title: Schema & Validation API
description: "API reference for schema validation in JustAPI, the FastAPI alternative — Schema, Field, and validation functions."
keywords: [schema validation, fastapi alternative, justapi, schema class, field, validation, pydantic]

JustAPI provides two validation paths: the built-in Schema class (validated in Rust) and Pydantic models (validated in Python).

Schema Class

Define a validation schema using the built-in Schema class:

from justapi import JustAPIApp, Schema
from pydantic import Field

class ProductCreate(Schema):
    name: str = Field(..., min_length=1, max_length=100)
    price: float = Field(..., gt=0, le=100000)
    quantity: int = Field(0, ge=0)

Schema subclasses can be used with body_schema to validate in Rust.

Field() Constraints

from pydantic import Field

Field(
    default=...,
    gt=None,           # Greater than
    ge=None,           # Greater than or equal
    lt=None,           # Less than
    le=None,           # Less than or equal
    min_length=None,   # Min string length
    max_length=None,   # Max string length
    regex=None,        # Regular expression pattern
    format=None,       # Named format (email, uri, uuid, date-time, date, hostname, ipv4)
    enum=None,         # List of allowed values
    description=None,  # Field description for OpenAPI docs
)

Supported Formats

Using with body_schema

@app.post("/products", body_schema=ProductCreate)
def create_product(request):
    data = request.json()
    return {"status": "created", "product": data}

When body_schema is set:

1. The request body is parsed and validated in Rust against the schema
2. Invalid bodies receive 422 without Python involvement
3. Valid bodies are passed to the handler as parsed JSON

Native Fast Path

With native=True, the entire handler executes in Rust:

@app.post("/fast-products", body_schema=ProductCreate, native=True)
def fast_create(request):
    return {"status": "ok"}

Nested Schemas

class Address(Schema):
    street: str
    city: str
    zip_code: str = Field(..., regex=r"^\d{5}(-\d{4})?$")

class Customer(Schema):
    name: str
    address: Address
    tags: list[str]

Nested schemas are compiled into a single JSON Schema with $defs and $ref references.

validate_value() Function

Validate arbitrary data against a schema at runtime:

from justapi._justapi import validate_value

schema = {
    "type": "object",
    "properties": {
        "name": {"type": "string"},
        "age": {"type": "integer", "minimum": 0},
    },
    "required": ["name", "age"],
}

result = validate_value({"name": "Alice", "age": 30}, schema)
# Returns: {"valid": true, "errors": []}

Pydantic Bridge

Standard Pydantic models work with JustAPI routes without modification:

from pydantic import BaseModel

class Item(BaseModel):
    name: str
    price: float

@app.post("/items")
def create_item(request, item: Item):
    return item.model_dump()

See Also

• Request Body & Validation Tutorial — Usage guide
• Native Fast Path — Rust-native execution
• JustAPIApp — body_schema parameter on route decorators