Event Sourcing Store in Python: Append-Only Log Mock

Mock an append-only event store in Python — record events, list them, and fetch by ID using a simple list-backed class.

Easy Python 3.9+ Aug 9, 2026 Microservices patterns 14 views 0 copies

event-sourcing microservices mock in-memory append-only

Python code

35 lines

Python 3.9+

class EventStore:
    def __init__(self):
        self._events = []

    def append(self, event):
        event_id = len(self._events) + 1
        stored_event = {"id": event_id, "data": event}
        self._events.append(stored_event)
        return stored_event

    def get_events(self):
        return list(self._events)

    def get_event(self, event_id):
        for event in self._events:
            if event["id"] == event_id:
                return event
        return None


if __name__ == "__main__":
    store = EventStore()
    store.append({"type": "UserCreated", "name": "Alice"})
    store.append({"type": "UserUpdated", "name": "Alice", "age": 30})
    event = store.append({"type": "UserDeleted", "name": "Alice"})

    print("All events:")
    for e in store.get_events():
        print(e)

    print("\nLatest event:")
    print(event)

    print("\nFetched event #2:")
    print(store.get_event(2))

Output

stdout

All events:
{'id': 1, 'data': {'type': 'UserCreated', 'name': 'Alice'}}
{'id': 2, 'data': {'type': 'UserUpdated', 'name': 'Alice', 'age': 30}}
{'id': 3, 'data': {'type': 'UserDeleted', 'name': 'Alice'}}

Latest event:
{'id': 3, 'data': {'type': 'UserDeleted', 'name': 'Alice'}}

Fetched event #2:
{'id': 2, 'data': {'type': 'UserUpdated', 'name': 'Alice', 'age': 30}}

How it works

The EventStore class wraps a list _events that acts as an append-only log. Each append call computes the next sequential ID from the current list length, then stores a dict with both the ID and the raw event payload. get_events returns a shallow copy so callers can't mutate the internal list directly. get_event performs a simple linear scan over all events, which is fine for a mock or small in-memory store. The class simulates the core behavior of an event-sourcing system: you only append new facts, never update or delete history.

Common mistakes

Returning the internal list directly from get_events, allowing external mutation of the store
Forgetting that event IDs must be immutable — recomputing from list length can cause issues after future compaction or replay logic
Assuming get_event returns by value forever; with a shallow copy you get the same dict refs, so mutations leak back

Variations

Use a sqlite or Postgres table for durable storage with an AUTOINCREMENT ID column
Read from a JSON file on startup and re-append to an in-memory list, simulating event sourcing replay

Real-world use cases

Prototyping a microservice's event-sourcing layer before wiring up a real database.
Testing command handlers in unit tests with an in-memory event store that supports quick rollback.
Building a simple audit log for a legacy system without adding external infrastructure.

Event Sourcing Store in Python: Append-Only Log Mock

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Microservices patterns

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