shoobx.immutable 1.0.5

Immutable Types

shoobx.immutable – Immutable Types

This library provides a state-based implementation of immutable types, including lists, sets and dicts. It handles an arbitrarily deep structure of nested objects.

In addition, support for revisioned immutables is provided, which allows for full revision histories of an immutable. A sample implementation of a revisioned immutable maanger is also provided.

Optional: A pjpersist-based storage mechanism for revisioned immutables is provided, which allows for easy storage of versioned immutables.

Using Immutables

Immutable objects can make certain complex systems more reasonable, because they tightly control when an object is modified and how. It also guarantees that an object can never change for another accessor in a different subsystem.

Introduction

Let’s start with a simple dictionary:

>>> import shoobx.immutable as im
>>> answer = im.ImmutableDict({
...     'question': 'Answer to the ultimate question of life, ...',
...     'answer': 0
... })

>>> answer['answer']
0

But no value can be changed anymore:

>>> answer['answer'] = 42
Traceback (most recent call last):
...
AttributeError: Cannot update locked immutable object.

Immutable objects are updated through a special context manager that creates a new version of the object that can be modified within the context manager block.

>>> orig = answer
>>> with im.update(answer) as answer:
...     answer['answer'] = 42

>>> answer['answer']
42

Note that the answer dictionary is a completely new object and that the original object is still unmodified.

>>> orig is not answer
True
>>> orig['answer']
0

Of course we can also create complex object structures, for example by adding a list:

>>> with im.update(answer) as answer:
...     answer['witnesses'] = ['Arthur', 'Gag']

>>> answer['witnesses']
['Arthur', 'Gag']

Of course, the list has been converted to its immutable equal, so that items cannot be modified.

>>> isinstance(answer['witnesses'], im.ImmutableList)
True
>>> answer['witnesses'].append('Deep Thought')
Traceback (most recent call last):
...
AttributeError: Cannot update locked immutable object.

However, updating from an child/sub-object is not allowed, since creating a new version of a child would sematically modify its parent thus violating the immutability constraint:

>>> with im.update(answer['witnesses']) as witnesses:
...     pass
Traceback (most recent call last):
...
AttributeError: update() is only available for master immutables.

The system accomplishes this by assigning “master” and “slave” modes to the immutables. The root immutable is the master and any sub-objects below are slaves.

Immutable sets are also supported as a core immutable:

>>> data = im.ImmutableSet({6})
>>> data
{6}

>>> with im.update(data) as data:
...     data.discard(6)
...     data.add(9)
>>> data
{9}

Custom Immutables

Creating your own immutable objects is simple:

>>> class Answer(im.Immutable):
...     def __init__(self, question=None, answer=None, witnesses=None):
...         self.question = question
...         self.answer = answer
...         self.witnesses = witnesses

>>> answer = Answer('The Answer', 42, ['Arthur', 'Gag'])
>>> answer.answer
42

Note how the list is automatically converted to its immutable equivalent:

>>> isinstance(answer.witnesses, im.ImmutableList)
True

Of course you cannot modify an immutable other than the update context:

>>> answer.answer = 54
Traceback (most recent call last):
...
AttributeError: Cannot update locked immutable object.

>>> with im.update(answer) as answer:
...     answer.answer = 54
>>> answer.answer
54

Revisioned Immutables

Since mutables create a new object for every change, they are ideal for creating systems that have to keep track of their entire history. This package provides support for such systems by defining a revision manager API and revisioned immutable that are managed within it.

Let’s start by creating a custom revisioned immutable:

>>> class Answer(im.RevisionedImmutable):
...
...     def __init__(self, question=None, answer=None):
...         self.question = question
...         self.answer = answer

A simple implementation of the revision manager API is provided to demonstrate a possible implementation path.

>>> data = im.RevisionedMapping()
>>> data['a'] = answer = Answer('Answer to the ultimate question')

The answer is the current revision and has been added to the manager.

>>> data['a'] is answer
True

In addition to the usual immutability features, the Revisioned immutable has several additional attributes that help with the management of the revisions:

>>> answer.__im_start_on__
datetime.datetime(...)
>>> answer.__im_end_on__ is None
True
>>> answer.__im_manager__
<shoobx.immutable.revisioned.SimpleRevisionedImmutableManager ...>
>>> answer.__im_creator__ is None
True
>>> answer.__im_comment__ is None
True

The update API is extended to support setting the creator and comment of the change:

>>> answer_r1 = answer
>>> with im.update(answer, 'universe', 'Provide Answer') as answer:
...     answer.answer = 42

We now have a second revision of the answer that has the comemnt and creator set:

>>> answer.answer
42

>>> answer.__im_start_on__
datetime.datetime(...)
>>> answer.__im_end_on__ is None
True
>>> answer.__im_creator__
'universe'
>>> answer.__im_comment__
'Provide Answer'

The first revision is now retired and has an end date/time (which equals the start date/time of the new revision):

>>> answer_r1.__im_start_on__
datetime.datetime(...)
>>> answer_r1.__im_end_on__ == answer.__im_start_on__
True
>>> answer_r1.__im_state__ == im.interfaces.IM_STATE_RETIRED
True

The manager has APIs to manage the various revisions.

>>> revisions = data.getRevisionManager('a')
>>> len(revisions.getRevisionHistory())
2

>>> revisions.getCurrentRevision(answer_r1) is answer
True

We can even roll back to a previous revision:

>>> revisions.rollbackToRevision(answer_r1)

>>> len(revisions.getRevisionHistory())
1
>>> answer_r1.__im_end_on__ is None
True
>>> answer_r1.__im_state__ == im.interfaces.IM_STATE_LOCKED
True

Optional pjpersist Support

A more serious and production-ready implementation of the revision manager API is provided in shoobx.immutable.pjpersist which utilizes pjpersist to store all data.

Notes

A technical discussion on the system’s inner workings is located in the doc strings of the corresponding interfaces. In addition, the tests covera a lot of special cases not dicsussed here.

CHANGES

1.0.5 (2019-05-31)

• Minor fixes, docs added, test hook added.

1.0.4 (2019-05-30)

• Add API documentation.

1.0.3 (2019-05-30)

• Moved documentation to Read the Docs.

1.0.2 (2019-05-30)

• Add some readable documentation.

• Added high-level shoobx.immutable.update(im, *args, **kw) function.

• Implemented __repr__() for ImmutableSet to mimic behavior of ImmutableDict and ImmutableList.

1.0.1 (2019-05-30)

• Fix package description.

1.0.0 (2019-05-30)

• Immutable Types, Immutable Dict, Immutable Set, Immutable List

• Revisioned Immutable with Revision Manager sample implementation

• Optional: pjpersist support for immutables. Requires pjpersist>=1.7.0.

• Initial Release