dispatch() raises UnboundLocalError instead of NumberOfModesInvalidError for an out-of-domain mode count

[robertodr]

🤖 AI text below 🤖

What

The dispatch() function emitted by tools/generate-dispatch.py (_core_dispatch_source) has no
terminal case, so an argument matching none of its arms falls through to a read of an unbound local:

def dispatch(num_modes: int) -> type[_SimulatorAdapter]:
    match num_modes:
        case n if n <= 0:
            raise NumberOfModesInvalidError(errmsg)
        case 1:
            cls = MonomialPropagator001
        ...
        case 250:
            cls = MonomialPropagator250
        case n if n > 250:
            raise NumberOfModesInvalidError(errmsg)

    return cls          # <-- UnboundLocalError if nothing matched

Why this is a problem

The arms cover n <= 0, the integer literals 1..MAX, and n > MAX. Anything else — a non-integral
value, or None — reaches return cls with cls never assigned, and the user sees

UnboundLocalError: cannot access local variable 'cls' where it is not associated with a value

instead of the NumberOfModesInvalidError the module defines for exactly this purpose.

None is reachable in practice: MajoranaOperator._from_terms(..., num_modes=None)
(src/monoprop/majorana.py) leaves num_modes unset, and _init_simulator passes
majorana_operator.num_modes straight into dispatch()
(src/monoprop/monomial_propagator.py:105,121).

Suggested fix

Add a terminal arm that raises NumberOfModesInvalidError with the received value and its type.

Note this overlaps with the dispatch-module simplification (see the separate issue on collapsing the
generated _dispatch.py): that change replaces the match with an explicit guard, at which point
this becomes a plain if. If both land in the same pass, fix it there rather than twice.

Verification

pytest case asserting NumberOfModesInvalidError for dispatch(None) and for a non-integral value.

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Found by a code-reading review of the repository at 29a8050. No build tree was available, so the
analysis is from source inspection and should be confirmed against a build.