mirror of
https://github.com/Brandon-Rozek/website.git
synced 2024-11-13 20:27:29 -05:00
147 lines
4 KiB
Markdown
147 lines
4 KiB
Markdown
---
|
|
title: "Python Patterns: Subscribe"
|
|
date: 2020-04-14T07:53:46-04:00
|
|
draft: false
|
|
tags: ["Python"]
|
|
medium_enabled: true
|
|
---
|
|
|
|
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](/blog/pydecorators/) and standard methods.
|
|
|
|
## 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
|
|
from typing import Callable, List
|
|
class Application:
|
|
def __init__(self):
|
|
self.callbacks: List[Callable] = []
|
|
def subscribe(self, func: Callable):
|
|
if not callable(func):
|
|
raise ValueError("Argument func must be callable.")
|
|
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)
|
|
|
|
def test2(message):
|
|
print("Function 2:", message)
|
|
|
|
app.subscribe(test2)
|
|
|
|
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
|
|
from typing import Callable, Optional
|
|
class Application:
|
|
def __init__(self):
|
|
self.callbacks: Dict[str, List[Callable]] = defaultdict(list)
|
|
def on(self, event: str, func: Optional[Callable] = None):
|
|
def subscribe(func: Callable):
|
|
if not callable(func):
|
|
raise ValueError("Argument func must be callable.")
|
|
self.callbacks[event].append(func)
|
|
return func
|
|
if func is None:
|
|
return subscribe
|
|
subscribe(func)
|
|
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)
|
|
|
|
def test3(message):
|
|
print("Function 3:", message)
|
|
|
|
app.on('event1', test3)
|
|
```
|
|
|
|
Now to subscribe a couple events to `event2`
|
|
|
|
```python
|
|
# Subscribed to event 2
|
|
@app.on('event2')
|
|
def test2(message):
|
|
print("Function 2:", message)
|
|
|
|
def test4(message):
|
|
print("Function 4:", message)
|
|
|
|
app.on('event2', test4)
|
|
```
|
|
|
|
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!
|
|
```
|
|
|
|
## Alternative: Observer Pattern
|
|
|
|
The above approaches assume that the callbacks would be needed for the lifetime of the program. An altnerative is to use the [Observer design pattern](https://en.wikipedia.org/wiki/Observer_pattern).
|
|
|
|
This assumes that we have two classes, `Observable` and `Observer`. The latter class implements the `notify` method and when it is constructed, it adds itself to the collection contained in the `Observable` class.
|
|
|
|
[Martin](https://martinheinz.dev/blog/112) wrote a great blog post on how you can use Python weak references so that when your `Observer` class goes out of scope, a copy isn't stored in the `Observable` class and instead gets safely removed from the collection.
|
|
|
|
|