Use a signal's connect method to connect a function to a signal. When a signal is sent, each connected function is called with the sender and any named arguments the signal provides.

from flask import template_rendered

def log_template(sender, template, context, **kwargs):
    sender.logger.info(
        'Rendered template %(template)r with context %(context)r.',
        template=template, context=context
    )

template_rendered.connect(log_template)

See the documentation on built-in signals for information about what arguments they provides. A useful pattern is adding a **kwargs argument to catch any unexpected arguments.

If you want to create and send signals in your own code (for example, if you are writing an extension), create a new Signal instance and call send when the subscribers should be notified. Signals are created using a Namespace.

from flask import current_app
from flask.signals import Namespace

namespace = Namespace()
message_sent = namespace.signal('mail_sent')

def message_response(recipient, body):
    ...
    message_sent.send(
        current_app._get_current_object(),
        recipient=recipient,
        body=body
    )

@message_sent.connect
def log_message(app, recipient, body):
    ...

Prefer using Flask's signal support over using Blinker directly. It wraps the library so that signals remain optional if developers using your extension have not opted to install Blinker.