CircuitInstruction

There is some possible confusion in the names of this class, Instruction, and Operation, and this class’s attribute operation. Our preferred terminology is by analogy to assembly languages, where an “instruction” is made up of an “operation” and its “operands”.

Historically, Instruction came first, and originally contained the qubits it operated on and any parameters, so it was a true “instruction”. Over time, QuantumCircuit became responsible for tracking qubits and clbits, and the class became better described as an “operation”. Changing the name of such a core object would be a very unpleasant API break for users, and so we have stuck with it.

This class was created to provide a formal “instruction” context object in QuantumCircuit.data, which had long been made of ad-hoc tuples. With this, and the advent of the Operation interface for adding more complex objects to circuits, we took the opportunity to correct the historical naming. For the time being, this leads to an awkward case where CircuitInstruction.operation is often an Instruction instance (Instruction implements the Operation interface), but as the Operation interface gains more use, this confusion will hopefully abate.

This is a lightweight internal class and there is minimal error checking; you must respect the type hints when using it. It is the user’s responsibility to ensure that direct mutations of the object do not invalidate the types, nor the restrictions placed on it by its context. Typically this will mean, for example, that qubits must be a sequence of distinct items, with no duplicates.