The original PR #325 was split into 2, for the previous set of changes look at that PR description.

• Defined __slots__ for all the Abstract Gate classes and standard gate classes for better memory efficiency and faster lookup time.

• Added boolean is_controlled, is_parametric method to the Gate class.

• Added the concept and implementation of namespaces for operations, this is done because at several places earlier gate.name was being used for comparision. Each operation (gate for now) must now be defined in a namespace or if it is a temporary gate, namespace can be set to None. This allows the user to define faulty version of standard gates or follow their own convention for certain Parametric gates. The concept of namespace is much broader. Currently the parent namespace is "std", under it only "gates" is set as of now. In future we can make other namespaces under "std" like for Arithmetic Ops, Ansatz Ops etc. For the idea of Ops look at Refactor the Gate class objects #263 (comment) Though it hasn't been implemented yet. In a way std namespace will work like a standard library for different operations and the user can also define his own implementation for operations in a different namespace.

• Added caching in get_qobj method for standard gates. This is done for two reasons.

@staticmethod
def get_qobj() -> Qobj:
    return Qobj([[1, 0], [0, -1]])

if you run it multiple times and check id() for each obj, they will be different. This leads to higher memory consumption for large circuit simulations, even though all of them represent the Z gate's qobj in this case. Additonally different qobjs will each need to be garbage collected. Memory and lookup is an even larger bottleneck in VQA circuits where several times many gates share the same parameter.

• Added dtype to get_qobj(dtype="dense") method, . This might be useful if the circuit simulation in run on a different backend like JAX, CUDA.

• Made Gate non initialised by default i.e. __init__ raise will raise an Error. ObviouslyParametricGate and controlled version of them are initialisable. This makes sense because target, controls have been removed from gates and x1=X(), x2=X() now represent the same thing. Also good from memory point of view.

• Added Sdag, Tdag, SQRTXdag, ISWAPdag, SQRTSWAPdag, BERKELEYdag to the standard gates. They have been added to the documentation table and in circuit draw (colour config for them).

• Added inverse method for Gate class. Also implemented inverse for all standard gates and tests for the same. For controlled gates, the inverse is auto calculated based on the target gate. Closes Add a gate inversion function #301

• Renamed unitary_gate to get_unitary_gate and added checks for all the input arguments.

• The control value for a gate is now baked in to the control gate itself. The user can always generate his own version of controlled gate e.g. controlled(gates.X, num_ctrl_qubits=2, control_value=0b10). For more detail check out Refactor Gate class #308 (comment)

• For all Controlled gates, the (target_gate, num_ctrl_qubit, ctrl_value) is used as another key in the namespace to reference to that controlled gate. This is done in order to prevent regeneration of controlled gate class for a given target gate with known (num_ctrl_qubit, ctrl_value). Since namespace is used by default, there isn't any overhead.

• Renamed controlled_gate to controlled_gate_unitary and resolved the bug which didn't allow for generating the correct qobj if number of controls > 1.

• Added gate equivalence checking for ParametricGate, ControlledGate. And added __eq__, __hash__ in Gate metaclass as well. Replaced gate.name = "X" search with gate == X now.

• Corrected inconsistency between Gate and Type[Gate] from previous PRs. Gate refers to an instantiated object while type[Gate] to a Gate class or subclass in accordance with Python's typehinting.

• Added several more checks in __init_subclass__, add_gate method.

• Added tests in test_gates.py and test_circuit.py mostly incorrect cases like num_qubits being negative etc. which should raise an Error. Since Gates are used through the codebase, their normal usage is already mostly covered.

Maintainence and Bug Fixes:

• gate_product had a clear bug that tensor_list was not defined but was being used, fixed that and also moved the entire gate_product logic to operations which was split in circuit-simulator utils, operations but was always being imported from operations as per __init__ even in mat_mul_simulator.py.

• Removed depreciated arguments, and functions which were depreciated 3 years back. This was checked in the git commit history.

• Remove mutable default values from function arguments across the codebase, this is a common Python bug. Something like targets = [], can often lead to error. For details check out this blog https://medium.com/@matbrizolla/beware-of-the-hidden-trap-understanding-mutable-default-arguments-in-python-d10e8b0a7b72

• Corrected several isInstance checks in the codebase. Iterable and Sequence were being used interchangeably at several places especially in qasm. This led to several unexpected behaviour and errors during gate refactoring.

• Moved pytest.ini to the root, this is what pytest themselves recommend https://docs.pytest.org/en/stable/explanation/goodpractices.html#choosing-a-test-layout-import-rules . The reason was placing pytest.ini in tests/ lead to certain sys path issues, for reference check https://docs.pytest.org/en/stable/explanation/pythonpath.html. This caused an error when standard gates were designated to a namespace and pytest was trying to redefine them and reassign them to the same namespace.

Edit: Some more changes

• Separated IDLE (meant for Pulse level simulation) and Identity gate (standard Identity matrix).

• In Pulse compiler instead of gate.name, gates are themselves being used as keys.

• Removed expand argument from inverse() in Parametric Gates.

• Renamed controlled to get_controlled_gate.