Function from_bytecode

qiskit/qasm2/parse.py:205–315

Loop through the Rust bytecode iterator `bytecode` producing a :class:`~qiskit.circuit.QuantumCircuit` instance from it. All the hard work is done in Rust space where operations are faster; here, we're just about looping through the instructions as fast as possible, doing as little calc

(bytecode, custom_instructions: Iterable[CustomInstruction])

def from_bytecode(bytecode, custom_instructions: Iterable[CustomInstruction]):
    """Loop through the Rust bytecode iterator `bytecode` producing a
    :class:`~qiskit.circuit.QuantumCircuit` instance from it.  All the hard work is done in Rust
    space where operations are faster; here, we're just about looping through the instructions as
    fast as possible, doing as little calculation as we can in Python space.  The Python-space
    components are the vast majority of the runtime.

    The "bytecode", and so also this Python function, is very tightly coupled to the output of the
    Rust parser.  The bytecode itself is largely defined by Rust; from Python space, the iterator is
    over essentially a 2-tuple of `(opcode, operands)`.  The `operands` are fixed by Rust, and
    assumed to be correct by this function.

    The Rust code is responsible for all validation.  If this function causes any errors to be
    raised by Qiskit (except perhaps for some symbolic manipulations of `Parameter` objects), we
    should consider that a bug in the Rust code."""
    # The method `QuantumCircuit._append` is a semi-public method, so isn't really subject to
    # "protected access".

    qc = QuantumCircuit()
    qubits = []
    clbits = []
    gates = []
    has_u, has_cx = False, False
    for custom in custom_instructions:
        gates.append(custom.constructor)
        if custom.name == "U":
            has_u = True
        elif custom.name == "CX":
            has_cx = True
    if not has_u:
        gates.append(lib.UGate)
    if not has_cx:
        gates.append(lib.CXGate)
    # Pull this out as an explicit iterator so we can manually advance the loop in `DeclareGate`
    # contexts easily.
    bc = iter(bytecode)
    for op in bc:
        # We have to check `op.opcode` so many times, it's worth pulling out the extra attribute
        # access.  We should check the opcodes in order of their likelihood to be in the OQ2 program
        # for speed.  Gate applications are by far the most common for long programs.  This function
        # is deliberately long and does not use hashmaps or function lookups for speed in
        # Python-space.
        opcode = op.opcode
        # `OpCode` is an `enum` in Rust, but its instances don't have the same singleton property as
        # Python `enum.Enum` objects.
        if opcode == OpCode.Gate:
            gate_id, parameters, op_qubits = op.operands
            qc._append(
                CircuitInstruction(gates[gate_id](*parameters), [qubits[q] for q in op_qubits])
            )
        elif opcode == OpCode.ConditionedGate:
            gate_id, parameters, op_qubits, creg, value = op.operands
            with qc.if_test((qc.cregs[creg], value)):
                gate = gates[gate_id](*parameters)
                qc.append(gate, [qubits[q] for q in op_qubits])
        elif opcode == OpCode.Measure:
            qubit, clbit = op.operands
            qc._append(CircuitInstruction(Measure(), (qubits[qubit],), (clbits[clbit],)))