Support default_factory in form fields.

demo/forms.py

@@ -2,7 +2,7 @@
 
 import enum
 from collections import defaultdict
-from datetime import date
+from datetime import date, timedelta
 from typing import Annotated, Literal, TypeAlias
 
 from fastapi import APIRouter, Request, UploadFile
@@ -160,6 +160,7 @@ class BigModel(BaseModel):
         Annotated[int, Field(description='X Coordinate')],
         Annotated[int, Field(description='Y Coordinate')],
     ]
+    start_data: date = Field(default_factory=lambda: date.today() + timedelta(days=1), title='Start Date')
 
     @field_validator('name')
     def name_validator(cls, v: str | None) -> str:

src/python-fastui/fastui/json_schema.py

@@ -2,8 +2,11 @@
 import re
 import typing as _t
 
+import pydantic_core
 import typing_extensions as _ta
 from pydantic import BaseModel
+from pydantic.json_schema import GenerateJsonSchema, JsonSchemaValue
+from pydantic_core import core_schema
 
 from .components.forms import (
     FormField,
@@ -24,8 +27,49 @@
 __all__ = 'model_json_schema_to_fields', 'SchemeLocation'
 
 
+class GenerateJsonSchemaWithDefaultFactory(GenerateJsonSchema):
+    """Custom JSON schema including default_factory value as in
+    https://github.com/pydantic/pydantic/blob/main/pydantic/json_schema.py#L1046
+    """
+
+    def default_schema(self, schema: core_schema.WithDefaultSchema) -> JsonSchemaValue:
+        """Generates a JSON schema that matches a schema with a default value.
+
+        Args:
+            schema: The core schema.
+
+        Returns:
+            The generated JSON schema.
+        """
+        json_schema = self.generate_inner(schema['schema'])
+
+        if 'default' in schema:
+            default = schema['default']
+        elif 'default_factory' in schema:
+            default = schema['default_factory']()
+        else:
+            return json_schema
+
+        try:
+            encoded_default = self.encode_default(default)
+        except pydantic_core.PydanticSerializationError:
+            self.emit_warning(
+                'non-serializable-default',
+                f'Default value {default} is not JSON serializable; excluding default from JSON schema',
+            )
+            # Return the inner schema, as though there was no default
+            return json_schema
+
+        if '$ref' in json_schema:
+            # Since reference schemas do not support child keys, we wrap the reference schema in a single-case allOf:
+            return {'allOf': [json_schema], 'default': encoded_default}
+        else:
+            json_schema['default'] = encoded_default
+            return json_schema
+
+
 def model_json_schema_to_fields(model: _t.Type[BaseModel]) -> _t.List[FormField]:
-    schema = _t.cast(JsonSchemaObject, model.model_json_schema())
+    schema = _t.cast(JsonSchemaObject, model.model_json_schema(schema_generator=GenerateJsonSchemaWithDefaultFactory))
     defs = schema.get('$defs', {})
     return list(json_schema_obj_to_fields(schema, [], [], defs))
 

src/python-fastui/tests/test_forms.py

@@ -1,9 +1,11 @@
 import enum
 from contextlib import asynccontextmanager
+from datetime import date
 from io import BytesIO
 from typing import List, Tuple, Union
 
 import pytest
+from dirty_equals import IsDate
 from fastapi import HTTPException
 from fastui import components
 from fastui.forms import FormFile, Textarea, fastui_form
@@ -522,3 +524,28 @@ def test_form_description_leakage():
         'submitUrl': '/foobar/',
         'type': 'ModelForm',
     }
+
+
+class FormFieldsDefaultFactory(BaseModel):
+    start_date: date = Field(title='Start Date', default_factory=lambda: date.today())
+
+
+def test_form_fields_default_factory():
+    m = components.ModelForm(model=FormFieldsDefaultFactory, submit_url='/foobar/')
+
+    assert m.model_dump(by_alias=True, exclude_none=True) == {
+        'formFields': [
+            {
+                'htmlType': 'date',
+                'initial': IsDate(approx=date.today(), iso_string=True),
+                'locked': False,
+                'name': 'start_date',
+                'required': False,
+                'title': ['Start Date'],
+                'type': 'FormFieldInput',
+            }
+        ],
+        'method': 'POST',
+        'submitUrl': '/foobar/',
+        'type': 'ModelForm',
+    }