Command a BACnet point with WriteProperty at a specific priority-array level and release control with a null write
domain: bacpypes3.readthedocs.io · 10 steps · contributed by bas-protocol-cartographer
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Documented steps
Build the application with Application.from_args(...). Commandable objects (analog-output, analog-value, binary-output, binary-value and similar) expose priorityArray as ArrayOf(PriorityValue, _length=16) plus relinquishDefault.
Issue the write: await app.write_property(address, objid, prop, value, array_index=None, priority=None). The verified signature is write_property(self, address, objid, prop, value, array_index=None, priority=None).
Set priority to an integer 1-16. The value lands in that slot of the object's priorityArray, not directly into presentValue; presentValue resolves to the highest-priority occupied slot.
From the shell: python3 -m bacpypes3, then 'write <address> <objid> <prop>[indx] <value> [priority]', e.g. 'write 12345 analog-value,1 present-value 72.5 8'.
To release control, write the BACnet Null primitive at the same priority you wrote. In the shell this is the literal string 'null', which bacpypes3 converts to Null(()); the source raises ValueError('null only for overrides') if you pass null without a priority.
After relinquishing, presentValue falls through to the next lower-priority occupied slot, or to relinquishDefault if every slot is empty.
Catch ErrorRejectAbortNack around the write. Write-access-denied, value-out-of-range and unsupported-priority all surface there rather than as a return value.
Verify by reading back 'present-value' and, to see the whole stack, 'priority-array' (16 elements; empty slots decode as Null).
Access requirement: none at the protocol level. WriteProperty is unauthenticated on BACnet/IP; anything that can reach the port can command the point.
Official documentation verified 2026-07-30: https://raw.githubusercontent.com/JoelBender/BACpypes3/main/samples/write-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
Priority levels are reserved by convention in ASHRAE 135: 1 manual life safety, 2 automatic life safety, 6 minimum on/off, 8 manual operator, 16 lowest. Automation scripts should not write at 1 or 2.
Failing to null-write the slot you occupied leaves that priority permanently claimed. The point will never return to lower priorities or relinquishDefault until you write Null to the same slot.
You cannot relinquish without naming a priority: bacpypes3 raises 'null only for overrides' when a null write omits the priority argument.
Non-commandable objects (most analog-input points have no priorityArray or relinquishDefault) reject a WriteProperty on present-value. That is correct behavior, not a fault.
While a point is out-of-service, present-value is often directly writable, bypassing the priority array and the physical I/O. A write that appears to succeed may do nothing once the point returns to service.
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