---
id: marshmallow-union
version: "0.1.15.post1"
license: unclear
license_treatment: permissive
maintenance: aging
---

# marshmallow-union — Union fields for marshmallow.

License: permissive · Maintenance: aging · Downloads: 1.2M/mo

## What it is and what it does

marshmallow-union extends marshmallow's schema validation by providing a Union field that can accept multiple field types. Instead of defining a schema field as strictly one type, you pass a list of candidate fields, and the union tries each one in order during deserialization and serialization, using the first field that succeeds without raising an error.

The library is straightforward but has a documented caveat: because it relies on trial-and-error field matching rather than type inspection, field order matters and unexpected type coercion can occur. The documentation notes that an Integer field accepting string representations will deserialize string inputs as integers. The package suggests marshmallow-polyfield as an alternative if you need stricter type control.

Use it for:

- Accept API payloads where a field may be either an integer ID or a full object representation
- Handle legacy data where a field type changed over time but both formats must be supported
- Deserialize configuration files with flexible value types across multiple formats
- Build schemas that tolerate multiple input formats without explicit type conversion logic
- Serialize polymorphic data structures where the same logical field holds different types

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Adds union field support to marshmallow, allowing a single field to deserialize and serialize values that match any of several candidate field types.

Yes, if you are already using marshmallow and need union field support with low friction. The package is stable and permissively licensed, but be aware that it has not been actively maintained since 2020 and relies on field-order heuristics rather than strict type matching—test your specific use case carefully, especially if field order could cause unexpected coercion.

## Install

pip install marshmallow-union
uv add marshmallow-union
poetry add marshmallow-union

## Installing marshmallow-union

Before you install: Low install friction with a single runtime dependency on marshmallow. Maintenance is aging—last release was 2020-06-24 and the repository shows no recent commits, though it remains unarchived and has received no security vulnerabilities.

License in practice: MIT license (permissive) means you can use, modify, and distribute this package freely with minimal restrictions, making it safe for both open-source and commercial projects.

Quickstart:

pip install marshmallow-union

from marshmallow import Schema, fields
from marshmallow_union import Union

class MySchema(Schema):
    value = Union([fields.Integer(), fields.String()])

schema = MySchema()
result = schema.load({'value': 'example'})

Verify before relying:

- Whether the package works reliably with recent marshmallow versions (last tested in 2020)
- Current Python version support beyond the 3.6 and 3.7 classifiers listed
- Whether field ordering in the union list significantly impacts real-world performance

## Package facts

- License: not declared (permissive)
- Python support: unspecified
- Install friction: low
- Maintenance: aging
- Downloads: 1.2M/month (top 5,000 on PyPI)
- Known vulnerabilities: none known

## Tags

marshmallow union fields, polymorphic serialization marshmallow, multiple field types marshmallow, union type validation, marshmallow flexible fields, deserialize multiple types, marshmallow schema unions, marshmallow-plugin, schema-validation

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