Core Concepts

The core idea of this library is really simple. All you have are processors, filters and dependencies. Read on to learn more about each concept.


Processors are simply functions that you create to process a certain event. Which processor gets called for which event depends on the filters you use for that processor. More info on the processors page.


Filters are how you tell the library which processor to invoke for each event you want to process. There’s a few different kinds of filters, the most common being the Exists filter and the Eq filter. More info on the filters page.


It’s possible to have ambiguous filters (see Filters for details). To resolve them, take a look at Ranking Processors and Invocation Strategies.


Dependencies are a pretty key concept, because they allow your processors to depend on values obtained dynamically (which is super important if you want to use external APIs in your processors). It’s also possible to depend on the event (so you can have it injected into your processor). More info on the dependencies page.


This example shows how these three concepts click together to make event processing easy. For the example, we just use a stub for the SSM client and assume that the admin_email parameter has a value of

from event_processor import EventProcessor, Event, Depends
from event_processor.filters import Exists

event_processor = EventProcessor()

def get_ssm():
    return FakeSSMClient()

def user_is_admin(raw_event: Event, ssm_client: FakeSSMClient = Depends(get_ssm)) -> bool:
    ssm_response = ssm_client.get_parameter(Name="admin_email")
    admin_email = ssm_response["Parameter"]["Value"]
    return raw_event["user"]["email"] == admin_email

print(" is admin:", event_processor.invoke({"user": {"email": ""}}))
print(" is admin:", event_processor.invoke({"user": {"email": ""}})) is admin: True is admin: False

You can see that because the event contains a value at (i.e. this path Exists in the event), the processor was invoked. It also received the event by specifying a parameter with the Event type and received an SSM client by depending on the value returned by get_ssm.