Generic schema with full typing support and minimal boilerplate
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Generic schema with full typing support and minimal boilerplate


Source Code:

Extension for marshmallow to make deserialization to objects easier and improve type safety.

The main GenericSchema class extends marshmallow.Schema making it generic in terms of the class that data should be deserialized to, when calling load/loads.

With GenericSchema there is no need to explicitly write post_load hooks to initialize the object anymore. 🎉

If the "model" class is (for example) User, it just needs to be passed as the type argument, when subclassing GenericSchema. The output of the load/loads method will then be automatically inferred as either User or list[User] (depending on whether many is True or not) by any competent type checker.

Usage Example

from marshmallow_generic import GenericSchema, fields

class User:
    def __init__(self, name: str, email: str) -> None: = name = email

    def __repr__(self) -> str:
        return "<User(name={!r})>".format(self=self)


class UserSchema(GenericSchema[User]):
    name = fields.Str()
    email = fields.Email()

user_data = {"name": "Monty", "email": ""}
schema = UserSchema()
single_user = schema.load(user_data)
print(single_user)  # <User(name='Monty')>

json_data = '''[
    {"name": "Monty", "email": ""},
    {"name": "Ronnie", "email": ""}
multiple_users = schema.loads(json_data, many=True)
print(multiple_users)  # [<User(name='Monty')>, <User(name='Ronnie')>]

Adding reveal_type(single_user) and reveal_type(multiple_users) at the bottom and running that code through mypy would yield the following output:

# note: Revealed type is "User"
# note: Revealed type is "builtins.list[User]"

With the regular marshmallow.Schema, the output of mypy would instead be this:

# note: Revealed type is "Any"
# note: Revealed type is "Any"

This also means your IDE will be able to infer the types and thus provide useful auto-suggestions for the loaded objects. 👨‍💻

Here is PyCharm with the example from above:

Image title


pip install marshmallow-generic


Python Version 3.9+ and marshmallow (duh)