Custom error messages

To customize the schema-level error messages that load and loads use when raising a ValidationError, override the error_messages class variable:

class MySchema(Schema):
    error_messages = {
        "unknown": "Custom unknown field error message.",
        "type": "Custom invalid type error message.",
    }

Field-level error message defaults can be set on Field.default_error_messages.

from marshmallow import Schema, fields

fields.Field.default_error_messages["required"] = "You missed something!"


class ArtistSchema(Schema):
    name = fields.Str(required=True)
    label = fields.Str(required=True, error_messages={"required": "Label missing."})


print(ArtistSchema().validate({}))
# {'label': ['Label missing.'], 'name': ['You missed something!']}