Add a new QuantumCircuit constructor QuantumCircuit.from_instructions

What should we add?

Creating a QuantumCircuit and appending operations into it is a very common pattern. It would be very convenient to be able to create a QuantumCircuit by directly passing an iterator of operations, something like:

@staticmethod
def from_ops(ops: Iterator[tuple[Instruction, Sequence[Qubit], Sequence[Clbit]]]) -> QuantumCircuit:
    circuit = QuantumCircuit()
    for op in ops:
        circuit.append(*ops)
    return circuit

Currently this is not possible as written but it would be if my proposal https://github.com/Qiskit/qiskit-terra/issues/8707 were implemented.

[jakelishman]

It’s important for it to be a sequence, because it defines the order that bits are applied to it when a circuit is used as an Instruction definition, for example - we associate CircuitInstruction.qubits with the QuantumCircuit.qubits by equal indices in the sequence. We do actually have the set view already, it’s just a private attribute - it’s QuantumCircuit._qubit_indices, which is a dict. (I forgot about that when I was saying we’d need to track the set separately.)

[jakelishman]

For a community member wanting to have a go at this, the rough interface would look like

class QuantumCircuit
    @classmethod
    def from_instructions(cls, instructions: Iterable[CircuitInstruction], *, name=None, metadata=None, global_phase=None):
        out = cls(name=name, metadata=metadata, global_phase=global_phase)
        # ...

CircuitInstruction always contains only Qubit and Clbit instances, so we’d need to track the bits used in a set, and append them to the circuit before appending the instruction if they’re not already there. We’d also need to take care to ensure that any bits or registers that appear in the condition field of an Instruction instance are added as well. For control-flow instructions, the method would need to inspect the blocks field (which contains QuantumCircuit instances, and ensure any registers in there are also in the top-level circuit.