marshmallow 2.0 Released

One alpha, five betas, and two release candidates after marshmallow's last 1.x release, marshmallow 2.0 is published to the PyPI.

What is marshmallow?

Marshmallow is a Python library for

  1. Validating input data against a schema,
  2. Deserializing data to application objects (e.g., ORM objects), and
  3. Serializing application objects to simple types.

Think Django REST Framework's Serializers, without the Django.

For more info, see the homepage:

Benefits of upgrading

Here are a few immediate benefits of upgrading to 2.0:

  • Dump-only and load-only fields (analagous to "read-only" and "write-only" in the context of a CRUD app).
  • More consistent treatment of null and missing values.
  • Powerful, pre- and post-processing API.
from marshmallow import Schema, fields, pre_load, post_dump, post_load

class UserSchema(Schema):
    name = fields.Str()
    email = fields.Email()

    def get_envelope_key(many):
        """Helper to get the envelope key."""
        return 'users' if many else 'user'

    def unwrap_envelope(self, data, many):
        key = self.get_envelope_key(many)
        return data[key]

    def wrap_with_envelope(self, data, many):
        key = self.get_envelope_key(many)
        return {key: data}

    def make_user(self, data):
        return User(**data)
  • More consistent default error messages and a better API for overriding error messages.
errors = schema.validate(invalid_data)
# {
#     'str_field': ['Not a valid string.'],
#     'bool_field': ['Not a valid boolean.'],
#     'int_field': ['Not a valid integer.'],
#     'decimal_field': ['Not a valid number.'],
#     'float_field': ['Not a valid number.']
#     'datetim_field': ['Not a valid datetime.'],
#     'email_field': ['Not a valid email address.'],
#     'url_field': ['Not a valid URL.'],
#     'list_field': ['Not a valid list.'],
# }
  • Better error bundling when validating multiple objects.
  • Improved strict mode.
  • Field and schema validators can be written as methods.


A number of supporting libraries emerged since the last major release.

What will happen to marshmallow 1?

I will continue to merge bug fixes into the 1.2 release line. No new 1.x feature releases are planned. New libraries in the marshmallow-code GitHub organization will likely support marshmallow>=2.0 only.

And moreā€¦

Several refinements were made to the marshmallow API. For details, see the changelog. For a guide on upgrading from marshmallow 1, see the upgrading guide.

Special thanks to everyone who contributed and reported issues. You made this release awesome.

Please send comments by email. I welcome your feedback, advice, and criticism.