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qiskit.opflow.expectations.ExpectationFactory.build

static ExpectationFactory.build(operator, backend=None, include_custom=True)[source]

A factory method for convenient automatic selection of an Expectation based on the Operator to be converted and backend used to sample the expectation value.

Parameters
  • operator (OperatorBase) – The Operator whose expectation value will be taken.

  • backend (Union[Backend, QuantumInstance, None]) – The backend which will be used to sample the expectation value.

  • include_custom (bool) – Whether the factory will include the (Aer) specific custom expectations if their behavior against the backend might not be as expected. For instance when using Aer qasm_simulator with paulis the Aer snapshot can be used but the outcome lacks shot noise and hence does not intuitively behave overall as people might expect when choosing a qasm_simulator. It is however fast as long as the more state vector like behavior is acceptable.

Return type

ExpectationBase

Returns

The expectation algorithm which best fits the Operator and backend.

Raises

ValueError – If operator is not of a composition for which we know the best Expectation method.