# Layout

*class *`qiskit.transpiler.Layout(input_dict=None)`

Bases: `object`

Two-ways dict to represent a Layout.

construct a Layout from a bijective dictionary, mapping virtual qubits to physical qubits

## Methods

### add

`add(virtual_bit, physical_bit=None)`

Adds a map element between bit and physical_bit. If physical_bit is not defined, bit will be mapped to a new physical bit.

**Parameters**

### add_register

`add_register(reg)`

Adds at the end physical_qubits that map each bit in reg.

**Parameters**

**reg** (*Register*) – A (qu)bit Register. For example, QuantumRegister(3, ‘qr’).

### combine_into_edge_map

`combine_into_edge_map(another_layout)`

Combines self and another_layout into an “edge map”.

For example:

```
self another_layout resulting edge map
qr_1 -> 0 0 <- q_2 qr_1 -> q_2
qr_2 -> 2 2 <- q_1 qr_2 -> q_1
qr_3 -> 3 3 <- q_0 qr_3 -> q_0
```

The edge map is used to compose dags via, for example, compose.

**Parameters**

**another_layout** (*Layout*) – The other layout to combine.

**Returns**

A “edge map”.

**Return type**

**Raises**

**LayoutError** – another_layout can be bigger than self, but not smaller. Otherwise, raises.

### compose

`compose(other, qubits)`

Compose this layout with another layout.

If this layout represents a mapping from the P-qubits to the positions of the Q-qubits, and the other layout represents a mapping from the Q-qubits to the positions of the R-qubits, then the composed layout represents a mapping from the P-qubits to the positions of the R-qubits.

**Parameters**

**other**(*Layout*) – The existing`Layout`

to compose this`Layout`

with.**qubits**(*List**[**Qubit**]*) – A list of`Qubit`

objects over which`other`

is defined, used to establish the correspondence between the positions of the`other`

qubits and the actual qubits.

**Returns**

A new layout object the represents this layout composed with the `other`

layout.

**Return type**

### copy

### from_dict

`from_dict(input_dict)`

Populates a Layout from a dictionary.

The dictionary must be a bijective mapping between virtual qubits (tuple) and physical qubits (int).

**Parameters**

**input_dict** (*dict*) –

e.g.:

```
{(QuantumRegister(3, 'qr'), 0): 0,
(QuantumRegister(3, 'qr'), 1): 1,
(QuantumRegister(3, 'qr'), 2): 2}
Can be written more concisely as follows:
* virtual to physical::
{qr[0]: 0,
qr[1]: 1,
qr[2]: 2}
* physical to virtual::
{0: qr[0],
1: qr[1],
2: qr[2]}
```

### from_intlist

*static *`from_intlist(int_list, *qregs)`

Converts a list of integers to a Layout mapping virtual qubits (index of the list) to physical qubits (the list values).

**Parameters**

**int_list**(*list*) – A list of integers.***qregs**(*QuantumRegisters*) – The quantum registers to apply the layout to.

**Returns**

The corresponding Layout object.

**Return type**

**Raises**

**LayoutError** – Invalid input layout.

### from_qubit_list

*static *`from_qubit_list(qubit_list, *qregs)`

Populates a Layout from a list containing virtual qubits, Qubit or None.

**Parameters**

**qubit_list**(*list*) – e.g.: [qr[0], None, qr[2], qr[3]]***qregs**(*QuantumRegisters*) – The quantum registers to apply the layout to.

**Returns**

the corresponding Layout object

**Return type**

**Raises**

**LayoutError** – If the elements are not Qubit or None

### generate_trivial_layout

*static *`generate_trivial_layout(*regs)`

Creates a trivial (“one-to-one”) Layout with the registers and qubits in regs.

**Parameters**

***regs** (*Registers, Qubits*) – registers and qubits to include in the layout.

**Returns**

A layout with all the regs in the given order.

**Return type**

### get_physical_bits

`get_physical_bits()`

Returns the dictionary where the keys are physical (qu)bits and the values are virtual (qu)bits.

### get_registers

`get_registers()`

Returns the registers in the layout [QuantumRegister(2, ‘qr0’), QuantumRegister(3, ‘qr1’)] :returns: A set of Registers in the layout :rtype: Set

### get_virtual_bits

`get_virtual_bits()`

Returns the dictionary where the keys are virtual (qu)bits and the values are physical (qu)bits.

### inverse

`inverse(source_qubits, target_qubits)`

Finds the inverse of this layout.

This is possible when the layout is a bijective mapping, however the input and the output qubits may be different (in particular, this layout may be the mapping from the extended-with-ancillas virtual qubits to physical qubits). Thus, if this layout represents a mapping from the P-qubits to the positions of the Q-qubits, the inverse layout represents a mapping from the Q-qubits to the positions of the P-qubits.

**Parameters**

**source_qubits**(*List**[**Qubit**]*) – A list of`Qubit`

objects representing the domain of the layout.**target_qubits**(*List**[**Qubit**]*) – A list of`Qubit`

objects representing the image of the layout.

**Returns**

A new layout object the represents the inverse of this layout.

### order_based_on_type

*static *`order_based_on_type(value1, value2)`

decides which one is physical/virtual based on the type. Returns (virtual, physical)

### reorder_bits

`reorder_bits(bits)`

Given an ordered list of bits, reorder them according to this layout.

The list of bits must exactly match the virtual bits in this layout.

**Parameters**

**bits** (*list**[**Bit**]*) – the bits to reorder.

**Returns**

ordered bits.

**Return type**

List

### swap

`swap(left, right)`

Swaps the map between left and right.

**Parameters**

**Raises**

**LayoutError** – If left and right have not the same type.

### to_permutation

`to_permutation(qubits)`

Creates a permutation corresponding to this layout.

This is possible when the layout is a bijective mapping with the same source and target qubits (for instance, a “final_layout” corresponds to a permutation of the physical circuit qubits). If this layout is a mapping from qubits to their new positions, the resulting permutation describes which qubits occupy the positions 0, 1, 2, etc. after applying the permutation.

For example, suppose that the list of qubits is `[qr_0, qr_1, qr_2]`

, and the layout maps `qr_0`

to `2`

, `qr_1`

to `0`

, and `qr_2`

to `1`

. In terms of positions in `qubits`

, this maps `0`

to `2`

, `1`

to `0`

and `2`

to `1`

, with the corresponding permutation being `[1, 2, 0]`

.