PyDIGI — read DIGI scales from Python

pydigi reads weight, price and status from DIGI retail scales over RS-232.

Supported models:

• DIGI DS-781 (Type B / Standard command protocol).

Install

pip install pydigi

Quick start

from pydigi import DigiDS781

with DigiDS781.open("/dev/ttyUSB0") as scale:      # COM3 on Windows
    r = scale.read()
    print(r.weight_net_kg, "kg", "stable" if r.is_stable else "moving")

DigiDS781.open(port, baudrate=..., parity=..., stopbits=..., timeout=..., rtscts=...) — extra keywords pass through to the serial config. open() returns a ready scale and is also a context manager.

What a reading contains

scale.read() returns an immutable ScaleReading:

A weight the scale can't report (overflow / underflow) comes back as None, never a fake 0.0. Prices are None only when the field is blank (e.g. total price during an overflow); with no PLU the scale sends real zeros, so they read 0.0. reading.as_dict() gives a JSON-friendly view.

Continuous reading & change-watch

for reading in scale.stream(interval=0.2):
    print(reading.weight_gross_kg)

watch() yields only when a field you care about changes — you tick the fields with a ChangeFilter:

from pydigi import ChangeFilter, Field

watch_weight = ChangeFilter.weight(min_delta_kg=0.005, stable_only=True)  # the default
for reading in scale.watch(interval=0.2, change_filter=watch_weight):
    print("weight ->", reading.weight_gross_kg, "kg")

ChangeFilter([Field.NET, Field.TARE, Field.UNIT_PRICE])   # several fields
ChangeFilter.everything()                                 # any field, flags included

For long-running loops, stream(..., ignore_errors=True, on_error=cb) keeps going through transient timeouts instead of raising.

Runnable examples: single_reading.py, continuous_reading.py, and offline_demo.py (no hardware).

Errors

All exceptions derive from PyDigiError:

PyDigiError
├── TransportError        # port open/read/write failed
│   └── ScaleTimeout      # no response within timeout / after retries
└── FrameError            # a frame arrived but was malformed
    ├── ShortFrameError · HeaderError · FieldParseError · ParityError

Catch PyDigiError broadly, ScaleTimeout to retry, or FrameError to log-and-skip a garbled frame in a stream without tearing down the port.

Testing without hardware

The serial layer is a pluggable Transport. Swap in LoopbackTransport (which replays canned bytes) and bind() to a scale — no port required:

from pydigi import DigiDS781, LoopbackTransport

with DigiDS781.bind(LoopbackTransport(my_frame_bytes)) as scale:
    print(scale.read().weight_net_kg)

This is exactly how the test suite runs. A complete, runnable version (which synthesises frames) is examples/offline_demo.py; the testing workflow is in TESTING.md.

Command line

pydigi --port /dev/ttyUSB0 read                       # one reading
pydigi --port /dev/ttyUSB0 read --json                # machine-readable
pydigi --port /dev/ttyUSB0 stream --interval 0.5 --count 10
pydigi --port /dev/ttyUSB0 watch --stable-only        # weight changes only
pydigi --port /dev/ttyUSB0 watch --field tare --field unit-price
pydigi --port /dev/ttyUSB0 watch --all-fields
pydigi --port /dev/ttyUSB0 watch --forever            # never stop; ride out device outages
pydigi --port /dev/ttyUSB0 --retries 5 read           # retry each poll up to 5 times
pydigi --model ds781 --port COM3 read                 # -v / -vv for logging
pydigi list-models

Extending

The library is three seams; a new device touches only one and needs no changes to existing code:

• New model (same protocol, different defaults) — subclass ScaleModel, set the class attributes, @register:

  from pydigi import ScaleModel, register
  from pydigi import TypeBProtocol
  
  @register
  class DigiDS782(ScaleModel):
      name = "ds782"
      protocol_class = TypeBProtocol
      default_baudrate = 9600
      max_weight_kg = 15.0
  

  DigiDS782.open(port) and pydigi --model ds782 work immediately.

• New protocol (Type A/C, another vendor) — subclass Protocol (poll_request / read_frame / parse) and point a model at it.

• New transport (TCP bridge, USB HID) — subclass Transport and pass it to Model.bind(transport).

Details and rationale in DESIGN.md.

Scale is not thread-safe — a serial port is a single shared resource. Use one Scale per thread, or guard it with your own lock.

Development

make test           # hardware-free test suite
make build          # wheel into ./dist
make docker-build   # same, isolated in Docker

Testing (hardware-free and on a real scale) is documented in TESTING.md; architecture in DESIGN.md.

Vibe-code disclosure

This library is written heavily with Claude. The design is decided upon by the initial human-author. All hands-on tests are conducted by a human-operator.

The human-conducted code review (HCCR) is an optimisation of a sort. It will be done in the right time.

License

MIT — see LICENSE.