# QPY serialization (qiskit.circuit.qpy_serialization)¶

## Using QPY¶

Using QPY is defined to be straightforward and mirror the user API of the serializers in Python's standard library, pickle and json. There are 2 user facing functions: qiskit.circuit.qpy_serialization.dump() and qiskit.circuit.qpy_serialization.load() which are used to dump QPY data to a file object and load circuits from QPY data in a file object respectively. For example:

from qiskit.circuit import QuantumCircuit
from qiskit.circuit import qpy_serialization

qc = QuantumCircuit(2, name='Bell', metadata={'test': True})
qc.h(0)
qc.cx(0, 1)
qc.measure_all()

with open('bell.qpy', 'wb') as fd:
qpy_serialization.dump(qc, fd)

with open('bell.qpy', 'rb') as fd:


### API documentation¶

 load(file_obj) Load a QPY binary file dump(circuits, file_obj) Write QPY binary data to a file

### QPY Compatibility¶

The QPY format is designed to be backwards compatible moving forward. This means you should be able to load a QPY with any newer Qiskit version than the one that generated it. However, loading a QPY file with an older Qiskit version is not supported and may not work.

For example, if you generated a QPY file using qiskit-terra 0.18.1 you could load that QPY file with qiskit-terra 0.19.0 and a hypothetical qiskit-terra 0.29.0. However, loading that QPY file with 0.18.0 is not supported and may not work.

## QPY Format¶

The QPY serialization format is a portable cross-platform binary serialization format for QuantumCircuit objects in Qiskit. The basic file format is as follows:

A QPY file (or memory object) always starts with the following 7 byte UTF8 string: QISKIT which is immediately followed by the overall file header. The contents of the file header as defined as a C struct are:

struct {
uint8_t qpy_version;
uint8_t qiskit_major_version;
uint8_t qiskit_minor_version;
uint8_t qiskit_patch_version;
uint64_t num_circuits;
}


All values use network byte order 1 (big endian) for cross platform compatibility.

The file header is immediately followed by the circuit payloads. Each individual circuit is composed of the following parts:

HEADER | METADATA | REGISTERS | CUSTOM_DEFINITIONS | INSTRUCTIONS

There is a circuit payload for each circuit (where the total number is dictated by num_circuits in the file header). There is no padding between the circuits in the data.

The METADATA field is a UTF8 encoded JSON string. After reading the HEADER (which is a fixed size at the start of the QPY file) and the name string you then read themetadata_size number of bytes and parse the JSON to get the metadata for the circuit.

### REGISTERS¶

The contents of REGISTERS is a number of REGISTER object. If num_registers is > 0 then after reading METADATA you read that number of REGISTER structs defined as:

struct {
char type;
_Bool standalone;
uint32_t size;
uint16_t name_size;
}


type can be 'q' or 'c'.

Immediately following the REGISTER struct is the utf8 encoded register name of size name_size. After the name utf8 bytes there is then an array of uint32_t values of size size that contains a map of the register's index to the circuit's qubit index. For example, array element 0's value is the index of the register[0]'s position in the containing circuit's qubits list.

The standalone boolean determines whether the register is constructed as a standalone register that was added to the circuit or was created from existing bits. A register is considered standalone if it has bits constructed solely as part of it, for example:

qr = QuantumRegister(2)
qc = QuantumCircuit(qr)


the register qr would be a standalone register. While something like:

bits = [Qubit(), Qubit()]
qr = QuantumRegister(bits=bits)
qc = QuantumCircuit(bits=bits)


qr would have standalone set to False.

### CUSTOM_DEFINITIONS¶

This section specifies custom definitions for any of the instructions in the circuit.

CUSTOM_DEFINITION_HEADER contents are defined as:

struct {
uint64_t size;
}


If size is greater than 0 that means the circuit contains custom instruction(s). Each custom instruction is defined with a CUSTOM_INSTRUCTION block defined as:

struct {
uint16_t name_size;
char type;
_Bool custom_definition;
uint64_t size;
}


Immediately following the CUSTOM_INSTRUCTION struct is the utf8 encoded name of size name_size.

If custom_definition is True that means that the immediately following size bytes contains a QPY circuit data which can be used for the custom definition of that gate. If custom_definition is False then the instruction can be considered opaque (ie no definition).

### INSTRUCTIONS¶

The contents of INSTRUCTIONS is a list of INSTRUCTION metadata objects

struct {
uint16_t name_size;
uint16_t label_size;
uint16_t num_parameters;
uint32_t num_qargs;
uint32_t num_cargs;
_Bool has_conditional;
uint16_t conditional_reg_name_size;
int64_t conditional_value;
}


This metadata object is immediately followed by name_size bytes of utf8 bytes for the name. name here is the Qiskit class name for the Instruction class if it's defined in Qiskit. Otherwise it falls back to the custom instruction name. Following the name bytes there are label_size bytes of utf8 data for the label if one was set on the instruction. Following the label bytes if has_conditional is True then there are conditional_reg_name_size bytes of utf8 data for the name of the conditional register name. In case of single classical bit conditions the register name utf8 data will be prefixed with a null character "x00" and then a utf8 string integer representing the classical bit index in the circuit that the condition is on.

This is immediately followed by the INSTRUCTION_ARG structs for the list of arguments of that instruction. These are in the order of all quantum arguments (there are num_qargs of these) followed by all classical arguments (num_cargs of these).

The contents of each INSTRUCTION_ARG is:

struct {
char type;
uint32_t index;
}


type can be 'q' or 'c'.

After all arguments for an instruction the parameters are specified with num_parameters INSTRUCTION_PARAM structs.

The contents of each INSTRUCTION_PARAM is:

struct {
char type;
uint64_t size;
}


After each INSTRUCTION_PARAM the next size bytes are the parameter's data. The type field can be 'i', 'f', 'p', 'e', 's', 'c' or 'n' which dictate the format. For 'i' it's an integer, 'f' it's a double, 's' if it's a string (encoded as utf8), 'c' is a complex and the data is represented by the struct format in the PARAMETER_EXPR section. 'p' defines a Parameter object which is represented by a PARAM struct (see below), e defines a ParameterExpression object (that's not a Parameter) which is represented by a PARAM_EXPR struct (see below), and 'n' represents an object from numpy (either an ndarray or a numpy type) which means the data is .npy format 2 data.

### PARAMETER¶

A PARAMETER represents a Parameter object the data for a INSTRUCTION_PARAM. The contents of the PARAMETER are defined as:

struct {
uint16_t name_size;
char uuid[16];
}


which is immediately followed by name_size utf8 bytes representing the parameter name.

### PARAMETER_EXPR¶

A PARAMETER_EXPR represents a ParameterExpression object that the data for an INSTRUCTION_PARAM. The contents of a PARAMETER_EXPR are defined as:

The PARAMETER_EXPR data starts with a header:

struct {
uint64_t map_elements;
uint64_t expr_size;
}


Immediately following the header is expr_size bytes of utf8 data containing the expression string, which is the sympy srepr of the expression for the parameter expression. Follwing that is a symbol map which contains map_elements elements with the format

struct {
char type;
uint64_t size;
}


Which is followed immediately by PARAMETER object (both the struct and utf8 name bytes) for the symbol map key. That is followed by size bytes for the data of the symbol. The data format is dependent on the value of type. If type is p then it represents a Parameter and size will be 0, the value will just be the same as the key. If type is f then it represents a double precision float. If type is c it represents a double precision complex, which is represented by:

struct {
double real;
double imag;
}


this matches the internal C representation of Python's complex type. 3 Finally, if type is i it represents an integer which is an int64_t`.

1

https://tools.ietf.org/html/rfc1700

2

https://numpy.org/doc/stable/reference/generated/numpy.lib.format.html

3

https://docs.python.org/3/c-api/complex.html#c.Py_complex