Supported Types

msgspec uses Python type annotations to describe the expected types. Most combinations of the following types are supported (with a few restrictions):

Builtin Types

Msgspec types

Standard Library Types

Typing module types

Abstract types

Third-Party Libraries

Additional types may be supported through extensions.

Note that except where explicitly stated, subclasses of these types are not supported by default (see Extending for how to add support yourself).

Here we document how msgspec maps Python objects to/from the various supported protocols.

None

None maps to the null value in all supported protocols. Note that TOML lacks a null value, attempted to encode a message containing None to TOML will result in an error.

>>> msgspec.json.encode(None)
b'null'

>>> msgspec.json.decode(b'null')
None

If strict=False is specified, a string value of "null" (case insensitive) may also be coerced to None. See “Strict” vs “Lax” Mode for more information.

>>> msgspec.json.decode(b'"null"', type=None, strict=False)
None

bool

Booleans map to their corresponding true/false values in both all supported protocols.

>>> msgspec.json.encode(True)
b'true'

>>> msgspec.json.decode(b'true')
True

If strict=False is specified, values of "true"/"1"/1 or "false"/"0"/0 (case insensitive for strings) may also be coerced to True/False respectively. See “Strict” vs “Lax” Mode for more information.

>>> msgspec.json.decode(b'"false"', type=bool, strict=False)
False

>>> msgspec.json.decode(b'"TRUE"', type=bool, strict=False)
True

>>> msgspec.json.decode(b'1', type=bool, strict=False)
True

int

Integers map to integers in all supported protocols.

Support for large integers varies by protocol:

  • msgpack only supports encoding/decoding integers within [-2**63, 2**64 - 1], inclusive.

  • json, yaml, and toml have no restrictions on encode or decode.

>>> msgspec.json.encode(123)
b"123"

>>> msgspec.json.decode(b"123", type=int)
123

If strict=False is specified, string values may also be coerced to integers, following the same restrictions as above. Likewise floats that have an exact integer representation (i.e. no decimal component) may also be coerced as integers. See “Strict” vs “Lax” Mode for more information.

>>> msgspec.json.decode(b'"123"', type=int, strict=False)
123

>>> msgspec.json.decode(b'123.0', type=int, strict=False)
123

float

Floats map to floats in all supported protocols. Note that per RFC8259, JSON doesn’t support nonfinite numbers (nan, infinity, -infinity); msgspec.json handles this by encoding these values as null. The msgpack, toml, and yaml protocols lack this restriction, and can accurately roundtrip any IEEE754 64 bit floating point value.

For all protocols, if a float type is specified and an int value is provided, the int will be automatically converted.

>>> msgspec.json.encode(123.0)
b"123.0"

>>> # JSON doesn't support nonfinite values, these serialize as null
... msgspec.json.encode(float("nan"))
b"null"

>>> msgspec.json.decode(b"123.0", type=float)
123.0

>>> # Ints are automatically converted to floats
... msgspec.json.decode(b"123", type=float)
123.0

If strict=False is specified, string values may also be coerced to floats. Note that in this case the strings "nan", "inf"/"infinity", "-inf"/"-infinity" (case insensitive) will coerce to nan/inf/-inf. See “Strict” vs “Lax” Mode for more information.

>>> msgspec.json.decode(b'"123.45"', type=float, strict=False)
123.45

>>> msgspec.json.decode(b'"-inf"', type=float, strict=False)
-inf

str

Strings map to strings in all supported protocols.

Note that for JSON, only the characters required by RFC8259 are escaped to ascii; unicode characters (e.g. "𝄞") are not escaped and are serialized directly as UTF-8 bytes.

>>> msgspec.json.encode("Hello, world!")
b'"Hello, world!"'

>>> msgspec.json.encode("𝄞 is not escaped")
b'"\xf0\x9d\x84\x9e is not escaped"'

>>> msgspec.json.decode(b'"Hello, world!"')
"Hello, world!"

bytes / bytearray / memoryview

Bytes-like objects map to base64-encoded strings in JSON, YAML, and TOML. The bin type is used for MessagePack.

>>> msg = msgspec.json.encode(b"\xf0\x9d\x84\x9e")

>>> msg
b'"85+Eng=="'

>>> msgspec.json.decode(msg, type=bytes)
b'\xf0\x9d\x84\x9e'

>>> msgspec.json.decode(msg, type=bytearray)
bytearray(b'\xf0\x9d\x84\x9e')

Note

For the msgpack protocol, memoryview objects will be decoded as direct views into the larger buffer containing the input message being decoded. This may be useful for implementing efficient zero-copy handling of large binary messages, but is also a potential footgun. As long as a decoded memoryview remains in memory, the input message buffer will also be persisted, potentially resulting in unnecessarily large memory usage. The usage of memoryview types in this manner is considered an advanced topic, and should only be used when you know their usage will result in a performance benefit.

For all other protocols memoryview objects will still result in a copy, and will likely be slightly slower than decoding into a bytes object

datetime

The encoding used for datetime.datetime objects depends on both the protocol and whether these objects are timezone-aware or timezone-naive:

  • JSON: Timezone-aware datetimes are encoded as RFC3339 compatible strings. Timezone-naive datetimes are encoded the same, but lack the timezone component (making them not strictly RFC3339 compatible, but still ISO8601 compatible).

  • MessagePack: Timezone-aware datetimes are encoded using the timestamp extension. Timezone-naive datetimes are encoded the same, but lack the timezone component (making them not strictly RFC3339 compatible, but still ISO8601 compatible). During decoding, both string and timestamp-extension values are supported for flexibility.

  • YAML: Datetimes are encoded using YAML’s native datetime type. Both timezone-aware and timezone-naive datetimes are supported.

  • TOML: Datetimes are encoded using TOML’s native datetime type. Both timezone-aware and timezone-naive datetimes are supported.

Note that you can require a datetime.datetime object to be timezone-aware or timezone-naive by specifying a tz constraint (see Datetime Constraints for more information).

>>> import datetime

>>> tz = datetime.timezone(datetime.timedelta(hours=6))

>>> tz_aware = datetime.datetime(2021, 4, 2, 18, 18, 10, 123, tzinfo=tz)

>>> msg = msgspec.json.encode(tz_aware)

>>> msg
b'"2021-04-02T18:18:10.000123+06:00"'

>>> msgspec.json.decode(msg, type=datetime.datetime)
datetime.datetime(2021, 4, 2, 18, 18, 10, 123, tzinfo=datetime.timezone(datetime.timedelta(seconds=21600)))

>>> tz_naive = datetime.datetime(2021, 4, 2, 18, 18, 10, 123)

>>> msg = msgspec.json.encode(tz_naive)

>>> msg
b'"2021-04-02T18:18:10.000123"'

>>> msgspec.json.decode(msg, type=datetime.datetime)
datetime.datetime(2021, 4, 2, 18, 18, 10, 123)

>>> msgspec.json.decode(b'"oops"', type=datetime.datetime)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid RFC3339 encoded datetime

Additionally, if strict=False is specified, all protocols will decode ints, floats, or strings containing ints/floats as timezone-aware datetimes, interpreting the value as seconds since the epoch in UTC (a Unix Timestamp). See “Strict” vs “Lax” Mode for more information.

>>> msgspec.json.decode(b"1617405490.000123", type=datetime.datetime)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `datetime`, got `float`

>>> msgspec.json.decode(b"1617405490.000123", type=datetime.datetime, strict=False)
datetime.datetime(2021, 4, 2, 18, 18, 10, 123, tzinfo=datetime.timezone.utc)

date

datetime.date values map to:

  • JSON: RFC3339 encoded strings

  • MessagePack: RFC3339 encoded strings

  • YAML: YAML’s native date type

  • TOML TOML’s native date type

>>> import datetime

>>> date = datetime.date(2021, 4, 2)

>>> msg = msgspec.json.encode(date)

>>> msg
b'"2021-04-02"'

>>> msgspec.json.decode(msg, type=datetime.date)
datetime.date(2021, 4, 2)

>>> msgspec.json.decode(b'"oops"', type=datetime.date)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid RFC3339 encoded date

time

The encoding used for datetime.time objects is dependent on both the protocol and whether these objects are timezone-aware or timezone-naive:

  • JSON, MessagePack, and YAML: Timezone-aware times are encoded as RFC3339 compatible strings. Timezone-naive times are encoded the same, but lack the timezone component (making them not strictly RFC3339 compatible, but still ISO8601 compatible).

  • TOML: Timezone-naive times are encoded using TOML’s native time type. Timezone-aware times aren’t supported.

Note that you can require a datetime.time object to be timezone-aware or timezone-naive by specifying a tz constraint (see Datetime Constraints for more information).

>>> import datetime

>>> tz = datetime.timezone(datetime.timedelta(hours=6))

>>> tz_aware = datetime.time(18, 18, 10, 123, tzinfo=tz)

>>> msg = msgspec.json.encode(tz_aware)

>>> msg
b'"18:18:10.000123+06:00"'

>>> msgspec.json.decode(msg, type=datetime.time)
datetime.time(18, 18, 10, 123, tzinfo=datetime.timezone(datetime.timedelta(seconds=21600)))

>>> tz_naive = datetime.time(18, 18, 10, 123)

>>> msg = msgspec.json.encode(tz_naive)

>>> msg
b'"18:18:10.000123"'

>>> msgspec.json.decode(msg, type=datetime.time)
datetime.time(18, 18, 10, 123)

>>> msgspec.json.decode(b'"oops"', type=datetime.time)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid RFC3339 encoded time

timedelta

datetime.timedelta values map to extended ISO 8601 duration strings in all protocols.

The format as described in the ISO specification is fairly lax and a bit underspecified, leading most real-world implementations to implement a stricter subset.

The duration format used here is as follows:

[+/-]P[#D][T[#H][#M][#S]]

  • The format starts with an optional sign (- or +). If negative, the whole duration is negated.

  • The letter P follows (case insensitive)

  • There are then four segments, each consisting of a number and unit. The units are D, H, M, S (case insensitive) for days, hours, minutes, and seconds respectively. These segments must occur in this order.

    • If a segment would have a 0 value it may be omitted, with the caveat that at least one segment must be present.

    • If a time (hour, minute, or second) segment is present then the letter T (case insensitive) must precede the first time segment. Likewise if a T is present, there must be at least 1 segment after the T.

    • Each segment is composed of 1 or more digits, followed by the unit. Leading 0s are accepted. The final segment may include a decimal component if needed.

A few examples:

"P0D"                # 0 days
"P1D"                # 1 Day
"PT1H30S"            # 1 Hour and 30 minutes
"PT1.5H"             # 1 Hour and 30 minutes
"-PT1M30S"           # -90 seconds
"PT1H30M25.5S"       # 1 Hour, 30 minutes, and 25.5 seconds

While msgspec will decode duration strings making use of the H (hour) or M (minute) units, durations encoded by msgspec will only consist of D (day) and S (second) segments.

The implementation in msgspec is compatible with the ones in:

  • Java’s time.Duration.parse (docs)

  • Javascript’s proposed Temporal.Duration standard API (docs)

  • Python libraries like pendulum or pydantic.

Duration strings produced by msgspec should be interchangeable with these libraries, as well as similar ones in other language ecosystems.

>>> from datetime import timedelta

>>> msgspec.json.encode(timedelta(seconds=123))
b'"PT123S"'

>>> msgspec.json.encode(timedelta(days=1, seconds=30, microseconds=123))
b'"P1DT30.000123S"'

>>> msgspec.json.decode(b'"PT123S"', type=timedelta)
datetime.timedelta(seconds=123)

>>> msgspec.json.decode(b'"PT1.5M"', type=timedelta)
datetime.timedelta(seconds=90)

>>> msgspec.json.decode(b'"oops"', type=datetime.timedelta)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid ISO8601 duration

Additionally, if strict=False is specified, all protocols will decode ints, floats, or strings containing ints/floats as timedeltas, interpreting the value as total seconds. See “Strict” vs “Lax” Mode for more information.

>>> msgspec.json.decode(b"123.4", type=datetime.timedelta)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `duration`, got `float`

>>> msgspec.json.decode(b"123.4", type=datetime.timedelta, strict=False)
datetime.timedelta(seconds=123, microseconds=400000)

uuid

uuid.UUID values are serialized as RFC4122 encoded canonical strings in all protocols by default. Subclasses of uuid.UUID are also supported for encoding only.

>>> import uuid

>>> u = uuid.UUID("c4524ac0-e81e-4aa8-a595-0aec605a659a")

>>> msgspec.json.encode(u)
b'"c4524ac0-e81e-4aa8-a595-0aec605a659a"'

>>> msgspec.json.decode(b'"c4524ac0-e81e-4aa8-a595-0aec605a659a"', type=uuid.UUID)
UUID('c4524ac0-e81e-4aa8-a595-0aec605a659a')

>>> msgspec.json.decode(b'"oops"', type=uuid.UUID)
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid UUID

Alternative formats are also supported by the JSON and MessagePack encoders. The format may be selected by passing it to uuid_format when creating an Encoder. The following options are supported:

  • canonical: UUIDs are encoded as RFC4122 canonical strings (same as str(uuid)). This is the default.

  • hex: UUIDs are encoded as RFC4122 hex strings (same as uuid.hex).

  • bytes: UUIDs are encoded as binary values of the uuid’s big-endian 128-bit integer representation (same as uuid.bytes). This is only supported by the MessagePack encoder.

When decoding, any of the above formats are accepted.

>>> enc = msgspec.json.Encoder(uuid_format="hex")

>>> uuid_hex = enc.encode(u)

>>> uuid_hex
b'"c4524ac0e81e4aa8a5950aec605a659a"'

>>> msgspec.json.decode(uuid_hex, type=uuid.UUID)
UUID('c4524ac0-e81e-4aa8-a595-0aec605a659a')

>>> enc = msgspec.msgpack.Encoder(uuid_format="bytes")

>>> uuid_bytes = enc.encode(u)

>>> msgspec.msgpack.decode(uuid_bytes, type=uuid.UUID)
UUID('c4524ac0-e81e-4aa8-a595-0aec605a659a')

decimal

decimal.Decimal values are encoded as their string representation in all protocols by default. This ensures no precision loss during serialization, as would happen with a float representation.

>>> import decimal

>>> x = decimal.Decimal("1.2345")

>>> msg = msgspec.json.encode(x)

>>> msg
b'"1.2345"'

>>> msgspec.json.decode(msg, type=decimal.Decimal)
Decimal('1.2345')

>>> msgspec.json.decode(b'"oops"', type=decimal.Decimal)
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid decimal string

For JSON and MessagePack you may instead encode decimal values the same as numbers by creating a Encoder and specifying decimal_format='number'.

>>> encoder = msgspec.json.Encoder(decimal_format="number")

>>> encoder.encode(x)
b'1.2345'

This setting is not yet supported for YAML or TOML - if this option is important for you please open an issue.

All protocols will also decode decimal.Decimal values from int or float inputs. For JSON the value is parsed directly from the serialized bytes, avoiding any precision loss:

>>> msgspec.json.decode(b"1.3", type=decimal.Decimal)
Decimal('1.3')

>>> msgspec.json.decode(b"1.300", type=decimal.Decimal)
Decimal('1.300')

>>> msgspec.json.decode(b"0.1234567891234567811", type=decimal.Decimal)
Decimal('0.1234567891234567811')

Other protocols will coerce float inputs to the shortest decimal value that roundtrips back to the corresponding IEEE754 float representation (this is effectively equivalent to decimal.Decimal(str(float_val))). This may result in precision loss for some inputs! In general we recommend avoiding parsing decimal.Decimal values from anything but strings.

>>> msgspec.yaml.decode(b"1.3", type=decimal.Decimal)
Decimal('1.3')

>>> msgspec.yaml.decode(b"1.300", type=decimal.Decimal)  # trailing 0s truncated!
Decimal('1.3')

>>> msgspec.yaml.decode(b"0.1234567891234567811", type=decimal.Decimal)  # precision loss!
Decimal('0.12345678912345678')

list / tuple / set / frozenset

list, tuple, set, and frozenset objects map to arrays in all protocols. An error is raised if the elements don’t match the specified element type (if provided).

Subclasses of these types are also supported for encoding only. To decode into a list subclass you’ll need to implement a dec_hook (see Extending).

>>> msgspec.json.encode([1, 2, 3])
b'[1,2,3]'

>>> msgspec.json.encode({1, 2, 3})
b'[1,2,3]'

>>> msgspec.json.decode(b'[1,2,3]', type=set)
{1, 2, 3}

>>> from typing import Set

>>> # Decode as a set of ints
... msgspec.json.decode(b'[1, 2, 3]', type=Set[int])
{1, 2, 3}

>>> # Oops, all elements should be ints
... msgspec.json.decode(b'[1, 2, "oops"]', type=Set[int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str` - at `$[2]`

NamedTuple

typing.NamedTuple types map to arrays in all protocols. An error is raised during decoding if the type doesn’t match or if any required fields are missing.

Note that msgspec supports both typing.NamedTuple and collections.namedtuple, although the latter lacks a way to specify field types.

When possible we recommend using msgspec.Struct (possibly with array_like=True and frozen=True) instead of NamedTuple for specifying schemas - Structs are faster, more ergonomic, and support additional features. Still, you may want to use a NamedTuple if you’re already using them elsewhere, or if you have downstream code that requires a tuple instead of an object.

>>> from typing import NamedTuple

>>> class Person(NamedTuple):
...     name: str
...     age: int

>>> ben = Person("ben", 25)

>>> msg = msgspec.json.encode(ben)

>>> msgspec.json.decode(msg, type=Person)
Person(name='ben', age=25)

>>> wrong_type = b'["chad", "twenty"]'

>>> msgspec.json.decode(wrong_type, type=Person)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str` - at `$[1]`

Other types that duck-type as NamedTuple are also supported, such as os.stat_result.

>>> import os

>>> import sys

>>> result = os.stat(sys.executable)

>>> result
os.stat_result(st_mode=33261, st_ino=5396073, st_dev=105, st_nlink=1, st_uid=0, st_gid=0, st_size=18440, st_atime=1760547094, st_mtime=1760547094, st_ctime=1760907672)

>>> msgspec.json.encode(result)
b'[33261,5396073,105,1,0,0,18440,1760547094,1760547094,1760907672]'

dict

Dicts encode/decode as objects/maps in all protocols.

Dict subclasses (collections.OrderedDict, for example) are also supported for encoding only. To decode into a dict subclass you’ll need to implement a dec_hook (see Extending).

JSON and TOML only support key types that encode as strings or numbers (for example str, int, float, enum.Enum, datetime.datetime, uuid.UUID, …). MessagePack and YAML support any hashable for the key type.

An error is raised during decoding if the keys or values don’t match their respective types (if specified).

>>> msgspec.json.encode({"x": 1, "y": 2})
b'{"x":1,"y":2}'

>>> from typing import Dict

>>> # Decode as a Dict of str -> int
... msgspec.json.decode(b'{"x":1,"y":2}', type=Dict[str, int])
{"x": 1, "y": 2}

>>> # Oops, there's a mistyped value
... msgspec.json.decode(b'{"x":1,"y":"oops"}', type=Dict[str, int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str` - at `$[...]`

TypedDict

typing.TypedDict provides a way to specify different types for different values in a dict, rather than a single value type (the int in Dict[str, int], for example). At runtime these are just standard dict types, the TypedDict type is only there to provide the schema information during decoding. Note that msgspec supports both typing.TypedDict and typing_extensions.TypedDict (a backport).

typing.TypedDict types map to objects/maps in all protocols. During decoding, any extra fields are ignored. An error is raised during decoding if the type doesn’t match or if any required fields are missing.

When possible we recommend using msgspec.Struct instead of TypedDict for specifying schemas - Structs are faster, more ergonomic, and support additional features. Still, you may want to use a TypedDict if you’re already using them elsewhere, or if you have downstream code that requires a dict instead of an object.

>>> from typing import TypedDict

>>> class Person(TypedDict):
...     name: str
...     age: int

>>> ben = {"name": "ben", "age": 25}

>>> msg = msgspec.json.encode(ben)

>>> msgspec.json.decode(msg, type=Person)
{'name': 'ben', 'age': 25}

>>> wrong_type = b'{"name": "chad", "age": "twenty"}'

>>> msgspec.json.decode(wrong_type, type=Person)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str` - at `$.age`

dataclasses

dataclasses map to objects/maps in all protocols.

During decoding, any extra fields are ignored. An error is raised if a field’s type doesn’t match or if any required fields are missing.

If a __post_init__ method is defined on the dataclass, it is called after the object is decoded. Note that “Init-only parameters” (i.e. InitVar fields) are _not_ supported.

When possible we recommend using msgspec.Struct instead of dataclasses for specifying schemas - Structs are faster, more ergonomic, and support additional features.

>>> from dataclasses import dataclass

>>> @dataclass
... class Person:
...     name: str
...     age: int

>>> carol = Person(name="carol", age=32)

>>> msg = msgspec.json.encode(carol)

>>> msgspec.json.decode(msg, type=Person)
Person(name='carol', age=32)

>>> wrong_type = b'{"name": "doug", "age": "thirty"}'

>>> msgspec.json.decode(wrong_type, type=Person)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str` - at `$.age`

Other types that duck-type as dataclasses are also supported, such as pydantic dataclasses.

>>> from datetime import datetime

>>> from pydantic.dataclasses import dataclass

>>> @dataclass
... class User:
...     id: int
...     name: str = 'John Doe'
...     signup_ts: datetime | None = None

>>> user = User(id='42', signup_ts='2032-06-21T12:00')

>>> user
User(id=42, name='John Doe', signup_ts=datetime.datetime(2032, 6, 21, 12, 0))

>>> msgspec.json.encode(user)
b'{"id":42,"name":"John Doe","signup_ts":"2032-06-21T12:00:00"}'

attrs

attrs types map to objects/maps in all protocols.

During encoding, all attributes without a leading underscore ("_") are encoded.

During decoding, any extra fields are ignored. An error is raised if a field’s type doesn’t match or if any required fields are missing.

If the __attrs_pre_init__ or __attrs_post_init__ methods are defined on the class, they are called as part of the decoding process. Likewise, if a class makes use of attrs’ validators, the validators will be called, and a msgspec.ValidationError raised on error. Note that attrs’ converters are not currently supported.

When possible we recommend using msgspec.Struct instead of attrs types for specifying schemas - Structs are faster, more ergonomic, and support additional features.

>>> from attrs import define

>>> @define
... class Person:
...     name: str
...     age: int

>>> carol = Person(name="carol", age=32)

>>> msg = msgspec.json.encode(carol)

>>> msgspec.json.decode(msg, type=Person)
Person(name='carol', age=32)

>>> wrong_type = b'{"name": "doug", "age": "thirty"}'

>>> msgspec.json.decode(wrong_type, type=Person)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str` - at `$.age`

Struct

Structs are the preferred way of defining structured data types in msgspec. You can think of them as similar to dataclasses/attrs/pydantic, but much faster to create/compare/encode/decode. For more information, see the Structs page.

By default msgspec.Struct types map to objects/maps in all protocols. During decoding, any unknown fields are ignored (this can be disabled, see Forbidding Unknown Fields), and any missing optional fields have their default values applied. An error is raised during decoding if the type doesn’t match or if any required fields are missing.

>>> from typing import Set, Optional

>>> class User(msgspec.Struct):
...     name: str
...     groups: Set[str] = set()
...     email: Optional[str] = None

>>> alice = User("alice", groups={"admin", "engineering"})

>>> msgspec.json.encode(alice)
b'{"name":"alice","groups":["admin","engineering"],"email":null}'

>>> msg = b"""
... {
...     "name": "bob",
...     "email": "bob@company.com",
...     "unknown_field": [1, 2, 3]
... }
... """

>>> msgspec.json.decode(msg, type=User)
User(name='bob', groups=[], email="bob@company.com")

>>> wrong_type = b"""
... {
...     "name": "bob",
...     "groups": ["engineering", 123]
... }
... """

>>> msgspec.json.decode(wrong_type, type=User)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `str`, got `int` - at `$.groups[1]`

If you pass array_like=True when defining the struct type, they’re instead treated as array types during encoding/decoding. In this case fields are serialized in their field order. This can further improve performance at the cost of less human readable messaging. Like array_like=False (the default) structs, extra (trailing) fields are ignored during decoding, and any missing optional fields have their defaults applied. Type checking also still applies.

>>> from typing import Set, Optional

>>> class User(msgspec.Struct, array_like=True):
...     name: str
...     groups: Set[str] = set()
...     email: Optional[str] = None

>>> alice = User("alice", groups={"admin", "engineering"})

>>> msgspec.json.encode(alice)
b'["alice",["admin","engineering"],null]'

>>> msgspec.json.decode(b'["bob"]', type=User)
User(name="bob", groups=[], email=None)

>>> msgspec.json.decode(b'["carol", ["admin"], null, ["extra", "field"]]', type=User)
User(name="carol", groups=["admin"], email=None)

>>> msgspec.json.decode(b'["david", ["finance", 123]]')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `str`, got `int` - at `$[1][1]`

UNSET

msgspec.UNSET is a singleton object used to indicate that a field has no set value. This is useful for cases where you need to differentiate between a message where a field is missing and a message where the field is explicitly None.

>>> from msgspec import Struct, UnsetType, UNSET, json

>>> class Example(Struct):
...     x: int
...     y: int | None | UnsetType = UNSET  # a field, defaulting to UNSET

During encoding, any field containing UNSET is omitted from the message.

>>> json.encode(Example(1))  # y is UNSET
b'{"x":1}'

>>> json.encode(Example(1, UNSET))  # y is UNSET
b'{"x":1}'

>>> json.encode(Example(1, None))  # y is None
b'{"x":1,"y":null}'

>>> json.encode(Example(1, 2))  # y is 2
b'{"x":1,"y":2}'

During decoding, if a field isn’t explicitly set in the message, the default value of UNSET will be set instead. This lets downstream consumers determine whether a field was left unset, or explicitly set to None

>>> json.decode(b'{"x": 1}', type=Example)  # y defaults to UNSET
Example(x=1, y=UNSET)

>>> json.decode(b'{"x": 1, "y": null}', type=Example)  # y is None
Example(x=1, y=None)

>>> json.decode(b'{"x": 1, "y": 2}', type=Example)  # y is 2
Example(x=1, y=2)

UNSET fields are supported for msgspec.Struct, dataclasses, and attrs types. It is an error to use msgspec.UNSET or msgspec.UnsetType anywhere other than a field for one of these types.

Enum / IntEnum / StrEnum

Enum types (enum.Enum, enum.IntEnum, enum.StrEnum, …) encode as their member values in all protocols.

Any enum whose value is a supported type may be encoded, but only enums composed of all string or all integer values may be decoded.

An error is raised during decoding if the value isn’t the proper type, or doesn’t match any valid member.

>>> import enum

>>> class Fruit(enum.Enum):
...     APPLE = "apple"
...     BANANA = "banana"

>>> msgspec.json.encode(Fruit.APPLE)
b'"apple"'

>>> msgspec.json.decode(b'"apple"', type=Fruit)
<Fruit.APPLE: 'apple'>

>>> msgspec.json.decode(b'"grape"', type=Fruit)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid enum value 'grape'

>>> class JobState(enum.IntEnum):
...     CREATED = 0
...     RUNNING = 1
...     SUCCEEDED = 2
...     FAILED = 3

>>> msgspec.json.encode(JobState.RUNNING)
b'1'

>>> msgspec.json.decode(b'2', type=JobState)
<JobState.SUCCEEDED: 2>

>>> msgspec.json.decode(b'4', type=JobState)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid enum value 4

If the enum type includes a _missing_ method (docs), this method will be called to handle any missing values. It should return a valid enum member, or None if the value is invalid. One potential use case of this is supporting case-insensitive enums:

>>> import enum

>>> class Fruit(enum.Enum):
...     APPLE = "apple"
...     BANANA = "banana"
...
...     @classmethod
...     def _missing_(cls, name):
...         """Called to handle missing enum values"""
...         # Normalize value to lowercase
...         value = name.lower()
...         # Return valid enum value, or None if invalid
...         return cls._value2member_map_.get(value)

>>> msgspec.json.decode(b'"apple"', type=Fruit)
<Fruit.APPLE: "apple">

>>> msgspec.json.decode(b'"ApPlE"', type=Fruit)
<Fruit.APPLE: "apple">

>>> msgspec.json.decode(b'"grape"', type=Fruit)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid enum value 'grape'

Literal

typing.Literal types can be used to ensure that a decoded object is within a set of valid values. An enum.Enum or enum.IntEnum can be used for the same purpose, but with a typing.Literal the decoded values are literal int or str instances rather than enum objects.

A literal can be composed of any of the following objects:

An error is raised during decoding if the value isn’t in the set of valid values, or doesn’t match any of their component types.

>>> from typing import Literal

>>> msgspec.json.decode(b'1', type=Literal[1, 2, 3])
1

>>> msgspec.json.decode(b'"one"', type=Literal["one", "two", "three"])
'one'

>>> msgspec.json.decode(b'4', type=Literal[1, 2, 3])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Invalid enum value 4

>>> msgspec.json.decode(b'"bad"', type=Literal[1, 2, 3])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str`

NewType

typing.NewType types are treated identically to their base type. Their support here is purely to aid static analysis tools like mypy or pyright.

>>> from typing import NewType

>>> UserId = NewType("UserId", int)

>>> msgspec.json.encode(UserId(1234))
b'1234'

>>> msgspec.json.decode(b'1234', type=UserId)
1234

>>> msgspec.json.decode(b'"oops"', type=UserId)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str`

Type Aliases

For complex types, sometimes it can be nice to write the type once so you can reuse it later.

Point = tuple[float, float]

Here Point is a “type alias” for tuple[float, float] - msgspec will substitute in tuple[float, float] whenever the Point type is used in an annotation.

msgspec supports the following equivalent forms:

# Using variable assignment
Point = tuple[float, float]

# Using variable assignment, annotated as a `TypeAlias`
Point: TypeAlias = tuple[float, float]

# Using Python 3.12's new `type` statement. This only works on Python 3.12+
type Point = tuple[float, float]

To learn more about Type Aliases, see Python’s Type Alias docs here.

Generic Types

msgspec supports generic types, including user-defined generic types based on any of the following types:

Generic types may be useful for reusing common message structures.

To define a generic type:

  • Define one or more type variables (typing.TypeVar) to parametrize your type with.

  • Add typing.Generic as a base class when defining your type, parametrizing it by the relevant type variables.

  • When annotating the field types, use the relevant type variables instead of “concrete” types anywhere you want to be generic.

For example, here we define a generic Paginated struct type for storing extra pagination information in an API response.

import msgspec
from typing import Generic, TypeVar

# A type variable for the item type
T = TypeVar("T")

class Paginated(msgspec.Struct, Generic[T]):
    """A generic paginated API wrapper, parametrized by the item type."""
    page: int        # The current page number
    per_page: int    # Number of items per page
    total: int       # The total number of items found
    items: list[T]   # Items returned, up to `per_page` in length

This type is generic over the type of item contained in Paginated.items. This Paginated wrapper may then be used to decode a message containing a specific item type by parametrizing it with that type. When processing a generic type, the parametrized types are substituted for the type variables.

Here we define a User type, then use it to decode a paginated API response containing a list of users:

class User(msgspec.Struct):
    """A user model"""
    name: str
    groups: list[str] = []

json_str = """
{
    "page": 1,
    "per_page": 5,
    "total": 252,
    "items": [
        {"name": "alice", "groups": ["admin"]},
        {"name": "ben"},
        {"name": "carol", "groups": ["engineering"]},
        {"name": "dan", "groups": ["hr"]},
        {"name": "ellen", "groups": ["engineering"]}
    ]
}
"""

# Decode a paginated response containing a list of users
msg = msgspec.json.decode(json_str, type=Paginated[User])
print(msg)
#> Paginated(
#>     page=1, per_page=5, total=252,
#>     items=[
#>         User(name='alice', groups=['admin']),
#>         User(name='ben', groups=[]),
#>         User(name='carol', groups=['engineering']),
#>         User(name='dan', groups=['hr']),
#>         User(name='ellen', groups=['engineering'])
#>     ]
#> )

If instead we wanted to decode a paginated response of another type (say Team), we could do this by parametrizing Paginated with a different type.

# Decode a paginated response containing a list of teams
msgspec.json.decode(some_other_message, type=Paginated[Team])

Any unparametrized type variables will be treated as typing.Any when decoding.

# These are equivalent.
# The unparametrized version substitutes in `Any` for `T`
msgspec.json.decode(some_other_message, type=Paginated)
msgspec.json.decode(some_other_message, type=Paginated[Any])

However, if an unparametrized type variable has a bound (docs), then the bound type will be used instead.

from collections.abc import Sequence
S = TypeVar("S", bound=Sequence)  # Can be any sequence type

class Example(msgspec.Struct, Generic[S]):
    value: S

msg = b'{"value": [1, 2, 3]}'

# These are equivalent.
# The unparametrized version substitutes in `Sequence` for `S`
msgspec.json.decode(some_other_message, type=Example)
msgspec.json.decode(some_other_message, type=Example[Sequence])

See the official Python docs on generic types and the corresponding PEP for more information.

Abstract Types

msgspec supports several “abstract” types, decoding them as instances of their most common concrete type.

Decoded as lists

Decoded as sets

Decoded as dicts

>>> from typing import MutableMapping

>>> msgspec.json.decode(b'{"x": 1}', type=MutableMapping[str, int])
{"x": 1}

>>> msgspec.json.decode(b'{"x": "oops"}', type=MutableMapping[str, int])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `str` - at `$[...]`

Union / Optional

Type unions are supported, with a few restrictions. These restrictions are in place to remove any ambiguity during decoding - given an encoded value there must always be a single type in a given typing.Union that can decode that value.

Union restrictions are as follows:

>>> from typing import Union, List

>>> # A decoder expecting either an int, a str, or a list of strings
... decoder = msgspec.json.Decoder(Union[int, str, List[str]])

>>> decoder.decode(b'1')
1

>>> decoder.decode(b'"two"')
"two"

>>> decoder.decode(b'["three", "four"]')
["three", "four"]

>>> decoder.decode(b'false')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int | str | array`, got `bool`

Raw

msgspec.Raw is a buffer-like type containing an already encoded messages. They have two common uses:

1. Avoiding unnecessary encoding cost

Wrapping an already encoded buffer in msgspec.Raw lets the encoder avoid re-encoding the message, instead it will simply be copied to the output buffer. This can be useful when part of a message already exists in an encoded format (e.g. reading JSON bytes from a database and returning them as part of a larger message).

>>> import msgspec

>>> # Create a new `Raw` object wrapping a pre-encoded message
... fragment = msgspec.Raw(b'{"x": 1, "y": 2}')

>>> # Compose a larger message containing the pre-encoded fragment
... msg = {"a": 1, "b": fragment}

>>> # During encoding, the raw message is efficiently copied into
... # the output buffer, avoiding any extra encoding cost
... msgspec.json.encode(msg)
b'{"a":1,"b":{"x": 1, "y": 2}}'

2. Delaying decoding of part of a message

Sometimes the type of a serialized value depends on the value of other fields in a message. msgspec provides an optimized version of one common pattern (Tagged Unions), but if you need to do something more complicated you may find using msgspec.Raw useful here.

For example, here we demonstrate how to decode a message where the type of one field (point) depends on the value of another (dimensions).

>>> import msgspec

>>> from typing import Union

>>> class Point1D(msgspec.Struct):
...     x: int

>>> class Point2D(msgspec.Struct):
...     x: int
...     y: int

>>> class Point3D(msgspec.Struct):
...     x: int
...     y: int
...     z: int

>>> class Model(msgspec.Struct):
...     dimensions: int
...     point: msgspec.Raw  # use msgspec.Raw to delay decoding the point field

>>> def decode_point(msg: bytes) -> Union[Point1D, Point2D, Point3D]:
...     """A function for efficiently decoding the `point` field"""
...     # First decode the outer `Model` struct. Decoding of the `point`
...     # field is delayed, with the composite bytes stored as a `Raw` object
...     # on `point`.
...     model = msgspec.json.decode(msg, type=Model)
...
...     # Based on the value of `dimensions`, determine which type to use
...     # when decoding the `point` field
...     if model.dimensions == 1:
...         point_type = Point1D
...     elif model.dimensions == 2:
...         point_type = Point2D
...     elif model.dimensions == 3:
...         point_type = Point3D
...     else:
...         raise ValueError("Too many dimensions!")
...
...     # Now that we know the type of `point`, we can finish decoding it.
...     # Note that `Raw` objects are buffer-like, and can be passed
...     # directly to the `decode` method.
...     return msgspec.json.decode(model.point, type=point_type)

>>> decode_point(b'{"dimensions": 2, "point": {"x": 1, "y": 2}}')
Point2D(x=1, y=2)

>>> decode_point(b'{"dimensions": 3, "point": {"x": 1, "y": 2, "z": 3}}')
Point3D(x=1, y=2, z=3)

Any

When decoding a message with Any type (or no type specified), encoded types map to Python types in a protocol specific manner.

JSON

JSON types are decoded to Python types as follows:

MessagePack

MessagePack types are decoded to Python types as follows:

YAML

YAML types are decoded to Python types as follows:

TOML

TOML types are decoded to Python types as follows: