async-signals

Easy library to implement the observer pattern in async code.

Note: This library is a copy of the signals library from Django. I always felt like using the observer pattern in Django is pretty well crafted and liked the way Django did implement this. But when switching to FastAPI I missed this feature. So I decided to copy the signals library from Django and implement it for FastAPI and other async frameworks.
A big thanks to the nice people why built Django! And for using a BSD license to make this possible.

Changes from the original Django signals library

• Signal.send(...) and Signal.send_robust(...) are now async functions 🚀
• I added type annotations to all functions and classes, mypy is happy now 🧐
• I created tests for the signals library - without using any Django models 😎

Installation

Just use pip install async-signals to install the library.

Usage

from async_signals import Signal

# Create a signal
my_signal = Signal()

# Connect a function to the signal (can be async or sync, needs to receive **kwargs)
async def my_handler(sender, **kwargs):
    print("Signal received!")

my_signal.connect(my_handler)

# Send the signal
await my_signal.send("sender")

signal.send(...) will return a list of all called receivers and their return values.

About **kwargs

The **kwargs are mandatory for your receivers. This is because the signal will pass any arguments it receives to the receivers. This is useful if you want to pass additional information to the receivers. To allow adding additional arguments to the signal in the future, the receivers should is required to accept **kwargs.

About weak signals

The signal class will automatically remove signals when the receiver is garbage collected. This is done by using weak references. This means that you can use signals in long running applications without having to worry about memory leaks.

If you want to disable this behaviour you can set the weak parameter to False when connecting the receiver.

my_signal.connect(my_handler, weak=False)

# or

my_signal.connect(my_handler, weak=True)  # the default

About async signals

The signal class will automatically await async receivers. If your receiver is sync it will be executed normally.

About the sender

The sender is the object that sends the signal. It can be anything. It is passed to the receiver as the first argument. This is useful if you want to have multiple signals in your application and you want to know which signal was sent. Normally the sender is the object that triggers the signal.

You may also pass the sender when connecting a receiver. This is useful if you want to connect a receiver to a specific sender. If you do this the receiver will only be called when the sender is the same as the one you passed when connecting the receiver.

Note: I normally tend to use Pydantic models as the sender in FastAPI. But feel free to use whatever you want.

my_signal.connect(my_handler, sender="sender")

# This will not call the receiver
await my_signal.send("other_sender")

Using the receiver decorator

You can also use the receiver decorator to connect a receiver to a signal.

@receiver(my_signal)
async def my_handler(sender, **kwargs):
    print("Signal received!")

Or if you want to limit the receiver to a specific sender.

@receiver(my_signal, sender="sender")
async def my_handler(sender, **kwargs):
    print("Signal received!")

Handle exceptions

By default the signal class will raise exceptions raised by receivers. If you want the signal to catch the exceptions and continue to call the other receivers you can use send_robust(..) instead of send(). The return value will be a list of tuples containing the receiver and the return or the exception raised by the receiver. You will need to check the type of the return value to see if it is an exception or not.

await my_signal.send_robust("sender")

Contributing

If you want to contribute to this project, feel free to just fork the project, create a dev branch in your fork and then create a pull request (PR). If you are unsure about whether your changes really suit the project please create an issue first, to talk about this.