task(DOPE-584): Python function blocks reach class parity [phase 5]

[thiagoralves]

What this is

Phase 5 of DOPE-584: a Python function block now sees the same Variables Table an ST block does — the same gap phase 4 closed for C++, across a harder boundary.

Before this, a Python block could see its inputs and its outputs. A VAR, a VAR_IN_OUT or a VAR_EXTERNAL declared in the same table did not cross, so the block's code referred to a name the editor had accepted and Python raised NameError.

How each class travels

A Python block is a separate process, so nothing is passed by pointer — every variable is marshalled through two packed structs. The direction each class travels follows from what it means:

Class	In	Out	Why
input	✓		the caller supplies it
output		✓	the block produces it
inOut	✓	✓	what VAR_IN_OUT is
local	✓	✓	the PLC owns the storage — round-tripping is what makes a VAR the block's own state, keeps it debuggable, and lets it be retained
external	✓	✓	same, read and written through the global's own lock
temp	—	—	refused

VAR_TEMP means storage that does not survive the invocation, and Python has no such thing here: the block's variables are module globals in a process that outlives every scan. Marshalling it would not make it temporary, only a VAR wearing the wrong name. The refusal names the variable and says to declare it under VAR instead.

One selection rule

Four emitters had been filtering by class independently — the two structs, the copy loops, the format strings, and the LSP preamble. That is worse here than it was for C++: a field one side omits does not go missing, it shifts every later field's offset, so the corruption surfaces on unrelated variables. The rule now lives in block-interface and all four read it.

Widening to VAR proved immediately why the exclusion list matters. first_run, shm_in_ptr and shm_out_ptr are injected as locals, and the first hardware run swept them into the structs and handed the block its own mapped segment addresses. Python died every cycle and the loader respawned it dozens of times. They are named and excluded now, as hasBeenInitialized is on the C++ side.

Hardware verification

On slm-rp4, one block using all five classes. Over 195 Python cycles:

• the VAR accumulated to exactly 195 x 2
• the VAR_IN_OUT reached 195 and round-tripped to the calling program
• the VAR_EXTERNAL reached 195 x 2, with the configuration global agreeing

Sampled three times; every counter stayed exact. A VAR_TEMP is refused at compile time. The simulator path still stubs Python out and builds at 6% flash / 3% RAM.

Coverage gaps closed on the way

• Phase 2 added the WSTRING pack and unpack paths without tests, dropping injectPythonRuntime from 100%. Both directions are pinned now, including the truncation that must land on a code-unit boundary.
• generatePythonLspPreamble had never covered STRING or an array of an unmappable element type, and the list[ branch in its literal helper was dead code its own caller already owned. It is gone. This one predates the feature branch — src/frontend/utils/ was already below its 100% threshold on development.

Not in scope here

• CONSTANT / RETAIN / NON_RETAIN wait on NODE-94 (feat(retain): CONSTANT, RETAIN and NON_RETAIN [NODE-94] #1034), as for C++.
• Structures, enumerations and function block instances in a Python interface are still refused, by name and type. Phase 6.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UaSZK4LqFWtZpERcqnZ8uQ

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