Like SerializerMethodField, but formatting returned value with another serializer

custom_drf_serializer_fields.py

from __future__ import annotations

from rest_framework import serializers


class FormattedSerializerMethodField(serializers.SerializerMethodField):
    """
    Like SerializerMethodField, but allowing you to apply an optional
    formatter serializer to the field's output.
    """

    def __init__(self, formatter_serializer: serializers.Field | None = None, **kwargs):
        self.formatter_serializer = formatter_serializer
        super().__init__(**kwargs)

    def to_representation(self, value):
        unformatted_value = super().to_representation(value)

        if not self.formatter_serializer:
            return unformatted_value

        return self.formatter_serializer.to_representation(unformatted_value)


if __name__ == "__main__":
    from decimal import Decimal
    import random

    class TestSerializer(serializers.Serializer):
        random_decimal = FormattedSerializerMethodField(
            formatter_serializer=serializers.DecimalField(max_digits=3, decimal_places=1)
        )

        def get_random_decimal(self, instance) -> Decimal:
            """Returns a random decimal with two integer digits and three decimals"""
            integer_part = random.randrange(10, 99)
            decimal_part = random.randrange(100, 999)
            return Decimal(f"{integer_part}.{decimal_part}")


    serialized_data = TestSerializer(instance={}).data
    
    # length is 4: two digits + 1 dot + 1 decimal
    assert len(serialized_data["random_decimal"]) == 4