qiskit-documentation/docs/api/qiskit/qiskit.transpiler.passes.VF2PostLayout.mdx

---
title: VF2PostLayout
description: API reference for qiskit.transpiler.passes.VF2PostLayout
in_page_toc_min_heading_level: 1
python_api_type: class
python_api_name: qiskit.transpiler.passes.VF2PostLayout
---

# VF2PostLayout

<Class id="qiskit.transpiler.passes.VF2PostLayout" isDedicatedPage={true} github="https://github.com/Qiskit/qiskit/tree/stable/1.1/qiskit/transpiler/passes/layout/vf2_post_layout.py#L53-L419" signature="qiskit.transpiler.passes.VF2PostLayout(*args, **kwargs)" modifiers="class">
  Bases: [`AnalysisPass`](qiskit.transpiler.AnalysisPass "qiskit.transpiler.basepasses.AnalysisPass")

  A pass for improving an existing Layout after transpilation of a circuit onto a Coupling graph, as a subgraph isomorphism problem, solved by VF2++.

  Unlike the [`VF2Layout`](qiskit.transpiler.passes.VF2Layout "qiskit.transpiler.passes.VF2Layout") transpiler pass which is designed to find an initial layout for a circuit early in the transpilation pipeline this transpiler pass is designed to try and find a better layout after transpilation is complete. The initial layout phase of the transpiler doesn’t have as much information available as we do after transpilation. This pass is designed to be paired in a similar pipeline as the layout passes. This pass will strip any idle wires from the circuit, use VF2 to find a subgraph in the coupling graph for the circuit to run on with better fidelity and then update the circuit layout to use the new qubits. The algorithm used in this pass is described in [arXiv:2209.15512](https://arxiv.org/abs/2209.15512).

  If a solution is found that means there is a lower error layout available for the circuit. If a solution is found the layout will be set in the property set as `property_set['post_layout']`. However, if no solution or no better solution is found, no `property_set['post_layout']` is set. The stopping reason is set in `property_set['VF2PostLayout_stop_reason']` in all the cases and will be one of the values enumerated in `VF2PostLayoutStopReason` which has the following values:

  > *   `"solution found"`: If a solution was found.
  > *   `"no better solution found"`: If the initial layout of the circuit is the best solution.
  > *   `"nonexistent solution"`: If no solution was found.
  > *   `">2q gates in basis"`: If VF2PostLayout can’t work with the basis of the circuit.

  By default, this pass will construct a heuristic scoring map based on the error rates in the provided `target` (or `properties` if `target` is not provided). However, analysis passes can be run prior to this pass and set `vf2_avg_error_map` in the property set with a `ErrorMap` instance. If a value is `NaN` that is treated as an ideal edge For example if an error map is created as:

  ```python
  from qiskit.transpiler.passes.layout.vf2_utils import ErrorMap

  error_map = ErrorMap(3)
  error_map.add_error((0, 0), 0.0024)
  error_map.add_error((0, 1), 0.01)
  error_map.add_error((1, 1), 0.0032)
  ```

  that represents the error map for a 2 qubit target, where the avg 1q error rate is `0.0024` on qubit 0 and `0.0032` on qubit 1. Then the avg 2q error rate for gates that operate on (0, 1) is 0.01 and (1, 0) is not supported by the target. This will be used for scoring if it’s set as the `vf2_avg_error_map` key in the property set when [`VF2PostLayout`](#qiskit.transpiler.passes.VF2PostLayout "qiskit.transpiler.passes.VF2PostLayout") is run.

  Initialize a `VF2PostLayout` pass instance

  **Parameters**

  *   **target** ([*Target*](qiskit.transpiler.Target "qiskit.transpiler.Target")) – A target representing the backend device to run `VF2PostLayout` on. If specified it will supersede a set value for `properties` and `coupling_map`.
  *   **coupling\_map** ([*CouplingMap*](qiskit.transpiler.CouplingMap "qiskit.transpiler.CouplingMap")) – Directed graph representing a coupling map.
  *   **properties** ([*BackendProperties*](qiskit.providers.models.BackendProperties "qiskit.providers.models.BackendProperties")) – The backend properties for the backend. If [`readout_error()`](qiskit.providers.models.BackendProperties#readout_error "qiskit.providers.models.BackendProperties.readout_error") is available it is used to score the layout.
  *   **seed** ([*int*](https://docs.python.org/3/library/functions.html#int "(in Python v3.12)")) – Sets the seed of the PRNG. -1 Means no node shuffling.
  *   **call\_limit** ([*int*](https://docs.python.org/3/library/functions.html#int "(in Python v3.12)")) – The number of state visits to attempt in each execution of VF2.
  *   **time\_limit** ([*float*](https://docs.python.org/3/library/functions.html#float "(in Python v3.12)")) – The total time limit in seconds to run `VF2PostLayout`
  *   **strict\_direction** ([*bool*](https://docs.python.org/3/library/functions.html#bool "(in Python v3.12)")) – Whether the pass is configured to follow the strict direction in the coupling graph. If this is set to false, the pass will treat any edge in the coupling graph as a weak edge and the interaction graph will be undirected. For the purposes of evaluating layouts the avg error rate for each qubit and 2q link will be used. This enables the pass to be run prior to basis translation and work with any 1q and 2q operations. However, if `strict_direction=True` the pass expects the input [`DAGCircuit`](qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") object to [`run()`](#qiskit.transpiler.passes.VF2PostLayout.run "qiskit.transpiler.passes.VF2PostLayout.run") to be in the target set of instructions.
  *   **max\_trials** ([*int*](https://docs.python.org/3/library/functions.html#int "(in Python v3.12)")) – The maximum number of trials to run VF2 to find a layout. A value of `0` (the default) means ‘unlimited’.

  **Raises**

  [**TypeError**](https://docs.python.org/3/library/exceptions.html#TypeError "(in Python v3.12)") – At runtime, if neither `coupling_map` or `target` are provided.

  ## Attributes

  ### is\_analysis\_pass

  <Attribute id="qiskit.transpiler.passes.VF2PostLayout.is_analysis_pass">
    Check if the pass is an analysis pass.

    If the pass is an AnalysisPass, that means that the pass can analyze the DAG and write the results of that analysis in the property set. Modifications on the DAG are not allowed by this kind of pass.
  </Attribute>

  ### is\_transformation\_pass

  <Attribute id="qiskit.transpiler.passes.VF2PostLayout.is_transformation_pass">
    Check if the pass is a transformation pass.

    If the pass is a TransformationPass, that means that the pass can manipulate the DAG, but cannot modify the property set (but it can be read).
  </Attribute>

  ## Methods

  ### execute

  <Function id="qiskit.transpiler.passes.VF2PostLayout.execute" github="https://github.com/Qiskit/qiskit/tree/stable/1.1/qiskit/passmanager/base_tasks.py#L72-L118" signature="execute(passmanager_ir, state, callback=None)">
    Execute optimization task for input Qiskit IR.

    **Parameters**

    *   **passmanager\_ir** ([*Any*](https://docs.python.org/3/library/typing.html#typing.Any "(in Python v3.12)")) – Qiskit IR to optimize.
    *   **state** ([*PassManagerState*](qiskit.passmanager.PassManagerState "qiskit.passmanager.compilation_status.PassManagerState")) – State associated with workflow execution by the pass manager itself.
    *   **callback** ([*Callable*](https://docs.python.org/3/library/collections.abc.html#collections.abc.Callable "(in Python v3.12)") *| None*) – A callback function which is caller per execution of optimization task.

    **Returns**

    Optimized Qiskit IR and state of the workflow.

    **Return type**

    [tuple](https://docs.python.org/3/library/stdtypes.html#tuple "(in Python v3.12)")\[[*Any*](https://docs.python.org/3/library/typing.html#typing.Any "(in Python v3.12)"), [qiskit.passmanager.compilation\_status.PassManagerState](qiskit.passmanager.PassManagerState "qiskit.passmanager.compilation_status.PassManagerState")]
  </Function>

  ### name

  <Function id="qiskit.transpiler.passes.VF2PostLayout.name" github="https://github.com/Qiskit/qiskit/tree/stable/1.1/qiskit/passmanager/base_tasks.py#L68-L70" signature="name()">
    Name of the pass.

    **Return type**

    [str](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.12)")
  </Function>

  ### run

  <Function id="qiskit.transpiler.passes.VF2PostLayout.run" github="https://github.com/Qiskit/qiskit/tree/stable/1.1/qiskit/transpiler/passes/layout/vf2_post_layout.py#L154-L375" signature="run(dag)">
    run the layout method
  </Function>

  ### update\_status

  <Function id="qiskit.transpiler.passes.VF2PostLayout.update_status" github="https://github.com/Qiskit/qiskit/tree/stable/1.1/qiskit/passmanager/base_tasks.py#L120-L138" signature="update_status(state, run_state)">
    Update workflow status.

    **Parameters**

    *   **state** ([*PassManagerState*](qiskit.passmanager.PassManagerState "qiskit.passmanager.compilation_status.PassManagerState")) – Pass manager state to update.
    *   **run\_state** (*RunState*) – Completion status of current task.

    **Returns**

    Updated pass manager state.

    **Return type**

    [*PassManagerState*](qiskit.passmanager.PassManagerState "qiskit.passmanager.compilation_status.PassManagerState")
  </Function>
</Class>