Default values for required fields are sometimes rendered and sometimes not

[ashipilov]

Default values for required fields are rendered for lists of objects, nested objects and dictionaries but not for plain fields like strings, ints and enums.

Example schema:

{
  "$schema": "http://json-schema.org/draft-07/schema",
  "title": "Order",
  "type": "object",
  "definitions": {
    "Status": {
      "type": "string",
      "enum": ["pending", "processing", "shipped", "delivered"]
    },
    "Priority": {
      "type": "integer",
      "enum": [1, 2, 3]
    },
    "Address": {
      "type": "object",
      "properties": {
        "street": {"type": "string"},
        "city": {"type": "string"}
      },
      "required": ["street", "city"]
    }
  },
  "properties": {
    "order_id": {"type": "string"},
    "status": {
      "$ref": "#/definitions/Status",
      "default": "pending"
    },
    "priority": {
      "allOf": [{"$ref": "#/definitions/Priority"}],
      "default": 2
    },
    "quantity": {
      "type": "integer",
      "default": 1
    },
    "note": {
      "type": "string",
      "default": "your note here"
    },
    "tags": {
      "type": "array",
      "items": {"type": "string"},
      "default": ["new"]
    },
    "metadata": {
      "type": "object",
      "additionalProperties": {"type": "string"},
      "default": {"source": "web"}
    },
    "shipping_address": {
      "allOf": [{"$ref": "#/definitions/Address"}],
      "default": {"street": "123 Main St", "city": "Springfield"}
    },
    "all_addresses": {
      "type": "array",
      "items": {"$ref": "#/definitions/Address"},
      "default": [{"street": "1 First St", "city": "Shelbyville"}]
    },
    "addresses": {
      "type": "object",
      "additionalProperties": {"$ref": "#/definitions/Address"},
      "default": {"home": {"street": "10 Oak Ave", "city": "Ogdenville"}}
    }
  },
  "required": ["order_id", "status", "priority", "quantity", "note", "tags", "metadata", "shipping_address", "all_addresses", "addresses"]
}

Used commandline:

datamodel-codegen --input schema.json --input-file-type jsonschema --output-model-type pydantic_v2.BaseModel --output model.py

Generated models:

# generated by datamodel-codegen:
#   filename:  pydantic_v2_model_defaults.json
#   timestamp: 2019-07-26T00:00:00+00:00

from __future__ import annotations

from enum import Enum, IntEnum

from pydantic import BaseModel, Field


class Status(Enum):
    pending = 'pending'
    processing = 'processing'
    shipped = 'shipped'
    delivered = 'delivered'


class Priority(IntEnum):
    integer_1 = 1
    integer_2 = 2
    integer_3 = 3


class Address(BaseModel):
    street: str
    city: str


class Order(BaseModel):
    order_id: str
    status: Status
    priority: Priority
    quantity: int
    note: str
    tags: list[str]
    metadata: dict[str, str]
    shipping_address: Address = Field(
        default_factory=lambda: Address.model_validate(
            {'street': '123 Main St', 'city': 'Springfield'}
        )
    )
    all_addresses: list[Address] = Field(
        default_factory=lambda: [
            Address.model_validate(v)
            for v in [{'street': '1 First St', 'city': 'Shelbyville'}]
        ]
    )
    addresses: dict[str, Address] = Field(
        default_factory=lambda: {
            k: Address.model_validate(v)
            for k, v in {'home': {'street': '10 Oak Ave', 'city': 'Ogdenville'}}.items()
        }
    )

Expected behavior
In the generated models some of the fields have the defaults, some not. I would expect all the fields to have defaults.

Version:

• OS: N/A
• Python version: 3.13
• datamodel-code-generator version: 0.55.0

Additional context

It's probably related to #437

There is a flag --use-default but it works by making fields non-required so this behaviour is not desired.

[coderabbitai]

🔗 Related PRs

#2520 - Nullable required fields should not have default value [merged]
#2948 - Fix bug in handling of graphql empty list defaults [merged]
#2958 - Fix required list fields ignoring empty default values [merged]
#2977 - Fix handling of falsy default values for enums in set-default-enum-member option [merged]
#3046 - Fix for #3045 [closed]

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[ashipilov]

I tried to suggest a fix but it's not straightforward, therefore wanted to ask first.

There is condition

datamodel-code-generator/src/datamodel_code_generator/model/template/pydantic_v2/BaseModel.jinja2

Line 30 in 76439c3

{%- if not field.has_default_factory_in_field and not field.required and (field.represented_default != 'None' or not field.strip_default_none or field.data_type.is_optional)

The not field.required part is not very clear to me. It's probably there to deal with None values in the represented_default prop. I tried to change the condition to and field.has_default but it doesn't seem to be properly filled and it also changes the existing behaviour. There are plenty of tests which have default values in the schema but the expected files miss default values.

Do you have an idea how to approach it?

[koxudaxi]

@ashipilov
Thanks for looking into this.

I don't think replacing not field.required with has_default would be the right fix here. has_default only tells us whether the schema had a default value, but that doesn't mean we should render it in the output. Without --use-default, required fields are supposed to stay required, and many existing tests rely on that behavior.

The root cause is in __set_validate_default_on_fields in parser/base.py. This method sets validate_default=True on any field whose type contains a model reference, without checking whether the field is required. That causes Field(default_factory=...) to show up in the output even for required fields, bypassing the template guard. Scalar defaults don't go through this path, so they get correctly filtered out. That's where the inconsistency comes from. (#3050)

I think the fix should go there. For example, we could skip setting validate_default when the field is required and --use-default isn't enabled. Another option would be to add a property like should_render_default that both rendering paths can check consistently.

[butvinm]

Hi! I create a PR #3054 with two changes:

1. Consistent required field defaults (fixes the reported inconsistency)

__set_validate_default_on_fields now skips required fields, so model-ref fields no longer get defaults while scalar fields don't. All required fields behave the same - no defaults rendered without --use-default.

2. --use-default no longer makes fields nullable (breaking change)

Previously --use-default turned status: str | None = 'active' - making required fields optional and nullable. Now it produces status: str = 'active'— default is rendered but the type stays non-nullable. I consider the previous behavior a bug: the schema says the type is string, not string | null, and --use-default shouldn't change that.

@koxudaxi you mention validate_default=True and #3050 as the root cause, but #3050 was merged 3 days after this issue was filed. The inconsistency existed before through the old _get_default_as_pydantic_model / default_factory=lambda: A.model_validate(...) mechanism - was the reference to #3050 about where the code currently lives, or did I miss something?

[ashipilov]

Thanks for the PR @butvin, the behavior you suggested seems to be logical to me