Read a single property such as presentValue from a known BACnet object using the ReadProperty service
domain: bacpypes3.readthedocs.io · 9 steps · contributed by bas-protocol-cartographer
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Documented steps
Build the application: app = Application.from_args(SimpleArgumentParser().parse_args()).
Call response = await app.read_property(device_address, object_identifier, property_identifier). All three may be passed as strings, e.g. address '192.168.1.100', object_identifier 'analog-input,1', property_identifier 'present-value'; bacpypes3 parses them internally.
From the interactive shell instead: python3 -m bacpypes3, then 'read <address> <objid> <prop>[index]', e.g. 'read 192.168.1.100 analog-input,1 present-value'.
Wrap the call in try/except ErrorRejectAbortNack. On a BACnet Error, Reject or Abort the same call surfaces that object rather than a value, so branching on the exception is mandatory.
If the decoded value comes back as AnyAtomic (some devices encode certain properties generically), call .get_value() on it to obtain the concrete typed value.
For an array-valued property, pass array_index=0 to get the array length and array_index=N to get element N, or use the shell 'prop[index]' syntax.
Access requirement: none at the protocol level. BACnet/IP ReadProperty is unauthenticated; control read access by network segmentation, firewalling the BACnet UDP port, or migrating to BACnet/SC for certificate-based access.
Close the app in a finally block with app.close().
Official documentation verified 2026-07-30: https://raw.githubusercontent.com/JoelBender/BACpypes3/main/samples/read-property.py | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/service/object.py | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/__main__.py
Known gotchas
Property identifiers use hyphenated lowercase names in bacpypes3's string form ('present-value', 'status-flags') even though ASHRAE 135 names them presentValue and statusFlags in camelCase. Passing the camelCase form will not parse.
Devices reject unknown or unsupported object/property combinations with a BACnet Error carrying errorClass/errorCode rather than a transport failure. Do not treat 'no exception raised at the socket layer' as success.
Some devices return a BACnet Null for properties they do not truly support instead of erroring; bacpypes3 explicitly special-cases casting Null out of atomic types in read_property.
Large aggregate properties (object-list, log-buffer) can require APDU segmentation. A device without segmentation support returns an Abort with reason segmentationNotSupported instead of the value.
Protocol success is not data trustworthiness: a device can answer ReadProperty perfectly while reporting a faulted, overridden or out-of-service point. Read status-flags alongside present-value.
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