global_store, "foo")
>>> entry_b = Entry(global_store, "bar")
>>> (entry_a.get_state(), entry_b.get_state())
('foo', 'bar')
>>> # Modify entries.
>>> entry_a.set_state("FOO")
>>> entry_b.set_state("BAR")
>>> (entry_a.get_state(), entry_b.get_state())
('FOO', 'BAR')
>>> # Undo modifications.
>>> global_store.undo()
>>> (entry_a.get_state(), entry_b.get_state())
('FOO', 'bar')
>>> global_store.undo()
>>> (entry_a.get_state(), entry_b.get_state())
('foo', 'bar')
>>> # Redo modifications.
>>> global_store.redo()
>>> (entry_a.get_state(), entry_b.get_state())
('FOO', 'bar')
>>> global_store.redo()
>>> (entry_a.get_state(), entry_b.get_state())
('FOO', 'BAR')