Polyserde

Introduction

polyserde solves a common problem when saving Pydantic models to JSON: it preserves the exact class types. When you use Pydantic's built-in model_dump(), it loses information about subclasses - so if you have a list of Animal objects that are actually Cat and Dog instances, the JSON won't remember which is which. polyserde embeds type information (like "__class__": "myapp.Cat") directly into the JSON, so when you load it back later, it automatically imports the right classes and reconstructs your exact object structure - no manual type tracking needed. It even checks if the library version you're loading with is compatible with the one that saved the file, warning you if there might be breaking changes (using semantic versioning rules). This makes it perfect for saving complex configuration objects, ML pipelines, or any nested data where preserving polymorphic types matters.

Features

• Polymorphic type preservation — automatically remembers exact subclass types (e.g., Cat vs Dog when both inherit from Animal)
• Automatic class imports — reconstructs objects by importing the right modules when deserializing
• Semantic version checking — warns if the saved config may be incompatible with installed library versions
• Supports complex Python types — handles enums, class references, and dictionaries with non-string keys
• Human-readable JSON — produces self-describing, inspectable output that's easy to debug
• Minimal dependencies — only requires pydantic ≥ 2.0 and packaging

Installation

pip install polyserde

Requires Python ≥ 3.9 and Pydantic ≥ 2.0.

Quick Start

Below is a complete usage example. Suppose the following classes are defined inside a module called zoolib.

# zoolib/__init__.py
from pydantic import BaseModel
from enum import Enum

class Species(Enum):
    CAT = "cat"
    DOG = "dog"

class Animal(BaseModel):
    name: str
    species: Species

class Cat(Animal):
    lives_left: int = 9

class Dog(Animal):
    breed: str
    is_good_boy: bool = True

class Zoo(BaseModel):
    location: str
    animals: list[Animal]
    caretaker_class: type = dict  # just an example class reference

Now use polyserde to serialize and deserialize the structure:

from zoolib import Zoo, Cat, Dog, Species
from polyserde import PolymorphicSerde

zoo = Zoo(
    location="Berlin",
    animals=[
        Cat(name="Mittens", species=Species.CAT, lives_left=7),
        Dog(name="Rex", species=Species.DOG, breed="Labrador"),
    ]
)

# Serialize to dict with metadata
data = PolymorphicSerde.dump(zoo, lib="zoolib", version="1.2.3")

# Save to JSON file
import json
with open("zoo_config.json", "w") as f:
    json.dump(data, f, indent=2)

# Load from JSON file
with open("zoo_config.json") as f:
    data = json.load(f)

# Deserialize (with version checking)
restored = PolymorphicSerde.load(data)
print(restored)
print(type(restored.animals[0]))

Output:

Zoo(location='Berlin', animals=[Cat(...), Dog(...)], caretaker_class=<class 'dict'>)
<class 'zoolib.Cat'>

If the current environment doesn’t have the same library version, polyserde emits helpful warnings such as:

⚠️ Major version mismatch for zoolib: serialized=1.2.3, installed=2.0.0 (config may be incompatible)

How It Works

PolymorphicSerde recursively converts complex Python objects into a JSON-safe structure with embedded type metadata:

• Each Pydantic model is tagged with "__class__": "module.ClassName".
• Enums are represented as "__enum__": "module.EnumClass.MEMBER".
• Class references are stored as "__class_ref__": "module.Class".
• Non-string dict keys are safely represented via { "__dict__": [{"__key__": ..., "value": ...}, ...]}.

This makes every JSON file self-describing — you can reload it anywhere, and PolymorphicSerde will reconstruct the correct objects automatically.

Version Safety

When saving a configuration, you can specify both the library name and version:

data = PolymorphicSerde.dump(my_config, lib="docling", version="0.14.0")

At load time, polyserde:

• Looks up the installed version of the library,
• Parses both versions semantically (using PEP 440),
• Emits a warning if major or minor versions differ,
• Falls back to strict equality for non-semantic versions.

Example warning:

⚠️ Minor version difference for docling: serialized=0.14.0, installed=0.15.0 (review config compatibility)

Contributing

Contributions are welcome! If you’d like to improve the serializer, add features, or extend compatibility, feel free to open a PR or issue.

1. Fork the repo
2. Create a feature branch
3. Run tests (pytest)
4. Submit a PR

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Acknowledgments

Inspired by real-world serialization challenges in projects like Docling, FastAPI, and LangChain, where polymorphic configuration graphs are the norm.

polyserde brings predictable, portable, and version-safe serialization to any Pydantic-based system.