sentinel — create sentinel and singleton objects

Creates simple sentinel objects.

Install

Basic features:

pip install sentinel

with extra magic features powered by python-varname:

pip install 'sentinel[varname]'

What is a sentinel?

Sentinels are singleton objects that typically represent some terminating (end) condition or have a special, symbolic meaning. Python's built-in None is a sentinel. Python also has other sentinels like NotImplemented and Ellipsis.

If you want to create your own sentinels, use this library! Make your calls to dict.get() more meaningful! You can replace the object() idiom with a sentinel:

d = {"a": 1, "b": None}

# Before sentinel:
missing = object()
if d.get("c", missing) is missing:
    ... # do some stuff

# After sentinel:
Missing = sentinel.create()
if d.get("c", Missing) is Missing:
    ... # do some stuff

Features

• sentinels are unique
• sentinels are singletons — the only instance of their own anonymous class
• sentinels can be used with is comparisons
• sentinels can be used with pickle
• sentinels can be used with copy.deepcopy
• you can add arbitrary attributes and methods to sentinels
• sentinels have a nice, self-documenting __repr__!

Usage

Create a sentinel:

>>> import sentinel
>>> MySentinel = sentinel.create("MySentinel")
>>> MySentinel
MySentinel

If you have python-varname installed, or installed this module using pip install 'sentinel[varname]', sentinel.create() can infer the name from the assignment expression:

import sentinel

MySentinel = sentinel.create()

print(MySentinel)  # prints `MySentinel`

NOTE: this will not work in the interactive console!

>>> import sentinel
>>> # Fails because varname can't find the source code for the interactive console!
>>> MySentinel = sentinel.create("MySentinel")

Example

Sentinels are useful when other objects such as None, False, 0, -1, are valid values within some data structure. For example, setting default values when all other values are valid with: dict.setdefault():

d = {"stdout": None, "stdin": 0, "EOF": -1}

MissingEntry = sentinel.create()

[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

Since a new sentinel can never occur in the original dictionary, you can tell which entries are missing or unset in a dictionary in a self-documenting way:

Unset = sentinel.create()
if d.get("stdin", Unset) is Unset:
    stdin = 0  # some reasonable default

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 --extras varname

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

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