sentinel 0.2.1a0

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.

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)

>>> 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