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Fast, correct Python JSON library supporting dataclasses, datetimes, and numpy

Project description


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.

Its features and drawbacks compared to other Python JSON libraries:

  • serializes dataclass instances 40-50x as fast as other libraries
  • serializes datetime, date, and time instances to RFC 3339 format, e.g., "1970-01-01T00:00:00+00:00"
  • serializes numpy.ndarray instances 4-12x as fast with 0.3x the memory usage of other libraries
  • pretty prints 10x to 20x as fast as the standard library
  • serializes to bytes rather than str, i.e., is not a drop-in replacement
  • serializes str without escaping unicode to ASCII, e.g., "好" rather than "\\u597d"
  • serializes float 10x as fast and deserializes twice as fast as other libraries
  • serializes subclasses of str, int, list, and dict natively, requiring default to specify how to serialize others
  • serializes arbitrary types using a default hook
  • has strict UTF-8 conformance, more correct than the standard library
  • has strict JSON conformance in not supporting Nan/Infinity/-Infinity
  • has an option for strict JSON conformance on 53-bit integers with default support for 64-bit
  • does not provide load() or dump() functions for reading from/writing to file-like objects

orjson supports CPython 3.8, 3.9, 3.10, 3.11, and 3.12. It distributes amd64/x86_64, aarch64/armv8, arm7, POWER/ppc64le, and s390x wheels for Linux, amd64 and aarch64 wheels for macOS, and amd64 and i686/x86 wheels for Windows. orjson does not and will not support PyPy. orjson does not and will not support PEP 554 subinterpreters. Releases follow semantic versioning and serializing a new object type without an opt-in flag is considered a breaking change.

orjson is licensed under both the Apache 2.0 and MIT licenses. The repository and issue tracker is, and patches may be submitted there. 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. Memory
    3. Reproducing
  5. Questions
  6. Packaging
  7. License



To install a wheel from PyPI:

pip install --upgrade "pip>=20.3" # manylinux_x_y, universal2 wheel support
pip install --upgrade orjson

To build a wheel, see packaging.


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)
>>> orjson.loads(_)
{'type': 'job', 'created_at': '1970-01-01T00:00:00+00:00', 'status': '🆗', 'payload': [[1, 2], [3, 4]]}


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.


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

dumps() serializes Python objects to JSON.

It natively serializes str, dict, list, tuple, int, float, bool, None, dataclasses.dataclass, typing.TypedDict, datetime.datetime,, 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. 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 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__.


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)
>>> 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, rapidjson
def default(obj):
    if isinstance(obj, decimal.Decimal):
        return str(obj)

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


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.


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([])
>>> orjson.dumps([], option=orjson.OPT_APPEND_NEWLINE)

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. orjson is the fastest compared library at pretty printing and has much less of a slowdown to pretty print than the standard library does. This option is compatible with all other options.

>>> import orjson
>>> orjson.dumps({"a": "b", "c": {"d": True}, "e": [1, 2]})
>>> orjson.dumps(
    {"a": "b", "c": {"d": True}, "e": [1, 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": [

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

Library compact (ms) pretty (ms) vs. orjson
orjson 0.03 0.04 1
ujson 0.18 0.19 4.6
rapidjson 0.1 0.12 2.9
simplejson 0.25 0.89 21.4
json 0.18 0.71 17

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 0.59 0.71 1
ujson 2.9 3.59 5
rapidjson 1.81 2.8 3.9
simplejson 10.43 42.13 59.1
json 4.16 33.42 46.9

This can be reproduced using the pyindent script.


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),
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0),

Serialize dict keys of type other than str. This allows dict keys to be one of str, int, float, bool, None, datetime.datetime,, 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]},
>>> orjson.dumps(
        {datetime.datetime(1970, 1, 1, 0, 0, 0): [1, 2, 3]},
        option=orjson.OPT_NON_STR_KEYS | orjson.OPT_NAIVE_UTC,

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,, 1, 5): 2,, 1, 3): 3},
    option=orjson.OPT_NON_STR_KEYS | orjson.OPT_SORT_KEYS

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 1.53 2.16 4.29
ujson 3.07 5.65
rapidjson 4.29
simplejson 11.24 14.50 21.86
json 7.17 8.49

ujson is blank for sorting because it segfaults. json is blank because it raises TypeError on attempting to sort before converting all keys to str. rapidjson is blank because it does not support non-str keys. This can be reproduced using the pynonstr script.


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),
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0, 1),

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

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

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

>>> orjson.dumps(User("3b1", "asd", "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"),

Passthrough datetime.datetime,, 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)})
>>> 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)},
b'{"created_at":"Thu, 01 Jan 1970 00:00:00 GMT"}'

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


Passthrough subclasses of builtin types to default.

>>> import orjson
class Secret(str):

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

>>> orjson.dumps(Secret("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)

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


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.


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


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.


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

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

Library unsorted (ms) sorted (ms) vs. orjson
orjson 0.32 0.54 1
ujson 1.6 2.07 3.8
rapidjson 1.12 1.65 3.1
simplejson 2.25 3.13 5.8
json 1.78 2.32 4.3

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)

This is the same sorting behavior as the standard library, rapidjson, simplejson, and ujson.

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


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


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")),
>>> orjson.dumps(
        datetime.datetime(1970, 1, 1, 0, 0, 0, tzinfo=zoneinfo.ZoneInfo("UTC")),


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.

This is similar to RawJSON in rapidjson.


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 1024 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.

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



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 1.40 1.60 1
rapidjson 3.64 68.48 42
simplejson 14.21 92.18 57
json 13.28 94.90 59

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

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

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

>>> orjson.dumps(Object(1, "a", [Member(1, True), Member(2)]))


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"))
>>> orjson.dumps(
    datetime.datetime(2100, 9, 1, 21, 55, 2).replace(tzinfo=zoneinfo.ZoneInfo("UTC"))
>>> orjson.dumps(
    datetime.datetime(2100, 9, 1, 21, 55, 2)

datetime.datetime supports instances with a tzinfo that is None, datetime.timezone.utc, a timezone instance from the python3.9+ 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"' objects will always serialize.

>>> import orjson, datetime
>>> orjson.dumps(, 1, 2))

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.


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)
>>> orjson.dumps(DatetimeEnum.EPOCH, option=orjson.OPT_NAIVE_UTC)

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)


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, ujson, rapidjson, json
>>> orjson.dumps([float("NaN"), float("Infinity"), float("-Infinity")])
>>> ujson.dumps([float("NaN"), float("Infinity"), float("-Infinity")])
OverflowError: Invalid Inf value when encoding double
>>> rapidjson.dumps([float("NaN"), float("Infinity"), float("-Infinity")])
>>> json.dumps([float("NaN"), float("Infinity"), float("-Infinity")])
'[NaN, Infinity, -Infinity]'


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)
>>> 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


orjson natively serializes numpy.ndarray and individual numpy.float64, numpy.float32, 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 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]]),

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(
>>> orjson.dumps(
            orjson.OPT_SERIALIZE_NUMPY |
            orjson.OPT_NAIVE_UTC |

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 malformed, which is not expected, 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 194 99 1.0
rapidjson 3,048 309 15.7
simplejson 3,023 297 15.6
json 3,133 297 16.1

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 178 115 1.0
rapidjson 1,512 551 8.5
simplejson 1,606 504 9.0
json 1,506 503 8.4

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 157 120 1.0
rapidjson 710 327 4.5
simplejson 931 398 5.9
json 996 400 6.3

In these benchmarks, orjson serializes natively, ujson is blank because it does not support a default parameter, and the other libraries serialize 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.


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.

orjson and rapidjson are the only compared JSON libraries to consistently error on bad input.

>>> import orjson, ujson, rapidjson, json
>>> orjson.dumps('\ud800')
JSONEncodeError: str is not valid UTF-8: surrogates not allowed
>>> ujson.dumps('\ud800')
UnicodeEncodeError: 'utf-8' codec ...
>>> rapidjson.dumps('\ud800')
UnicodeEncodeError: 'utf-8' codec ...
>>> json.dumps('\ud800')
>>> orjson.loads('"\\ud800"')
JSONDecodeError: unexpected end of hex escape at line 1 column 8: line 1 column 1 (char 0)
>>> ujson.loads('"\\ud800"')
>>> rapidjson.loads('"\\ud800"')
ValueError: Parse error at offset 1: The surrogate pair in string is invalid.
>>> json.loads('"\\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"))


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

>>> import orjson, uuid
>>> orjson.dumps(uuid.UUID('f81d4fae-7dec-11d0-a765-00a0c91e6bf6'))
>>> orjson.dumps(uuid.uuid5(uuid.NAMESPACE_DNS, ""))


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. It also uses some tests from the ultrajson library.

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
ujson 38 0
rapidjson 6 0
simplejson 13 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.


Serialization and deserialization performance of orjson is better than ultrajson, rapidjson, simplejson, or json. The benchmarks are done on fixtures of real data:

  • twitter.json, 631.5KiB, results of a search on Twitter for "一", containing CJK strings, dictionaries of strings and arrays of dictionaries, indented.

  • github.json, 55.8KiB, a GitHub activity feed, containing dictionaries of strings and arrays of dictionaries, not indented.

  • citm_catalog.json, 1.7MiB, concert data, containing nested dictionaries of strings and arrays of integers, indented.

  • canada.json, 2.2MiB, coordinates of the Canadian border in GeoJSON format, containing floats and arrays, indented.


twitter.json serialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 0.33 3069.4 1
ujson 1.68 592.8 5.15
rapidjson 1.12 891 3.45
simplejson 2.29 436.2 7.03
json 1.8 556.6 5.52

twitter.json deserialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 0.81 1237.6 1
ujson 1.87 533.9 2.32
rapidjson 2.97 335.8 3.67
simplejson 2.15 463.8 2.66
json 2.45 408.2 3.03

github.json serialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 0.03 28817.3 1
ujson 0.18 5478.2 5.26
rapidjson 0.1 9686.4 2.98
simplejson 0.26 3901.3 7.39
json 0.18 5437 5.27

github.json deserialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 0.07 15270 1
ujson 0.19 5374.8 2.84
rapidjson 0.17 5854.9 2.59
simplejson 0.15 6707.4 2.27
json 0.16 6397.3 2.39

citm_catalog.json serialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 0.58 1722.5 1
ujson 2.89 345.6 4.99
rapidjson 1.83 546.4 3.15
simplejson 10.39 95.9 17.89
json 3.93 254.6 6.77

citm_catalog.json deserialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 1.76 569.2 1
ujson 3.5 284.3 1.99
rapidjson 5.77 173.2 3.28
simplejson 5.13 194.7 2.92
json 4.99 200.5 2.84

canada.json serialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 3.62 276.3 1
ujson 14.16 70.6 3.91
rapidjson 33.64 29.7 9.29
simplejson 57.46 17.4 15.88
json 35.7 28 9.86

canada.json deserialization

Library Median latency (milliseconds) Operations per second Relative (latency)
orjson 3.89 256.6 1
ujson 8.73 114.3 2.24
rapidjson 23.33 42.8 5.99
simplejson 23.99 41.7 6.16
json 21.1 47.4 5.42


orjson as of 3.7.0 has higher baseline memory usage than other libraries due to a persistent buffer used for parsing. Incremental memory usage when deserializing is similar to the standard library and other third-party libraries.

This measures, in the first column, RSS after importing a library and reading the fixture, and in the second column, increases in RSS after repeatedly calling loads() on the fixture.


Library import, read() RSS (MiB) loads() increase in RSS (MiB)
orjson 21.8 2.8
ujson 14.3 4.8
rapidjson 14.9 4.6
simplejson 13.4 2.4
json 13.1 2.3


Library import, read() RSS (MiB) loads() increase in RSS (MiB)
orjson 21.2 0.5
ujson 13.6 0.6
rapidjson 14.1 0.5
simplejson 12.5 0.3
json 12.4 0.3


Library import, read() RSS (MiB) loads() increase in RSS (MiB)
orjson 23 10.6
ujson 15.2 11.2
rapidjson 15.8 29.7
simplejson 14.4 24.7
json 13.9 24.7


Library import, read() RSS (MiB) loads() increase in RSS (MiB)
orjson 23.2 21.3
ujson 15.6 19.2
rapidjson 16.3 23.4
simplejson 15 21.1
json 14.3 20.9


The above was measured using Python 3.10.5 on Linux (amd64) with orjson 3.7.9, ujson 5.4.0, python-rapidson 1.8, and simplejson 3.17.6.

The latency results can be reproduced using the pybench and graph scripts. The memory results can be reproduced using the pymem script.


Why can't I install it from PyPI?

Probably pip needs to be upgraded to version 20.3 or later to support the latest manylinux_x_y or universal2 wheel formats.

"Cargo, the Rust package manager, is not installed or is not on PATH."

This happens when there are no binary wheels (like manylinux) for your platform on PyPI. You can install Rust through rustup or a package manager and then it will compile.

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.


To package orjson requires at least Rust 1.65 and the maturin build tool. The recommended build command is:

maturin build --release --strip

It benefits from also having a C build environment to compile a faster deserialization backend. See this project's manylinux_2_28 builds for an example using clang and LTO.

The project's own CI tests against nightly-2023-10-10 and stable 1.65. It is prudent to pin the nightly version because that channel can introduce breaking changes.

orjson is tested for amd64, aarch64, arm7, ppc64le, and s390x on Linux. It is tested for amd64 on macOS and cross-compiles for aarch64. For Windows it is tested on amd64 and i686.

There are no runtime dependencies other than libc.

The source distribution on PyPI contains all dependencies' source and can be built without network access. The file can be downloaded from${version}.tar.gz.

orjson's tests are included in the source distribution on PyPI. The requirements to run the tests are specified in test/requirements.txt. The tests should be run as part of the build. It can be run with pytest -q test.


orjson was written by ijl <>, copyright 2018 - 2023, licensed under both the Apache 2 and MIT licenses.

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