Decorators with arguments?

[Christopher de Vidal]

Peter Otten, Cameron Simpson, thank you for your detailed replies :-) I
confess, I didn't quite understand all you were saying. (Still only an
intermediate-level programmer.) But Cameron what you said questioning my
use of decorators and maybe a class instead got me thinking. I realized
what I needed was a function within a function. Couldn't have gotten there
without your help. Working code:

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

def bridge(col_name):
    def on_snapshot(col_snapshot, changes, read_time):
        data = dict()
        for doc in col_snapshot:
            serial = doc.id
            data[serial] = json.dumps(doc.to_dict()['value'])
        for change in changes:
            serial = change.document.id
            mqtt_topic = col_name + '/outgoing/' + serial
            if change.type.name in ['ADDED', 'MODIFIED']:
                contents = data[serial]
                mqtt.publish(mqtt_topic, contents)
            elif change.type.name == 'REMOVED':
                mqtt.publish(mqtt_topic, '')


    @mqtt.incoming
    def mqtt_subscribe(serial, value):
        # TODO Passing a None entry to delete from MQTT doesn't trigger
this
        #   callback, so it doesn't delete from Firestore. Need this ugly
        #   workaround 'clear_mqtt'.
        if value == 'clear_mqtt':
            value = None
            mqtt.publish(col_name + '/incoming/' + serial, None)
            mqtt.publish(col_name + '/outgoing/' + serial, None)
        db.collection(col_name).document(serial).set({'value': value})


    col_watch = db.collection(col_name).on_snapshot(on_snapshot)
    mqtt.subscribe(col_name + '/incoming/#', mqtt_subscribe)
    return col_watch


cred = credentials.Certificate("certs/firebase.json")
firebase_admin.initialize_app(cred)
db = firestore.Client()
mqtt.connect()
adapters = list()
for collection in ['door_status', 'cpu_temp']:
    adapters.append(bridge(collection))
while True:
    sleep(1)
for adapter in adapters:
    adapter.unsubscribe()

Christopher de Vidal

Would you consider yourself a good person? Have you ever taken the 'Good
Person' test? It's a fascinating five minute quiz. Google it.


On Fri, May 15, 2020 at 9:55 AM Peter Otten <__peter__ at web.de> wrote:

> Christopher de Vidal wrote:
>
> > 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
>
> You might also consider a contextmanager:
>
> https://docs.python.org/3/library/contextlib.html
>
> # untested
>
> @contextmanager
> def subscribe(name, col_snapshot, changes, read_time):
>     def status_on_snapshot(col_snapshot, changes, read_time):
>         on_snapshot(name, col_snapshot, changes, read_time)
>
>     status_col_ref = db.collection(name)
>     status_col_watch = status_col_ref.on_snapshot(door_status_on_snapshot)
>     try:
>         yield status_col_ref
>     finally:
>         status_col_watch.unsubscribe()
>
>
> with subscribe("door_status", ...) as door_status_col_ref:
>     with subscribe("cpu_temp", ...) as cpu_temp_col_ref:
>         ...
>
>
> If there are many uniform ones the nested with statements can be
> generalized:
>
> NAMES = "door_status", "cpu_temp", ...
> with ExitStack() as stack:
>     col_refs = [
>         stack.enter_context(subscribe(name)) for name in NAMES
>     ]
>
> And if you like Camoron's suggestion or the subscribe() generator above
> just
> gets too unwieldy: a custom class can act as a contextmanager, too.
>
> https://docs.python.org/3/reference/compound_stmts.html#with
>
> --
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>