Python RevisionDict

RevisionDict works like an ordinary dictionary with additional revision keeping of changes. It remembers the order when keys were updated (in contrast to the OrderedDict which is remembering the order when keys are inserted).

It is revision tracking the overall dictionary, but does not store each earlier value for each key in the dictionary. It's basically answering the following questions:

• What is the actual revision number?
• What changed since revsion number N?
• In which revision has value of key K changed the last time?

Use RevisionDict to build caching systems, interface multiple clients to a common database, publish/subscribe systems,...

Additional functionality compared to dict():

• .revision - returning the actual revision as integer (starting with 0)
• .base_revision - revision before oldest item changed (or 0 on empty dict)
• .key_to_revision(key) - return the revision when the given key was changed
• .checkout(start=0) - return a dict with changes since start

Install

pip install revisiondict

Example

>>> from revisiondict import RevisionDict
>>> d = RevisionDict()
>>> d.revision                    # get revision (is 0 at init)
0
>>> d.base_revision               # get revision before oldest change
0

Adding new items:

>>> d['a']=0; d['b']=1; d['c']=2  # make three updates
>>> d.revision                    # showing 3 changes
3
>>> d.base_revision               # get revision before oldest change
0

Inspecting content of RevisionDict:

>>> d.checkout()=={'a': 0, 'b': 1, 'c': 2} # get a dictionary with all changes
True
>>> d.checkout(2)                 # get all changes starting with rev. 2
{'c': 2}
>>> d.checkout(3)                 # all changes starting with actual revision
{}
>>> d.key_to_revision('b')        # revision where 'b' was changed last time
2
>>> d
RevisionDict([_Item(key='a', value=0, revision=1), _Item(key='b', value=1, revision=2), _Item(key='c', value=2, revision=3)])

Update items:

>>> d['a']=3                      # update value of 'a' (was 0 before)
>>> d.revision
4
>>> d.base_revision
1
>>> d.key_to_revision('a')
4
>>> d.checkout(3)                 # get all changes starting with rev. 3
{'a': 3}
>>> tuple(d.keys())               # iterate over keys (ordered by time of update)
('b', 'c', 'a')

UniqueRevisionDict

UniqueRevisionDict is a subclass of RevisionDict which does not create a new revision, when an element is updated with the same value.

>>> from revisiondict import UniqueRevisionDict
>>> d = UniqueRevisionDict(a=0)
>>> d.revision
1

>>> d['a']=0  # value is equal to the previous one
>>> d.revision
1

>>> d['a']=False  # value is still equal as 0 == False
>>> d.revision
1

>>> d['a']=100  # a new value is set, so revision is increased
>>> d.revision
2