serializing to JSON an array of data classes

Question

I have an array of data classes I need to serialize to JSON. I wrap the list in a class (Persons)

@attrs.frozen
class Person:
    name: str
    age: int
    grades: Dict[str, int]

@attrs.define
class Persons:
    persons: List[Person] = attrs.field(factory=list)
    def add(self, person: Person) -> None:
        self.persons.append(person)

def run(filename: str) -> None:

    college = Persons()
    college.add(Person("uzi" , 51, {"math": 100, "cs": 90}))
    college.add(Person("dave", 76, {"math": 94, "music": 92}))
    college.add(Person("dan",  22, {"bible": 98}))

    with open(filename, "w+") as fl:
        fl.write(f"{json.dumps(attrs.asdict(college), indent=4)}\n")

Can I somehow get rid of the wrapping class and still serialize a List[Person] easily?

Answer 1

You can use default argument of the json.dumps function and pass the function attrs.asdict to serialize it.

I have added the "persons" key so it matches the output of your code.

import json
import attrs


@attrs.frozen
class Person:
    name: str
    age: int
    grades: dict[str, int]


def run(filename: str) -> None:

    college = []
    college.append(Person("uzi", 51, {"math": 100, "cs": 90}))
    college.append(Person("dave", 76, {"math": 94, "music": 92}))
    college.append(Person("dan", 22, {"bible": 98}))

    with open(filename, "w+") as fl:
        fl.write(
            f"{json.dumps({'persons': college}, default=attrs.asdict, indent=4)}\n"
        )


run("my_json.json")

Output

{
    "persons": [
        {
            "name": "uzi",
            "age": 51,
            "grades": {
                "math": 100,
                "cs": 90
            }
        },
        {
            "name": "dave",
            "age": 76,
            "grades": {
                "math": 94,
                "music": 92
            }
        },
        {
            "name": "dan",
            "age": 22,
            "grades": {
                "bible": 98
            }
        }
    ]
}