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content/blog/pysubscribepattern.md

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+---
+title: "Python Patterns: Subscribe"
+date: 2020-04-14T07:53:46-04:00
+draft: false
+tags: []
+---
+
+It is common for larger applications to have modules that publishes and subscribes to events. This post will outline a couple ways to achieve this using [decorators](https://brandonrozek.com/blog/pydecorators/).
+
+## Single Event
+
+First let us concern ourselves with a single event since that's the easiest. Here we will create an application class that stores callbacks of functions through the subscribe decorator. Calling `emit` will send a message to all the functions stored in `self.callbacks`.
+
+```python
+class Application:
+    def __init__(self):
+        self.callbacks = []
+    def subscribe(self, func):
+        self.callbacks.append(func)
+        return func
+    def emit(self, message):
+        for callback in self.callbacks:
+            callback(message)
+```
+
+Here is an example of its usage:
+
+```python
+app = Application()
+
+@app.subscribe
+def test1(message):
+    print("Function 1:", message)
+
+@app.subscribe
+def test2(message):
+    print("Function 2:", message)
+
+app.emit('Hello World')
+```
+
+```
+Function 1: Hello World
+Function 2: Hello World
+```
+
+## Multiple Events
+
+Let's say you want the application to handle different types of events. Now `self.callbacks` is a dictionary of lists, where the key is the event and the list is the same as the last section. There's an additional layered function on top of `subscribe` this time in order to handle passing an argument into the decorator.
+
+```python
+from collections import defaultdict
+
+class Application:
+    def __init__(self):
+        self.callbacks = defaultdict(list)
+    def on(self, event):
+        def subscribe(func):
+            self.callbacks[event].append(func)
+            return func
+        return subscribe
+    def emit(self, event, message):
+        for callback in self.callbacks[event]:
+            callback(message)
+```
+
+To show its usage lets first create an instance of `Application`
+
+```python
+app = Application()
+```
+
+Now let's subscribe a couple functions to `event1`
+
+```python
+@app.on('event1')
+def test1(message):
+    print("Function 1:", message)
+
+@app.on('event1')
+def test3(message):
+    print("Function 3:", message)
+```
+
+Now to subscribe a couple events to `event2`
+
+```python
+# Subscribed to event 2
+@app.on('event2')
+def test2(message):
+    print("Function 2:", message)
+
+@app.on('event2')
+def test4(message):
+    print("Function 4:", message)
+```
+
+We can also subscribe to both events
+
+```python
+# Subscribed to both events
+@app.on('event1')
+@app.on('event2')
+def test5(message):
+    print("Function 5:", message)
+```
+
+```python
+app.emit('event1', 'Hello, World!')
+```
+
+```
+Function 1: Hello, World!
+Function 3: Hello, World!
+Function 5: Hello, World!
+```
+
+```python
+app.emit('event2', 'Goodbye, World!')
+```
+
+```
+Function 2: Goodbye, World!
+Function 4: Goodbye, World!
+Function 5: Goodbye, World!
+```
+