sentinel 0.2.0

Create sentinel objects, akin to None, NotImplemented, Ellipsis

Creates simple sentinel objects which are the only instance of their own anonymous class. As a singleton, there is a guarantee that there will only ever be one instance: they can be safely used with pickle and cPickle alike, as well as being able to be used properly with copy.deepcopy(). In addition, a self-documenting __repr__ is provided for free!

Usage

Sentinels are singleton objects that typically represent some end or terminating condition. Some singletons already exist in Python, like None, NotImplemented, and Ellipsis.

All that’s needed to create a sentinel is its name:

>>> import sentinel
>>> Nothing = sentinel.create('Nothing')
>>> Nothing
Nothing

This by itself is useful when other objects such as None, False, 0, -1, etc. are entirely valid values. For example, setting default values when all other values are valid with: dict.setdefault():

>>> MissingEntry = sentinel.create('MissingEntry')
>>> d = {'stdout': None, 'stdin': 0, 'EOF': -1}
>>> [d.setdefault(key, MissingEntry) for key in ('stdin', 'stdout', 'stderr')]
[0, None, MissingEntry]

Alternatively, using dict.get() when fetching values:

>>> d = {'stdout': None, 'stdin': 0, 'EOF': -1}
>>> d.get('stdout', MissingEntry)
None
>>> d.get('stdin', MissingEntry)
0
>>> d.get('stderr', MissingEntry)
MissingEntry

It’s known immediately which value was missing from the dictionary in a self-documenting manner.

Adding extra methods and class attributes

Sentinels may also inherit from base classes, or implement extra methods.

Consider a binary search tree with two kinds of nodes: interior nodes (Node) which contain some payload and leaves (Leaf), which simply terminate traversal.

To create singleton leaf which implements a search method and an is_leaf property, you may provide any extra class attributes in the cls_dict keyword argument. The following is a full example of both the singleton Leaf and its Node counterpart:

def _search_leaf(self, key):
    raise KeyError(key)

Leaf = sentinel.create('Leaf', cls_dict={
    'search': _search_leaf,
    'is_leaf': property(lambda self: True)
})

class Node(object):
    def __init__(self, key, payload, left=Leaf, right=Leaf):
        self.left = left
        self.right = right
        self.key = key
        self.payload = payload

    def search(self, key):
        if key < self.key:
            return self.left.search(key)
        elif key > self.key:
            return self.right.search(key)
        else:
            return self.payload

    is_leaf = property(lambda: false)

Example usage:

>>> tree = Node(2, 'bar', Node(1, 'foo'), Node(3, 'baz'))
>>> tree.search(1)
'foo'
>>> tree.search(4)
Traceback (most recent call last):
    ...
KeyError: 2

Contributing

This project uses Poetry. To contribute to the codebase, make sure to install poetry, With Poetry installed, clone then repo, then within the repo directory, install the developer dependencies:

$ poetry install

Next, I recommend you do all development tasks within the poetry shell:

$ poetry shell
(sentinel-nUnrocCf-py3.9) $ black .
(sentinel-nUnrocCf-py3.9) $ pytest