orjson

orjson is a fast, correct JSON library for Python. It benchmarks as the fastest Python library for JSON and is more correct than the standard json library or other third-party libraries. It serializes dataclass, datetime, numpy, and UUID instances natively.

orjson.dumps() is something like 10x as fast as json, serializes common types and subtypes, has a default parameter for the caller to specify how to serialize arbitrary types, and has a number of flags controlling output.

orjson.loads() is something like 2x as fast as json, and is strictly compliant with UTF-8 and RFC 8259 ("The JavaScript Object Notation (JSON) Data Interchange Format").

Reading from and writing to files, line-delimited JSON files, and so on is not provided by the library.

orjson supports CPython 3.10, 3.11, 3.12, 3.13, 3.14, and 3.15.

It distributes amd64/x86_64/x64, i686/x86, aarch64/arm64/armv8, and armv7 wheels for Linux, amd64 and aarch64 wheels for macOS, and amd64, i686, and aarch64 wheels for Windows.

Wheels published to PyPI for amd64 run on x86-64-v1 (2003) or later, but will at runtime use AVX-512 if available for a significant performance benefit; aarch64 wheels run on ARMv8-A (2011) or later.

orjson does not and will not support PyPy, embedded Python builds for Android/iOS, or PEP 554 subinterpreters.

orjson may support PEP 703 free-threading when it is stable.

Releases follow semantic versioning and serializing a new object type without an opt-in flag is considered a breaking change.

orjson contains source code licensed under the Mozilla Public License 2.0, Apache 2.0, and MIT licenses. The repository from which PyPI artifacts are published is github.com/ijl/orjson and an alternative repository is codeberg.org/ijl/orjson. There is no open issue tracker or pull requests due to signal-to-noise ratio. There is a CHANGELOG available in the repository.

1. Usage
   1. Install
   2. Quickstart
   3. Migrating
   4. Serialize
      1. default
      2. option
      3. Fragment
   5. Deserialize
2. Types
   1. dataclass
   2. datetime
   3. enum
   4. float
   5. int
   6. numpy
   7. str
   8. uuid
3. Testing
4. Performance
   1. Latency
   2. Reproducing
5. Questions
6. Packaging
7. License

Usage

Install

To install a wheel from PyPI, install the orjson package.

In requirements.in or requirements.txt format, specify:

orjson >= 3.10,<4

In pyproject.toml format, specify:

orjson = "^3.10"

To build a wheel, see packaging.

Quickstart

This is an example of serializing, with options specified, and deserializing:

>>> import orjson, datetime, numpy
>>> data = {
    "type": "job",
    "created_at": datetime.datetime(1970, 1, 1),
    "status": "🆗",
    "payload": numpy.array([[1, 2], [3, 4]]),
}
>>> orjson.dumps(data, option=orjson.OPT_NAIVE_UTC | orjson.OPT_SERIALIZE_NUMPY)
b'{"type":"job","created_at":"1970-01-01T00:00:00+00:00","status":"\xf0\x9f\x86\x97","payload":[[1,2],[3,4]]}'
>>> orjson.loads(_)
{'type': 'job', 'created_at': '1970-01-01T00:00:00+00:00', 'status': '🆗', 'payload': [[1, 2], [3, 4]]}

Migrating

orjson version 3 serializes more types than version 2. Subclasses of str, int, dict, and list are now serialized. This is faster and more similar to the standard library. It can be disabled with orjson.OPT_PASSTHROUGH_SUBCLASS.dataclasses.dataclass instances are now serialized by default and cannot be customized in a default function unless option=orjson.OPT_PASSTHROUGH_DATACLASS is specified. uuid.UUID instances are serialized by default. For any type that is now serialized, implementations in a default function and options enabling them can be removed but do not need to be. There was no change in deserialization.

To migrate from the standard library, the largest difference is that orjson.dumps returns bytes and json.dumps returns a str.

Users with dict objects using non-str keys should specify option=orjson.OPT_NON_STR_KEYS.

sort_keys is replaced by option=orjson.OPT_SORT_KEYS.

indent is replaced by option=orjson.OPT_INDENT_2 and other levels of indentation are not supported.

ensure_ascii is probably not relevant today and UTF-8 characters cannot be escaped to ASCII.

Serialize

def dumps(
    __obj: Any,
    default: Optional[Callable[[Any], Any]] = ...,
    option: Optional[int] = ...,
) -> bytes: ...

dumps() serializes Python objects to JSON.

It natively serializes str, dict, frozendict, list, tuple, int, float, bool, None, dataclasses.dataclass, typing.TypedDict, datetime.datetime, datetime.date, datetime.time, uuid.UUID, numpy.ndarray, and orjson.Fragment instances. It supports arbitrary types through default. It serializes subclasses of str, int, dict, list, dataclasses.dataclass, and enum.Enum. It does not serialize subclasses of tuple to avoid serializing namedtuple objects as arrays. It does not serialize subclasses of float or frozendict due to implementation details in CPython. To avoid serializing subclasses, specify the option orjson.OPT_PASSTHROUGH_SUBCLASS.

The output is a bytes object containing UTF-8.

The global interpreter lock (GIL) is held for the duration of the call.

It raises JSONEncodeError on an unsupported type. This exception message describes the invalid object with the error message Type is not JSON serializable: .... To fix this, specify default.

It raises JSONEncodeError on a str that contains invalid UTF-8.

It raises JSONEncodeError on an integer that exceeds 64 bits by default or, with OPT_STRICT_INTEGER, 53 bits.

It raises JSONEncodeError if a dict or frozendict has a key of a type other than str, unless OPT_NON_STR_KEYS is specified.

It raises JSONEncodeError if the output of default recurses to handling by default more than 254 levels deep.

It raises JSONEncodeError on circular references.

It raises JSONEncodeError if a tzinfo on a datetime object is unsupported.

JSONEncodeError is a subclass of TypeError. This is for compatibility with the standard library.

If the failure was caused by an exception in default then JSONEncodeError chains the original exception as __cause__.

default

To serialize a subclass or arbitrary types, specify default as a callable that returns a supported type. default may be a function, lambda, or callable class instance. To specify that a type was not handled by default, raise an exception such as TypeError.

>>> import orjson, decimal
>>>
def default(obj):
    if isinstance(obj, decimal.Decimal):
        return str(obj)
    raise TypeError

>>> orjson.dumps(decimal.Decimal("0.0842389659712649442845"))
JSONEncodeError: Type is not JSON serializable: decimal.Decimal
>>> orjson.dumps(decimal.Decimal("0.0842389659712649442845"), default=default)
b'"0.0842389659712649442845"'
>>> orjson.dumps({1, 2}, default=default)
orjson.JSONEncodeError: Type is not JSON serializable: set

The default callable may return an object that itself must be handled by default up to 254 times before an exception is raised.

It is important that default raise an exception if a type cannot be handled. Python otherwise implicitly returns None, which appears to the caller like a legitimate value and is serialized:

>>> import orjson, json
>>>
def default(obj):
    if isinstance(obj, decimal.Decimal):
        return str(obj)

>>> orjson.dumps({"set":{1, 2}}, default=default)
b'{"set":null}'
>>> json.dumps({"set":{1, 2}}, default=default)
'{"set":null}'

option

To modify how data is serialized, specify option. Each option is an integer constant in orjson. To specify multiple options, mask them together, e.g., option=orjson.OPT_STRICT_INTEGER | orjson.OPT_NAIVE_UTC.

OPT_APPEND_NEWLINE

Append \n to the output. This is a convenience and optimization for the pattern of dumps(...) + "\n". bytes objects are immutable and this pattern copies the original contents.

>>> import orjson
>>> orjson.dumps([])
b"[]"
>>> orjson.dumps([], option=orjson.OPT_APPEND_NEWLINE)
b"[]\n"

OPT_INDENT_2

Pretty-print output with an indent of two spaces. This is equivalent to indent=2 in the standard library. Pretty printing is slower and the output larger. This option is compatible with all other options.

>>> import orjson
>>> orjson.dumps({"a": "b", "c": {"d": True}, "e": [1, 2]})
b'{"a":"b","c":{"d":true},"e":[1,2]}'
>>> orjson.dumps(
    {"a": "b", "c": {"d": True}, "e": [1, 2]},
    option=orjson.OPT_INDENT_2
)
b'{\n  "a": "b",\n  "c": {\n    "d": true\n  },\n  "e": [\n    1,\n    2\n  ]\n}'

If displayed, the indentation and linebreaks appear like this:

{
  "a": "b",
  "c": {
    "d": true
  },
  "e": [
    1,
    2
  ]
}

This measures serializing the github.json fixture as compact (52KiB) or pretty (64KiB):

Library	compact (ms)	pretty (ms)	vs. orjson
orjson	0.01	0.02	1
json	0.13	0.54	34

This measures serializing the citm_catalog.json fixture, more of a worst case due to the amount of nesting and newlines, as compact (489KiB) or pretty (1.1MiB):

Library	compact (ms)	pretty (ms)	vs. orjson
orjson	0.25	0.45	1
json	3.01	24.42	54.4

This can be reproduced using the pyindent script.

OPT_NAIVE_UTC

Serialize datetime.datetime objects without a tzinfo as UTC. This has no effect on datetime.datetime objects that have tzinfo set.

>>> import orjson, datetime
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0),
    )
b'"1970-01-01T00:00:00"'
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0),
        option=orjson.OPT_NAIVE_UTC,
    )
b'"1970-01-01T00:00:00+00:00"'

OPT_NON_STR_KEYS

Serialize dict and frozendict keys of type other than str. This allows dict and frozendict keys to be one of str, int, float, bool, None, datetime.datetime, datetime.date, datetime.time, enum.Enum, and uuid.UUID. For comparison, the standard library serializes str, int, float, bool or None by default. orjson benchmarks as being faster at serializing non-str keys than other libraries. This option is slower for str keys than the default.

>>> import orjson, datetime, uuid
>>> orjson.dumps(
        {uuid.UUID("7202d115-7ff3-4c81-a7c1-2a1f067b1ece"): [1, 2, 3]},
        option=orjson.OPT_NON_STR_KEYS,
    )
b'{"7202d115-7ff3-4c81-a7c1-2a1f067b1ece":[1,2,3]}'
>>> orjson.dumps(
        {datetime.datetime(1970, 1, 1, 0, 0, 0): [1, 2, 3]},
        option=orjson.OPT_NON_STR_KEYS | orjson.OPT_NAIVE_UTC,
    )
b'{"1970-01-01T00:00:00+00:00":[1,2,3]}'

These types are generally serialized how they would be as values, e.g., datetime.datetime is still an RFC 3339 string and respects options affecting it. The exception is that int serialization does not respect OPT_STRICT_INTEGER.

This option has the risk of creating duplicate keys. This is because non-str objects may serialize to the same str as an existing key, e.g., {"1": true, 1: false}. The last key to be inserted to the dict will be serialized last and a JSON deserializer will presumably take the last occurrence of a key (in the above, false). The first value will be lost.

This option is compatible with orjson.OPT_SORT_KEYS. If sorting is used, note the sort is unstable and will be unpredictable for duplicate keys.

>>> import orjson, datetime
>>> orjson.dumps(
    {"other": 1, datetime.date(1970, 1, 5): 2, datetime.date(1970, 1, 3): 3},
    option=orjson.OPT_NON_STR_KEYS | orjson.OPT_SORT_KEYS
)
b'{"1970-01-03":3,"1970-01-05":2,"other":1}'

This measures serializing 589KiB of JSON comprising a list of 100 dict in which each dict has both 365 randomly-sorted int keys representing epoch timestamps as well as one str key and the value for each key is a single integer. In "str keys", the keys were converted to str before serialization, and orjson still specifes option=orjson.OPT_NON_STR_KEYS (which is always somewhat slower).

Library	str keys (ms)	int keys (ms)	int keys sorted (ms)
orjson	0.5	0.93	2.08
json	2.72	3.59

json is blank because it raises TypeError on attempting to sort before converting all keys to str. This can be reproduced using the pynonstr script.

OPT_OMIT_MICROSECONDS

Do not serialize the microsecond field on datetime.datetime and datetime.time instances.

>>> import orjson, datetime
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0, 1),
    )
b'"1970-01-01T00:00:00.000001"'
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0, 1),
        option=orjson.OPT_OMIT_MICROSECONDS,
    )
b'"1970-01-01T00:00:00"'

OPT_PASSTHROUGH_DATACLASS

Passthrough dataclasses.dataclass instances to default. This allows customizing their output but is much slower.

>>> import orjson, dataclasses
>>>
@dataclasses.dataclass
class User:
    id: str
    name: str
    password: str

def default(obj):
    if isinstance(obj, User):
        return {"id": obj.id, "name": obj.name}
    raise TypeError

>>> orjson.dumps(User("3b1", "asd", "zxc"))
b'{"id":"3b1","name":"asd","password":"zxc"}'
>>> orjson.dumps(User("3b1", "asd", "zxc"), option=orjson.OPT_PASSTHROUGH_DATACLASS)
TypeError: Type is not JSON serializable: User
>>> orjson.dumps(
        User("3b1", "asd", "zxc"),
        option=orjson.OPT_PASSTHROUGH_DATACLASS,
        default=default,
    )
b'{"id":"3b1","name":"asd"}'

OPT_PASSTHROUGH_DATETIME

Passthrough datetime.datetime, datetime.date, and datetime.time instances to default. This allows serializing datetimes to a custom format, e.g., HTTP dates:

>>> import orjson, datetime
>>>
def default(obj):
    if isinstance(obj, datetime.datetime):
        return obj.strftime("%a, %d %b %Y %H:%M:%S GMT")
    raise TypeError

>>> orjson.dumps({"created_at": datetime.datetime(1970, 1, 1)})
b'{"created_at":"1970-01-01T00:00:00"}'
>>> orjson.dumps({"created_at": datetime.datetime(1970, 1, 1)}, option=orjson.OPT_PASSTHROUGH_DATETIME)
TypeError: Type is not JSON serializable: datetime.datetime
>>> orjson.dumps(
        {"created_at": datetime.datetime(1970, 1, 1)},
        option=orjson.OPT_PASSTHROUGH_DATETIME,
        default=default,
    )
b'{"created_at":"Thu, 01 Jan 1970 00:00:00 GMT"}'

This does not affect datetimes in dict or frozendict keys if using OPT_NON_STR_KEYS.

OPT_PASSTHROUGH_SUBCLASS

Passthrough subclasses of builtin types to default.

>>> import orjson
>>>
class Secret(str):
    pass

def default(obj):
    if isinstance(obj, Secret):
        return "******"
    raise TypeError

>>> orjson.dumps(Secret("zxc"))
b'"zxc"'
>>> orjson.dumps(Secret("zxc"), option=orjson.OPT_PASSTHROUGH_SUBCLASS)
TypeError: Type is not JSON serializable: Secret
>>> orjson.dumps(Secret("zxc"), option=orjson.OPT_PASSTHROUGH_SUBCLASS, default=default)
b'"******"'

This does not affect serializing subclasses as dict or frozendict keys if using OPT_NON_STR_KEYS.

OPT_SERIALIZE_DATACLASS

This is deprecated and has no effect in version 3. In version 2 this was required to serialize dataclasses.dataclass instances. For more, see dataclass.

OPT_SERIALIZE_NUMPY

Serialize numpy.ndarray instances. For more, see numpy.

OPT_SERIALIZE_UUID

This is deprecated and has no effect in version 3. In version 2 this was required to serialize uuid.UUID instances. For more, see UUID.

OPT_SORT_KEYS

Serialize dict and frozendict keys in sorted order. The default is to serialize in an unspecified order. This is equivalent to sort_keys=True in the standard library.

This can be used to ensure the order is deterministic for hashing or tests. It has a substantial performance penalty and is not recommended in general.

>>> import orjson
>>> orjson.dumps({"b": 1, "c": 2, "a": 3})
b'{"b":1,"c":2,"a":3}'
>>> orjson.dumps({"b": 1, "c": 2, "a": 3}, option=orjson.OPT_SORT_KEYS)
b'{"a":3,"b":1,"c":2}'

This measures serializing the twitter.json fixture unsorted and sorted:

Library	unsorted (ms)	sorted (ms)	vs. orjson
orjson	0.11	0.3	1
json	1.36	1.93	6.4

The benchmark can be reproduced using the pysort script.

The sorting is not collation/locale-aware:

>>> import orjson
>>> orjson.dumps({"a": 1, "ä": 2, "A": 3}, option=orjson.OPT_SORT_KEYS)
b'{"A":3,"a":1,"\xc3\xa4":2}'

This is the same sorting behavior as the standard library.

dataclass also serialize as maps but this has no effect on them.

OPT_STRICT_INTEGER

Enforce 53-bit limit on integers. The limit is otherwise 64 bits, the same as the Python standard library. For more, see int.

OPT_UTC_Z

Serialize a UTC timezone on datetime.datetime instances as Z instead of +00:00.

>>> import orjson, datetime, zoneinfo
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0, tzinfo=zoneinfo.ZoneInfo("UTC")),
    )
b'"1970-01-01T00:00:00+00:00"'
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0, tzinfo=zoneinfo.ZoneInfo("UTC")),
        option=orjson.OPT_UTC_Z
    )
b'"1970-01-01T00:00:00Z"'

Fragment

orjson.Fragment includes already-serialized JSON in a document. This is an efficient way to include JSON blobs from a cache, JSONB field, or separately serialized object without first deserializing to Python objects via loads().

>>> import orjson
>>> orjson.dumps({"key": "zxc", "data": orjson.Fragment(b'{"a": "b", "c": 1}')})
b'{"key":"zxc","data":{"a": "b", "c": 1}}'

It does no reformatting: orjson.OPT_INDENT_2 will not affect a compact blob nor will a pretty-printed JSON blob be rewritten as compact.

The input must be bytes or str and given as a positional argument.

This raises orjson.JSONEncodeError if a str is given and the input is not valid UTF-8. It otherwise does no validation and it is possible to write invalid JSON. This does not escape characters. The implementation is tested to not crash if given invalid strings or invalid JSON.

Deserialize

def loads(__obj: Union[bytes, bytearray, memoryview, str]) -> Any: ...

loads() deserializes JSON to Python objects. It deserializes to dict, list, int, float, str, bool, and None objects.

bytes, bytearray, memoryview, and str input are accepted. If the input exists as a memoryview, bytearray, or bytes object, it is recommended to pass these directly rather than creating an unnecessary str object. That is, orjson.loads(b"{}") instead of orjson.loads(b"{}".decode("utf-8")). This has lower memory usage and lower latency.

The input must be valid UTF-8.

orjson maintains a cache of map keys for the duration of the process. This causes a net reduction in memory usage by avoiding duplicate strings. The keys must be at most 64 bytes to be cached and 2048 entries are stored.

The global interpreter lock (GIL) is held for the duration of the call.

It raises JSONDecodeError if given an invalid type or invalid JSON. This includes if the input contains NaN, Infinity, or -Infinity, which the standard library allows, but is not valid JSON.

It raises JSONDecodeError if a combination of array or object recurses 1024 levels deep.

It raises JSONDecodeError if unable to allocate a buffer large enough to parse the document.

JSONDecodeError is a subclass of json.JSONDecodeError and ValueError. This is for compatibility with the standard library.

Types

dataclass

orjson serializes instances of dataclasses.dataclass natively. It serializes instances 40-50x as fast as other libraries and avoids a severe slowdown seen in other libraries compared to serializing dict.

It is supported to pass all variants of dataclasses, including dataclasses using __slots__, frozen dataclasses, those with optional or default attributes, and subclasses. There is a performance benefit to not using __slots__.

Library	dict (ms)	dataclass (ms)	vs. orjson
orjson	0.43	0.95	1
json	5.81	38.32	40

This measures serializing 555KiB of JSON, orjson natively and other libraries using default to serialize the output of dataclasses.asdict(). This can be reproduced using the pydataclass script.

Dataclasses are serialized as maps, with every attribute serialized and in the order given on class definition:

>>> import dataclasses, orjson, typing

@dataclasses.dataclass
class Member:
    id: int
    active: bool = dataclasses.field(default=False)

@dataclasses.dataclass
class Object:
    id: int
    name: str
    members: typing.List[Member]

>>> orjson.dumps(Object(1, "a", [Member(1, True), Member(2)]))
b'{"id":1,"name":"a","members":[{"id":1,"active":true},{"id":2,"active":false}]}'

datetime

orjson serializes datetime.datetime objects to RFC 3339 format, e.g., "1970-01-01T00:00:00+00:00". This is a subset of ISO 8601 and is compatible with isoformat() in the standard library.

>>> import orjson, datetime, zoneinfo
>>> orjson.dumps(
    datetime.datetime(2018, 12, 1, 2, 3, 4, 9, tzinfo=zoneinfo.ZoneInfo("Australia/Adelaide"))
)
b'"2018-12-01T02:03:04.000009+10:30"'
>>> orjson.dumps(
    datetime.datetime(2100, 9, 1, 21, 55, 2).replace(tzinfo=zoneinfo.ZoneInfo("UTC"))
)
b'"2100-09-01T21:55:02+00:00"'
>>> orjson.dumps(
    datetime.datetime(2100, 9, 1, 21, 55, 2)
)
b'"2100-09-01T21:55:02"'

datetime.datetime supports instances with a tzinfo that is None, datetime.timezone.utc, a timezone instance from the standard library zoneinfo module, or a timezone instance from the third-party pendulum, pytz, or dateutil/arrow libraries.

It is fastest to use the standard library's zoneinfo.ZoneInfo for timezones.

datetime.time objects must not have a tzinfo.

>>> import orjson, datetime
>>> orjson.dumps(datetime.time(12, 0, 15, 290))
b'"12:00:15.000290"'

datetime.date objects will always serialize.

>>> import orjson, datetime
>>> orjson.dumps(datetime.date(1900, 1, 2))
b'"1900-01-02"'

Errors with tzinfo result in JSONEncodeError being raised.

To disable serialization of datetime objects specify the option orjson.OPT_PASSTHROUGH_DATETIME.

To use "Z" suffix instead of "+00:00" to indicate UTC ("Zulu") time, use the option orjson.OPT_UTC_Z.

To assume datetimes without timezone are UTC, use the option orjson.OPT_NAIVE_UTC.

enum

orjson serializes enums natively. Options apply to their values.

>>> import enum, datetime, orjson
>>>
class DatetimeEnum(enum.Enum):
    EPOCH = datetime.datetime(1970, 1, 1, 0, 0, 0)
>>> orjson.dumps(DatetimeEnum.EPOCH)
b'"1970-01-01T00:00:00"'
>>> orjson.dumps(DatetimeEnum.EPOCH, option=orjson.OPT_NAIVE_UTC)
b'"1970-01-01T00:00:00+00:00"'

Enums with members that are not supported types can be serialized using default:

>>> import enum, orjson
>>>
class Custom:
    def __init__(self, val):
        self.val = val

def default(obj):
    if isinstance(obj, Custom):
        return obj.val
    raise TypeError

class CustomEnum(enum.Enum):
    ONE = Custom(1)

>>> orjson.dumps(CustomEnum.ONE, default=default)
b'1'

float

orjson serializes and deserializes double precision floats with no loss of precision and consistent rounding.

orjson.dumps() serializes Nan, Infinity, and -Infinity, which are not compliant JSON, as null:

>>> import orjson, json
>>> orjson.dumps([float("NaN"), float("Infinity"), float("-Infinity")])
b'[null,null,null]'
>>> json.dumps([float("NaN"), float("Infinity"), float("-Infinity")])
'[NaN, Infinity, -Infinity]'

int

orjson serializes and deserializes 64-bit integers by default. The range supported is a signed 64-bit integer's minimum (-9223372036854775807) to an unsigned 64-bit integer's maximum (18446744073709551615). This is widely compatible, but there are implementations that only support 53-bits for integers, e.g., web browsers. For those implementations, dumps() can be configured to raise a JSONEncodeError on values exceeding the 53-bit range.

>>> import orjson
>>> orjson.dumps(9007199254740992)
b'9007199254740992'
>>> orjson.dumps(9007199254740992, option=orjson.OPT_STRICT_INTEGER)
JSONEncodeError: Integer exceeds 53-bit range
>>> orjson.dumps(-9007199254740992, option=orjson.OPT_STRICT_INTEGER)
JSONEncodeError: Integer exceeds 53-bit range

numpy

orjson natively serializes numpy.ndarray and individual numpy.float64, numpy.float32, numpy.float16 (numpy.half), numpy.int64, numpy.int32, numpy.int16, numpy.int8, numpy.uint64, numpy.uint32, numpy.uint16, numpy.uint8, numpy.uintp, numpy.intp, numpy.datetime64, and numpy.bool instances.

orjson is compatible with both numpy v1 and v2.

orjson is faster than all compared libraries at serializing numpy instances. Serializing numpy data requires specifying option=orjson.OPT_SERIALIZE_NUMPY.

>>> import orjson, numpy
>>> orjson.dumps(
        numpy.array([[1, 2, 3], [4, 5, 6]]),
        option=orjson.OPT_SERIALIZE_NUMPY,
)
b'[[1,2,3],[4,5,6]]'

The array must be a contiguous C array (C_CONTIGUOUS) and one of the supported datatypes.

Note a difference between serializing numpy.float32 using ndarray.tolist() or orjson.dumps(..., option=orjson.OPT_SERIALIZE_NUMPY): tolist() converts to a double before serializing and orjson's native path does not. This can result in different rounding.

numpy.datetime64 instances are serialized as RFC 3339 strings and datetime options affect them.

>>> import orjson, numpy
>>> orjson.dumps(
        numpy.datetime64("2021-01-01T00:00:00.172"),
        option=orjson.OPT_SERIALIZE_NUMPY,
)
b'"2021-01-01T00:00:00.172000"'
>>> orjson.dumps(
        numpy.datetime64("2021-01-01T00:00:00.172"),
        option=(
            orjson.OPT_SERIALIZE_NUMPY |
            orjson.OPT_NAIVE_UTC |
            orjson.OPT_OMIT_MICROSECONDS
        ),
)
b'"2021-01-01T00:00:00+00:00"'

If an array is not a contiguous C array, contains an unsupported datatype, or contains a numpy.datetime64 using an unsupported representation (e.g., picoseconds), orjson falls through to default. In default, obj.tolist() can be specified.

If an array is not in the native endianness, e.g., an array of big-endian values on a little-endian system, orjson.JSONEncodeError is raised.

If an array is malformed, orjson.JSONEncodeError is raised.

This measures serializing 92MiB of JSON from an numpy.ndarray with dimensions of (50000, 100) and numpy.float64 values:

Library	Latency (ms)	RSS diff (MiB)	vs. orjson
orjson	105	105	1
json	1,481	295	14.2

This measures serializing 100MiB of JSON from an numpy.ndarray with dimensions of (100000, 100) and numpy.int32 values:

Library	Latency (ms)	RSS diff (MiB)	vs. orjson
orjson	68	119	1
json	684	501	10.1

This measures serializing 105MiB of JSON from an numpy.ndarray with dimensions of (100000, 200) and numpy.bool values:

Library	Latency (ms)	RSS diff (MiB)	vs. orjson
orjson	50	125	1
json	573	398	11.5

In these benchmarks, orjson serializes natively and json serializes ndarray.tolist() via default. The RSS column measures peak memory usage during serialization. This can be reproduced using the pynumpy script.

orjson does not have an installation or compilation dependency on numpy. The implementation is independent, reading numpy.ndarray using PyArrayInterface.

str

orjson is strict about UTF-8 conformance. This is stricter than the standard library's json module, which will serialize and deserialize UTF-16 surrogates, e.g., "\ud800", that are invalid UTF-8.

If orjson.dumps() is given a str that does not contain valid UTF-8, orjson.JSONEncodeError is raised. If loads() receives invalid UTF-8, orjson.JSONDecodeError is raised.

>>> import orjson, json
>>> orjson.dumps('\ud800')
JSONEncodeError: str is not valid UTF-8: surrogates not allowed
>>> json.dumps('\ud800')
'"\\ud800"'
>>> orjson.loads('"\\ud800"')
JSONDecodeError: unexpected end of hex escape at line 1 column 8: line 1 column 1 (char 0)
>>> json.loads('"\\ud800"')
'\ud800'

To make a best effort at deserializing bad input, first decode bytes using the replace or lossy argument for errors:

>>> import orjson
>>> orjson.loads(b'"\xed\xa0\x80"')
JSONDecodeError: str is not valid UTF-8: surrogates not allowed
>>> orjson.loads(b'"\xed\xa0\x80"'.decode("utf-8", "replace"))
'���'

uuid

orjson serializes uuid.UUID instances to RFC 4122 format, e.g., "f81d4fae-7dec-11d0-a765-00a0c91e6bf6".

>>> import orjson, uuid
>>> orjson.dumps(uuid.uuid5(uuid.NAMESPACE_DNS, "python.org"))
b'"886313e1-3b8a-5372-9b90-0c9aee199e5d"'

Testing

The library has comprehensive tests. There are tests against fixtures in the JSONTestSuite and nativejson-benchmark repositories. It is tested to not crash against the Big List of Naughty Strings. It is tested to not leak memory. It is tested to not crash against and not accept invalid UTF-8. There are integration tests exercising the library's use in web servers (gunicorn using multiprocess/forked workers) and when multithreaded.

orjson is the most correct of the compared libraries. This graph shows how each library handles a combined 342 JSON fixtures from the JSONTestSuite and nativejson-benchmark tests:

Library	Invalid JSON documents not rejected	Valid JSON documents not deserialized
orjson	0	0
json	17	0

This shows that all libraries deserialize valid JSON but only orjson correctly rejects the given invalid JSON fixtures. Errors are largely due to accepting invalid strings and numbers.

The graph above can be reproduced using the pycorrectness script.

Performance

Serialization and deserialization performance of orjson is consistently better than the standard library's json. The graphs below illustrate a few commonly used documents.

Latency

twitter.json serialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	0.1	8453	1
json	1.3	765	11.1

twitter.json deserialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	0.5	1889	1
json	2.2	453	4.2

github.json serialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	0.01	103693	1
json	0.13	7648	13.6

github.json deserialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	0.04	23264	1
json	0.1	10430	2.2

citm_catalog.json serialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	0.3	3975	1
json	3	338	11.8

citm_catalog.json deserialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	1.3	781	1
json	4	250	3.1

canada.json serialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	2.5	399	1
json	29.8	33	11.9

canada.json deserialization

Library	Median latency (milliseconds)	Operations per second	Relative (latency)
orjson	3	333	1
json	18	55	6

Reproducing

The above was measured using Python 3.11.10 in a Fedora 42 container on an x86-64-v4 machine using the orjson-3.10.11-cp311-cp311-manylinux_2_17_x86_64.manylinux2014_x86_64.whl artifact on PyPI. The latency results can be reproduced using the pybench script.

Questions

Will it deserialize to dataclasses, UUIDs, decimals, etc or support object_hook?

No. This requires a schema specifying what types are expected and how to handle errors etc. This is addressed by data validation libraries a level above this.

Will it serialize to str?

No. bytes is the correct type for a serialized blob.

Will it support NDJSON or JSONL?

No. orjsonl may be appropriate.

Will it support JSON5 or RJSON?

No, it supports RFC 8259.

How do I depend on orjson in a Rust project?

orjson is only shipped as a Python module. The project should depend on orjson in its own Python requirements and should obtain pointers to functions and objects using the normal PyImport_* APIs.

Packaging

To package orjson requires at least Rust 1.95, a C compiler, and the maturin build tool. The recommended build command is:

maturin build --release --strip

The project's own CI tests against nightly-2026-10-01 and stable 1.95. It is prudent to pin the nightly version because that channel can introduce breaking changes. There is a significant performance benefit to using nightly.

orjson is tested, on Linux, using native hardware for amd64 and aarch64, aarch64 to run an armv7 container, amd64 to run an i686 container; on macOS, native hardware for aarch64 and a cross-compilation for amd64; on Windows, native hardware for amd64 and aarch64, and amd64 for i686.

The library does not require any other host-level or Python package to be installed.

The source distribution on PyPI contains all dependencies' source and can be built without network access. The file can be downloaded from https://files.pythonhosted.org/packages/source/o/orjson/orjson-${version}.tar.gz.

orjson's tests are included in the source distribution on PyPI. The tests require only pytest. There are optional packages such as pytz and numpy listed in test/requirements.txt and used in ~10% of tests. Not having these dependencies causes the tests needing them to skip. Tests can be run with pytest -q test.

License

orjson was written by ijl <ijl@mailbox.org>, copyright 2018 - 2026, with some source files available under the Mozilla Public License 2.0 and some available under your choice of the Apache 2 license or MIT license.