lib: Add a class for doing the Publisher / Subscriber pattern

This will eventually replace my current notifications system. Hopefully
it'll be a little easier to work with and maintain

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

emmental.py

@@ -1,5 +1,6 @@
 #!/usr/bin/python
 # Copyright 2019 (c) Anna Schumaker.
+import lib
 import curds
 import rind
 import sys

lib/__init__.py

@@ -0,0 +1,2 @@
+# Copyright 2020 (c) Anna Schumaker.
+from . import publisher

lib/publisher.py

@@ -0,0 +1,18 @@
+# Copyright 2020 (c) Anna Schumaker.
+
+class Publisher:
+    def __init__(self):
+        self.subscribers = set()
+
+    def publish(self, *args):
+        for func in self.subscribers:
+            func(*args)
+
+    def register(self, func):
+        self.subscribers.add(func)
+
+    def reset(self):
+        self.subscribers.clear()
+
+    def unregister(self, func):
+        self.subscribers.discard(func)

lib/test_publisher.py

@@ -0,0 +1,30 @@
+# Copyright 2020 (c) Anna Schumaker.
+from . import publisher
+import unittest
+
+class TestPublisher(unittest.TestCase):
+    def on_test(self, text):
+        self.test_arg = text
+
+    def test_publisher_init(self):
+        pub = publisher.Publisher()
+        self.assertIsInstance(pub.subscribers, set)
+        self.assertEqual(pub.subscribers, set())
+
+    def test_publisher_register(self):
+        pub = publisher.Publisher()
+        pub.register(self.on_test)
+        self.assertEqual(pub.subscribers, { self.on_test })
+
+        pub.unregister(self.on_test)
+        self.assertEqual(pub.subscribers, set())
+
+        pub.subscribers = set([ 1, 2, 3 ])
+        pub.reset()
+        self.assertEqual(pub.subscribers, set())
+
+    def test_publisher_publish(self):
+        pub = publisher.Publisher()
+        pub.register(self.on_test)
+        pub.publish("Test Arg")
+        self.assertEqual(self.test_arg, "Test Arg")