43 lines
2.3 KiB
Plaintext
43 lines
2.3 KiB
Plaintext
---
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title: ExpectationFactory
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description: API reference for qiskit.opflow.expectations.ExpectationFactory
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in_page_toc_min_heading_level: 1
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python_api_type: class
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python_api_name: qiskit.opflow.expectations.ExpectationFactory
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---
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# ExpectationFactory
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<Class id="qiskit.opflow.expectations.ExpectationFactory" isDedicatedPage={true} github="https://github.com/qiskit/qiskit/tree/stable/0.21/qiskit/opflow/expectations/expectation_factory.py" signature="ExpectationFactory" modifiers="class">
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Bases: `object`
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A factory class for convenient automatic selection of an Expectation based on the Operator to be converted and backend used to sample the expectation value.
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## Methods Defined Here
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### build
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<Function id="qiskit.opflow.expectations.ExpectationFactory.build" signature="ExpectationFactory.build(operator, backend=None, include_custom=True)" modifiers="static">
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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.
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**Parameters**
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* **operator** ([`OperatorBase`](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")) – The Operator whose expectation value will be taken.
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* **backend** (`Union`\[[`Backend`](qiskit.providers.Backend "qiskit.providers.backend.Backend"), [`QuantumInstance`](qiskit.utils.QuantumInstance "qiskit.utils.quantum_instance.QuantumInstance"), `None`]) – The backend which will be used to sample the expectation value.
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* **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.
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**Return type**
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[`ExpectationBase`](qiskit.opflow.expectations.ExpectationBase "qiskit.opflow.expectations.expectation_base.ExpectationBase")
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**Returns**
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The expectation algorithm which best fits the Operator and backend.
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**Raises**
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**ValueError** – If operator is not of a composition for which we know the best Expectation method.
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</Function>
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</Class>
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