Pydantic request bodies — automatic validation

Define a Pydantic model with typed fields. FastAPI validates the JSON body, returns 422 if invalid.

main.py v3.1

from fastapi import FastAPI
from pydantic import BaseModel, Field, EmailStr
from typing import Optional
from datetime import datetime

app = FastAPI()

class UserCreate(BaseModel):
    username: str = Field(..., min_length=3, max_length=20)
    email: EmailStr
    age: int = Field(..., ge=0, le=150)
    bio: Optional[str] = Field(None, max_length=500)
    is_active: bool = True
    created_at: datetime = Field(default_factory=datetime.utcnow)

@app.post('/users')
async def create_user(user: UserCreate):
    # FastAPI has ALREADY validated the body before this runs.
    # If validation fails, it returns 422 without touching your code.
    return {
        'id': 1,
        'username': user.username,
        'email': user.email,
        'age': user.age,
        'is_active': user.is_active,
    }

# Try with curl:
# curl -X POST http://localhost:8000/users \
#   -H 'Content-Type: application/json' \
#   -d '{"username":"ada","email":"ada@x.com","age":36}'
# -> 200, returns the user dict
#
# Validation failures (each returns 422):
# {"username":"ab"}            -> min_length=3 violated
# {"username":"ada","email":"not-an-email"} -> invalid email
# {"age": 200}                  -> le=150 violated
# {"username":"ada"}            -> email + age required

# Response model — validates the OUTPUT too
class UserResponse(BaseModel):
    id: int
    username: str
    email: EmailStr

@app.post('/users2', response_model=UserResponse)
async def create_user2(user: UserCreate):
    return {'id': 1, **user.model_dump(), 'extra_field': 'ignored'}
    # response_model strips extra_field automatically

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🧠 How it works

Pydantic BaseModel defines the SHAPE of the JSON body. Field(...) — required. Field(default) — optional with default. Constraints: min_length, max_length, ge (>=), le (<=), gt (>), lt (<), regex. Special types: EmailStr, HttpUrl, IPvAnyAddress, conint(ge=0, le=100). Optional[T] = None — explicitly optional, defaults to None. Type hints VALIDATE: wrong type → 422 with detailed error pointing at the field. response_model on the route validates the OUTPUT too — strips extra fields, validates types. user.model_dump() converts Pydantic model → dict (v2). In v1 it was .dict(). Benefits: no manual validation, free /docs, type-safe serialization.

✏️ Try these changes

Change min_length=3 to min_length=5
→ usernames shorter than 5 chars now 422
Add a new field password: str = Field(..., min_length=8)
→ password now required
Change is_active: bool = True to is_active: bool = False
→ default flips
Remove the response_model from /users2
→ extra_field is now included in the response

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