# Protocols This module provides various protocols used throughout the package. A [`typing.Protocol`](https://docs.python.org/3/library/typing.html#typing.Protocol) is an interface that a class can fulfill structurally, without explicitly inheriting from it. For example, any class that implements a `__len__` method satisfies [`Sized`](https://docs.python.org/3/library/typing.html#typing.Sized), whether or not it says so anywhere in its class hierarchy. The protocols in this module follow the same idea, but each one centers on a single, otherwise unremarkable dunder-style method (for example `_majorana_operator_`). A class implementing that one method thereby satisfies the corresponding protocol ([`SupportsMajoranaOperator`](../stubs/qiskit_fermions.protocols.SupportsMajoranaOperator.html.md#qiskit_fermions.protocols.SupportsMajoranaOperator) in this example) and becomes usable wherever that protocol is expected, without any explicit registration. Note that “protocol” is sometimes also used loosely to refer to a type-agnostic dispatch function built on top of a protocol (for example [`fermion_operator()`](../stubs/qiskit_fermions.mappers.library.fermion_operator.html.md#qiskit_fermions.mappers.library.fermion_operator)) rather than the interface itself; the distinction should be clear from context. Some protocols below are paired with a small, type-agnostic **helper function** that dispatches to the protocol method on whatever object it is given. | Protocol | Method | Helper function | |----------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [`SupportsFermionOperator`](../stubs/qiskit_fermions.protocols.SupportsFermionOperator.html.md#qiskit_fermions.protocols.SupportsFermionOperator) | `_fermion_operator_(self)` | [`fermion_operator()`](../stubs/qiskit_fermions.mappers.library.fermion_operator.html.md#qiskit_fermions.mappers.library.fermion_operator) | | [`SupportsMajoranaOperator`](../stubs/qiskit_fermions.protocols.SupportsMajoranaOperator.html.md#qiskit_fermions.protocols.SupportsMajoranaOperator) | `_majorana_operator_(self)` | [`majorana_operator()`](../stubs/qiskit_fermions.mappers.library.majorana_operator.html.md#qiskit_fermions.mappers.library.majorana_operator) | | [`SupportsApplyUnitaryPlaced`](../stubs/qiskit_fermions.protocols.SupportsApplyUnitaryPlaced.html.md#qiskit_fermions.protocols.SupportsApplyUnitaryPlaced) | `_apply_unitary_placed_(self, vec, norb, nelec, copy, freg_indices)` | – | | [`SupportsCommutators`](../stubs/qiskit_fermions.protocols.SupportsCommutators.html.md#qiskit_fermions.protocols.SupportsCommutators) | `_commutator_`, `_anti_commutator_`, `_double_commutator_` (all `@staticmethod`) | [`commutator()`](../stubs/qiskit_fermions.operators.library.commutator.html.md#qiskit_fermions.operators.library.commutator), [`anti_commutator()`](../stubs/qiskit_fermions.operators.library.anti_commutator.html.md#qiskit_fermions.operators.library.anti_commutator), [`double_commutator()`](../stubs/qiskit_fermions.operators.library.double_commutator.html.md#qiskit_fermions.operators.library.double_commutator) | ## Protocols implemented from ffsim Simulation is [ffsim](https://qiskit-community.github.io/ffsim/)’s concern, and its protocols are defined there rather than restated here. The gates and operators of this package implement the following, so ffsim’s dispatchers accept them directly: | ffsim protocol | Method | Implemented by | |-----------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [`ffsim.SupportsApplyUnitary`](https://qiskit-community.github.io/ffsim/api/stubs/ffsim.SupportsApplyUnitary.html#ffsim.SupportsApplyUnitary) | `_apply_unitary_(self, vec, norb, nelec, copy)` | [`FermionicCircuit`](../stubs/qiskit_fermions.circuit.FermionicCircuit.html.md#qiskit_fermions.circuit.FermionicCircuit) and every [`FermionicGate`](../stubs/qiskit_fermions.circuit.FermionicGate.html.md#qiskit_fermions.circuit.FermionicGate) | | [`ffsim.SupportsLinearOperator`](https://qiskit-community.github.io/ffsim/api/stubs/ffsim.SupportsLinearOperator.html#ffsim.SupportsLinearOperator) | `_linear_operator_(self, norb, nelec)` | [`FermionOperator`](../stubs/qiskit_fermions.operators.FermionOperator.html.md#qiskit_fermions.operators.FermionOperator) | | [`ffsim.SupportsTrace`](https://qiskit-community.github.io/ffsim/api/stubs/ffsim.SupportsTrace.html#ffsim.SupportsTrace) | `_trace_(self, norb, nelec)` | [`FermionOperator`](../stubs/qiskit_fermions.operators.FermionOperator.html.md#qiskit_fermions.operators.FermionOperator) | #### CAUTION [`ffsim.SupportsFermionOperator`](https://qiskit-community.github.io/ffsim/api/stubs/ffsim.SupportsFermionOperator.html#ffsim.SupportsFermionOperator) shares the `_fermion_operator_` method name with this package’s [`SupportsFermionOperator`](../stubs/qiskit_fermions.protocols.SupportsFermionOperator.html.md#qiskit_fermions.protocols.SupportsFermionOperator), but the two are *not* the same contract: ours returns this package’s [`FermionOperator`](../stubs/qiskit_fermions.operators.FermionOperator.html.md#qiskit_fermions.operators.FermionOperator) rather than ffsim’s unrelated type. Since [`ffsim.fermion_operator()`](https://qiskit-community.github.io/ffsim/api/stubs/ffsim.fermion_operator.html#ffsim.fermion_operator) dispatches purely on the method name, calling it on an object of this package returns our type.