How should I add a field containing a list of dictionaries in Marshmallow Python?

Question

In Marshmallow in order to have a list field you can use:

include_in = fields.List(cls_or_instance=fields.Str(),
                         default=['sample1', 'sample2'])

This is OK, but I have a new requirement to have a list of dictionaries in a field. A sample payload:

[{
  "name": "Ali",
  "age": 20
},
{
  "name": "Hasan",
  "age": 32
}]

This payload is part of the bigger schema, so now the question is how should I add and validate such a field?

EDIT-1: I went a step further and could find out that there is a Dict field type in Marshmallow so until now I have the below code sample:

fields.List(fields.Dict(
        keys=fields.String(validate=OneOf(('name', 'age'))),
        values=fields.String(required=True)
))

Now new problem arise and I cannot set different data types for fields in the dictionary (name and age). I'd be happy if someone could shed some light on this.

You may want to look into https://github.com/Bachmann1234/marshmallow-polyfield or https://github.com/marshmallow-code/marshmallow-oneofschema, which both implement polymorphic fields, i.e. you can use a different schemas for different data shapes. — Steve L, Aug 13 '19 at 03:53

score 19 · Accepted Answer · answered Sep 05 '19 at 15:52

19

If the items in the list have the same shape, you can use a nested field within fields.List, like so:

class PersonSchema(Schema):
    name = fields.Str()
    age = fields.Int()

class RootSchema(Schema):
    people = fields.List(fields.Nested(PersonSchema))

answered Sep 05 '19 at 15:52

Steve L

1,704
1
20
29

Something like ```people = fields.List(fields.Nested(PersonSchema()))``` also works in my case – tjurkan Apr 21 '23 at 12:18

score 2 · Answer 2 · answered Aug 15 '20 at 13:30

Another approach for validating list of dictionaries in a field using one schema class.

from marshmallow import Schema, ValidationError


class PeopleSchema(Schema):
    name = fields.Str(required=True)
    age = fields.Int(required=True)


people = [{
    "name": "Ali",
    "age": 20
},
{
    "name": "Hasan",
    "age": 32
},
{
    "name": "Ali",
    "age": "twenty"  # Error: Not an int
}
]


def validate_people():
    try:
        validated_data = PeopleSchema(many=True).load(people)
    except ValidationError as err:
        print(err.messages)

validate_people()

Output:

{2: {'age': ['Not a valid integer.']}}

How should I add a field containing a list of dictionaries in Marshmallow Python?

2 Answers2

Linked