FastAPI Form Validation with Pydantic

Validate form data in FastAPI using Pydantic models with custom validators, field constraints, and automatic error response generation. Includes email.

Python Fastapi Form Validation Updated: 2026-01-15

Code

snippet

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel, EmailStr, field_validator

app = FastAPI()

class UserForm(BaseModel):
    email: EmailStr
    password: str
    name: str

    @field_validator('password')
    @classmethod
    def validate_password(cls, v):
        if len(v) < 8:
            raise ValueError('Password must be at least 8 characters')
        if not any(c.isupper() for c in v):
            raise ValueError('Password must contain uppercase')
        return v

@app.post('/register')
async def register(user: UserForm):
    return {'email': user.email, 'name': user.name}