custom_fields.rst

Custom Fields

There are three ways to create a custom-formatted field for a Schema:

• Create a custom :class:`Field <marshmallow.fields.Field>` class
• Use a :class:`Method <marshmallow.fields.Method>` field
• Use a :class:`Function <marshmallow.fields.Function>` field

The method you choose will depend on the manner in which you intend to reuse the field.

Creating A Field Class

To create a custom field class, create a subclass of :class:`marshmallow.fields.Field` and implement its :meth:`_serialize <marshmallow.fields.Field._serialize>` and/or :meth:`_deserialize <marshmallow.fields.Field._deserialize>` methods.

from marshmallow import fields, ValidationError


class PinCode(fields.Field):
    """Field that serializes to a string of numbers and deserializes
    to a list of numbers.
    """

    def _serialize(self, value, attr, obj, **kwargs):
        if value is None:
            return ""
        return "".join(str(d) for d in value)

    def _deserialize(self, value, attr, data, **kwargs):
        try:
            return [int(c) for c in value]
        except ValueError as error:
            raise ValidationError("Pin codes must contain only digits.") from error


class UserSchema(Schema):
    name = fields.String()
    email = fields.String()
    created_at = fields.DateTime()
    pin_code = PinCode()

Method Fields

A :class:`Method <marshmallow.fields.Method>` field will serialize to the value returned by a method of the Schema. The method must take an obj parameter which is the object to be serialized.

class UserSchema(Schema):
    name = fields.String()
    email = fields.String()
    created_at = fields.DateTime()
    since_created = fields.Method("get_days_since_created")

    def get_days_since_created(self, obj):
        return dt.datetime.now().day - obj.created_at.day

Function Fields

A :class:`Function <marshmallow.fields.Function>` field will serialize the value of a function that is passed directly to it. Like a :class:`Method <marshmallow.fields.Method>` field, the function must take a single argument obj.

class UserSchema(Schema):
    name = fields.String()
    email = fields.String()
    created_at = fields.DateTime()
    uppername = fields.Function(lambda obj: obj.name.upper())

Method and Function field deserialization

Both :class:`Function <marshmallow.fields.Function>` and :class:`Method <marshmallow.fields.Method>` receive an optional deserialize argument which defines how the field should be deserialized. The method or function passed to deserialize receives the input value for the field.

class UserSchema(Schema):
    # `Method` takes a method name (str), Function takes a callable
    balance = fields.Method("get_balance", deserialize="load_balance")

    def get_balance(self, obj):
        return obj.income - obj.debt

    def load_balance(self, value):
        return float(value)


schema = UserSchema()
result = schema.load({"balance": "100.00"})
result["balance"]  # => 100.0

Adding Context to Method and Function Fields

A :class:`Function <marshmallow.fields.Function>` or :class:`Method <marshmallow.fields.Method>` field may need information about its environment to know how to serialize a value.

In these cases, you can set the context attribute (a dictionary) of a Schema. :class:`Function <marshmallow.fields.Function>` and :class:`Method <marshmallow.fields.Method>` fields will have access to this dictionary.

As an example, you might want your UserSchema to output whether or not a User is the author of a Blog or whether a certain word appears in a Blog's title.

class UserSchema(Schema):
    name = fields.String()
    # Function fields optionally receive context argument
    is_author = fields.Function(lambda user, context: user == context["blog"].author)
    likes_bikes = fields.Method("writes_about_bikes")

    def writes_about_bikes(self, user):
        return "bicycle" in self.context["blog"].title.lower()


schema = UserSchema()

user = User("Freddie Mercury", "fred@queen.com")
blog = Blog("Bicycle Blog", author=user)

schema.context = {"blog": blog}
result = schema.dump(user)
result["is_author"]  # => True
result["likes_bikes"]  # => True

Customizing Error Messages

Validation error messages for fields can be configured at the class or instance level.

At the class level, default error messages are defined as a mapping from error codes to error messages.

from marshmallow import fields


class MyDate(fields.Date):
    default_error_messages = {"invalid": "Please provide a valid date."}

Note

A Field's default_error_messages dictionary gets merged with its parent classes' default_error_messages dictionaries.

Error messages can also be passed to a Field's constructor.

from marshmallow import Schema, fields


class UserSchema(Schema):

    name = fields.Str(
        required=True, error_messages={"required": "Please provide a name."}
    )

Next Steps

• Need to add schema-level validation, post-processing, or error handling behavior? See the :doc:`Extending Schemas <extending>` page.
• For example applications using marshmallow, check out the :doc:`Examples <examples>` page.