Marshmallow 2.0 Released

Sep 29, 2015

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, user-friendly 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.


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.

tagged in python marshmallow

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