Overview

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Marshmallow schemas for attrs classes

Automatic generation of marshmallow schemas from dataclasses.

This package is based on marshmallow-dataclass.

Specifying a schema to which your data should conform is very useful, both for (de)serialization and for documentation. However, using schemas in python often means having both a class to represent your data and a class to represent its schema, which means duplicated code that could fall out of sync. With the new features of python 3.6, types can be defined for class members, and that allows libraries like this one to generate schemas automatically.

An use case would be to document APIs (with flasgger, for instance) in a way that allows you to statically check that the code matches the documentation.

Installation

pip install marshmallow-attrs

Documentation

https://marshmallow-attrs.readthedocs.io/

Usage

You simply import `marshmallow_attrs.dataclass instead of attr.dataclass. It adds a Schema property to the generated class, containing a marshmallow Schema class.

If you need to specify custom properties on your marshmallow fields (such as attribute, error, validate, required, dump_only, error_messages, description ...) you can add them using the metadata argument of the attr.ib function.

import attr
from marshmallow_attrs import dataclass # Importing from marshmallow_attrs instead of attrs
import marshmallow.validate
from typing import List, Optional

@dataclass
class Building:
  # The field metadata is used to instantiate the marshmallow field
  height: float = attr.ib(metadata={'validate': marshmallow.validate.Range(min=0)})
  name: str = attr.ib(default="anonymous")


@dataclass
class City:
  name: Optional[str]
  buildings: List[Building] = attr.ib(factory=list)

# City.Schema contains a marshmallow schema class
city, _ = City.Schema().load({
    "name": "Paris",
    "buildings": [
        {"name": "Eiffel Tower", "height":324}
    ]
})

# Serializing city as a json string
city_json, _ = City.Schema().dumps(city)

The previous syntax is very convenient, as the only change you have to apply to your existing code is update the dataclass import.

However, as the .Schema property is added dynamically, it can confuse type checkers. If you want to avoid that, you can also use the standard attr.s decorator, and generate the schema manually using class_schema :

import attr


from datetime import datetime
import marshmallow_attrs

@attr.dataclass
class Person:
    name: str
    birth: datetime

PersonSchema = marshmallow_attrs.class_schema(Person)

You can also declare the schema as a ClassVar:

from marshmallow_attrs import dataclass
from marshmallow import Schema
from typing import ClassVar, Type

@dataclass
class Point:
  x:float
  y:float
  Schema: ClassVar[Type[Schema]] = Schema

You can specify the Meta just as you would in a marshmallow Schema:

from marshmallow_attrs import dataclass

@dataclass
class Point:
  x:float
  y:float
  class Meta:
    ordered = True

Installation

This package is hosted on pypi :

pip install marshmallow-attrs

Documentation

The project documentation is hosted on readthedocs.

Usage warning

This library depends on python's standard typing library, which is provisional.

Credits

This package is based on marshmallow-dataclass.