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Decorators with arguments?


Help please? Creating an MQTT-to-Firestore bridge and I know a decorator
would help but I'm stumped how to create one. I've used decorators before
but not with arguments.

The Firestore collection.on_snapshot() method invokes a callback and sends
it three parameters (collection_snapshot, changes, and read_time). I need
the callback to also know the name of the collection so that I can publish
to the equivalent MQTT topic name. I had thought to add a fourth parameter
and I believe a decorator is the right approach but am stumped how to add
that fourth parameter. How would I do this with the code below?

#!/usr/bin/env python3
from google.cloud import firestore
import firebase_admin
from firebase_admin import credentials
import json
import mqtt

firebase_admin.initialize_app(credentials.Certificate("certs/firebase.json"))
db = firestore.Client()
mqtt.connect()


def load_json(contents):
    try:
        return json.loads(contents)
    except (json.decoder.JSONDecodeError, TypeError):
        return contents


def on_snapshot(col_name, col_snapshot, changes, read_time):
    data = dict()
    for doc in col_snapshot:
        serial = doc.id
        contents = load_json(doc.to_dict()['value'])
        data[serial] = contents
    for change in changes:
        serial = change.document.id
        mqtt_topic = col_name + '/' + serial
        contents = data[serial]
        if change.type.name in ['ADDED', 'MODIFIED']:
            mqtt.publish(mqtt_topic, contents)
        elif change.type.name == 'REMOVED':
            mqtt.publish(mqtt_topic, None)


# Start repeated code section
# TODO Better to use decorators but I was stumped on how to pass arguments
def door_status_on_snapshot(col_snapshot, changes, read_time):
    on_snapshot('door_status', col_snapshot, changes, read_time)


door_status_col_ref = db.collection('door_status')
door_status_col_watch =
door_status_col_ref.on_snapshot(door_status_on_snapshot)

# Repetition...
def cpu_temp_on_snapshot(col_snapshot, changes, read_time):
    on_snapshot('cpu_temp', col_snapshot, changes, read_time)


cpu_temp_col_ref = db.collection('cpu_temp')
cpu_temp_col_watch = cpu_temp_col_ref.on_snapshot(cpu_temp_on_snapshot)
# End repeated code section

# Start repeated code section
door_status_col_watch.unsubscribe()
cpu_temp_col_watch.unsubscribe()
# Repetition...
# End repeated code section

Christopher de Vidal

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