Joseph DohertyS7 integration-test fixture — python-snap7
python-snap7 Server class
wrapped in a pinned python:3.12-slim-bookworm image. Docker is the
only supported launch path — a fresh clone needs Docker Desktop and
nothing else.
| File | Purpose |
|---|---|
Dockerfile |
python:3.12-slim-bookworm + python-snap7>=2.0 + the server shim + the profile JSONs |
docker-compose.yml |
One service per profile; currently only s7_1500 |
server.py |
Same Python shim the native fallback uses — copy kept in the build context |
profiles/*.json |
Area-seed definitions (DB1 / MB layouts with typed seeds) |
Run
From the repo root:
docker compose -f tests\Drivers\ZB.MOM.WW.OtOpcUa.Driver.S7.IntegrationTests\Docker\docker-compose.yml --profile s7_1500 up
Detached + stop:
docker compose -f tests\...\Docker\docker-compose.yml --profile s7_1500 up -d
docker compose -f tests\...\Docker\docker-compose.yml --profile s7_1500 down
⚠️ Editing a profile requires --build
Dockerfile COPYs profiles/ into the image, so the seeds are baked in at
build time — they are not bind-mounted. A plain up -d, and even
up -d --force-recreate, reuses the existing otopcua-python-snap7:1.0 image
and silently keeps serving the OLD seeds. After editing a profile JSON:
docker compose --profile s7_1500 up -d --build --force-recreate
Confirm it took: docker logs otopcua-python-snap7-s7_1500 prints
seeded DB1 size=1024 seeds=<n> at startup — check <n> matches the seed count
you just wrote. This is the fastest way to tell a stale image from a bad address.
On the shared Docker host the deployment mirror at /opt/otopcua-s7/ is
root-owned, so a plain rsync fails with status 23 and leaves the old
files in place while still printing the file list. Push with:
rsync -av --rsync-path="sudo rsync" \
tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.IntegrationTests/Docker/ \
dohertj2@10.100.0.35:/opt/otopcua-s7/
Endpoint
- Default:
localhost:1102(non-privileged; sidesteps Windows Firewall prompt + Linux's root-required bind on port 102). - Override with
S7_SIM_ENDPOINTto point at a real S7 CPU on:102. - The driver's S7DriverOptions.Port flows through S7netplus's 5-arg
Plc(CpuType, host, port, rack, slot)ctor so the non-standard port works end-to-end.
Run the integration tests
In a separate shell with the container up:
cd C:\Users\dohertj2\Desktop\lmxopcua
dotnet test tests\Drivers\ZB.MOM.WW.OtOpcUa.Driver.S7.IntegrationTests
Snap7ServerFixture probes localhost:1102 at collection init + records
a SkipReason when unreachable, so tests stay green on a fresh clone
without Docker running.
What's encoded in profiles/s7_1500.json
DB1 (1024 bytes) + MB (256 bytes) with typed seeds at known offsets:
| Address | Type | Seed | Purpose |
|---|---|---|---|
DB1.DBW0 |
u16 | 4242 |
read-back probe |
DB1.DBW10 |
i16 | -12345 |
smoke i16 read |
DB1.DBD20 |
i32 | 1234567890 |
smoke i32 read |
DB1.DBD30 |
f32 | 3.14159 |
smoke f32 read (big-endian) |
DB1.DBX50.3 |
bool | true |
smoke bool read at bit 3 |
DB1.DBW100 |
u16 | 0 |
scratch for write-then-read |
DB1.STRING[200] |
S7 STRING | "Hello" |
S7 STRING read |
MW0 |
u16 | 1 |
S7ProbeOptions.ProbeAddress default |
Seed types supported: u8, i8, u16, i16, u32, i32, f32,
bool (with "bit": 0..7), ascii (S7 STRING).
Span-planner gate seeds (offsets 300+, S7_1500SpanReadTests)
These exist to give S7ReadSpanPlanner a layout with a known partition under
the shipped 16-byte-gap / 220-byte-span defaults — the offsets are the test, so
moving one changes what the gate proves.
| Address | Type | Seed | Role in the partition |
|---|---|---|---|
DB1.DBW300 |
u16 | 51001 |
cluster A — merges with the four below into ONE 300..320 span |
DB1.DBW302 |
i16 | -21001 |
cluster A (gap 0) |
DB1.DBD304 |
i32 | 305419896 |
cluster A (gap 0) |
DB1.DBX310.5 |
bool | true |
cluster A (gap 2) |
DB1.DBD316 |
f32 | 2.7182817 |
cluster A (gap 5); seals the span at 20 bytes |
DB1.DBW400 |
u16 | 40201 |
cluster B — 80 bytes on, so the gap budget forces a SECOND span |
DB1.DBW402 |
u16 | 40202 |
cluster B's second member (keeps it off the PrefersPerTagRead demotion) |
DB1.DBW500 |
u16 | 60001 |
isolated ⇒ demoted single-member span ⇒ multi-var leftover |
DB1.DBW600 |
i16 | -32001 |
isolated ⇒ multi-var leftover |
MW10 |
u16 | 4097 |
multi-var leftover in a different area — a block read can't cross (area, DB), multi-var can |
Offsets 308, 311 and 312 carry poison (0xDEAD, 0x5A, 0xDEADBEEF)
inside the holes cluster A's span reads through. No tag maps to them; they are
there so a mis-computed slice offset returns garbage instead of a zero that could
match by luck. Every seed above is distinct and non-zero for the same reason.
Known limitations
From the snap7.server.Server docstring upstream:
"Legacy S7 server implementation. Emulates a Siemens S7 PLC for testing and development purposes. [...] pure Python emulator implementation that simulates PLC behaviour for protocol compliance testing rather than full industrial-grade functionality."
Not exercised here — needs a lab rig:
- S7-1500 Optimized-DB symbolic access
- PG / OP / S7-Basic session-type differentiation
- PUT/GET-disabled-by-default enforcement
See docs/drivers/S7-Test-Fixture.md
for the full coverage map.
References
- python-snap7 GitHub
docs/drivers/S7-Test-Fixture.md— coverage mapdocs/v2/dev-environment.md§Docker fixtures