Welcome to SlySerialize’s documentation!#

Convert JSON-like data structures into nice Python objects.

Key features:

  • Common, built-in types like set, tuple and Enum

  • Generic dataclasses and nested generics

  • Type aliases

  • Union types

  • Recursive types and delayed annotations

  • Custom deserialization

  • Asynchronous custom deserialization

  • Zero dependencies

In just one line:

assert myThing == from_json(MyClass[int], to_json(myThing))


pip install slyserialize

Basic usage#

Call from_json with a target type, with generic arguments, and some json data, such as returned from json.loads. Generic arguments are optional, but if you don’t provide them, you’ll get a TypeError if the target type requires them to be concrete. See the final line in the following example:

from typing import Generic, TypeVar, TypeAlias
from dataclasses import dataclass
from SlySerialize import from_json

ListOfIntegers: TypeAlias = list[int]
T = TypeVar("T")

class MyClass(Generic[T]):
    aliased: ListOfIntegers
    generic: T
    builtin: tuple[float, list[str]]
    union: dict[str, T] | None
    delayed: 'MyClass[T] | None'

my_obj = MyClass[int]([1, 2, 3], 42, (3.1, ["a"]), None, None)

# dataclasses.asdict(my_obj)
serialized = {
    "aliased": [1, 2, 3],   "generic": 42,
    "union":   None,        "delayed": None,
    "builtin": [3.1, ["a"]],

assert my_obj == from_json(MyClass[int], serialized)

Should I use this?#

The goal of this library is to handle deserialization cases for strongly typed dataclass and custom types, and to do so with as little code as possible.

If you only want fast and customizable serialization, but maybe not deserialization, you should use a library like orjson.

If you are using JSON-like types already, without generics or some other specific feature, there are also other libraries that will deserialize faster.

The serialization format for JSON in this library also prefers succinctness at the cost of certain edge cases related to overlapping type representations. See the Default Representations section for more details.


Type variables or mutually recursive types must be declared in the global scope if used in a delayed annotation.

SlySerialize.JsonType is an alias for the return type of json.loads (Python standard library), it represents the native JSON types.

If the type is not supported, or if the representation was different than what was expected, a TypeError will be raised.

dataclasses.asdict only supports serializing dataclasses, JsonType, tuple, and lists, tuples, or dicts of these. There will not be an error, but, the return type is not JsonType. Other types will be passed through unaffected. If you want to serialize a dataclass with a member that is not one of these types, you may want to implement your own serialization.

Default Representations#

The following types are supported by default:

  • NoneType, bool, int, float, and str as themselves

  • list and dict as arrays and maps, with or without their generic arguments

    • dict is only supported if the key type is str

    • The default value type used with no arguments is JsonType

  • tuple and set as an array

  • Dataclasses as maps

  • Enum types of str or int as their value

  • Union types as whichever case the value would otherwise be represented as

  • Generic types are substituted for their concrete type. If the type is not available, a ValueError is raised.

  • The covariant versions of list and map, collections.abc.Sequence and collections.abc.Mapping, are also supported.

    • Consequently, JsonType is a valid type to deserialize to. If you want to do nothing to a member during deserialization, use JsonType as the type.

Not all types are guaranteed to round-trip. For a simple example, list and set are both serialized as a JSON array, so if there is a union list | set, and an empty value, then the first case, list, will be selected during deserialization. Other examples would include dict and classes, or classes that have the same member names.


Implementations for serialization is provided. Some type loaders that are needed for deserialization do not have a corresponding serializer, such as TypeVar and TypeAlias, since it would be unusual for an instance value to be these types.

# ...
from SlySerialize import to_json

assert serialized == to_json(my_obj)

In most cases, to_json should be the inverse of from_json.

Custom Converters#

Custom converters can be created for any type. Subclass Loader or Unloader, or Converter for both. These have a generic argument for the domain type that they serialize to or from. For JSON, this is JsonType. Some of the loaders implemented by this library do not require a specific domain, so they may be combined with custom loaders for other domain types.

Loaders must implement the following two methods:

    def can_load(self, cls: type) -> bool: ...

    def des(self, ctx: DesCtx[Domain], value: Domain, cls: type[MyType]) -> MyType: ...

And unloaders must implement:

    def can_unload(self, cls: type) -> bool: ...

    def ser(self, ctx: SerCtx[Domain], obj: MyType) -> Domain: ...

Where DesCtx and SerCtx are used to recursively call the top level converter being used.

In most cases can_unload can be implemented in terms of can_load.

The converters implemented by this library are available in SlySerialize.COMMON_CONVERTERS, which is a ConverterCollection that implements Converter[JsonType].

To use a custom converter, pass any Converter to from_json or to_json:

# ...

custom_converter = COMMON_CONVERTERS.with_(MyConverter())

thing = from_json(MyClass[int], serialized, custom_converter)

It is not strictly necessary to use a ConverterCollection.

Async Loaders#

Async loaders are supported. They must implement the following methods:

    def can_load(self, cls: type) -> bool: ...

    async def des(self, ctx: DesCtx[Domain], value: Domain, cls: type[MyType]) -> MyType: ...

When a loader is async or part of a ConverterCollection, then from_json_async must be used instead of from_json or an error will be raised. There is no async version of to_json nor any async version of Unloader. It is OK to pass a ConverterCollection with async loaders to to_json, since it will only access converters that implement Loader.

Indices and tables#