How to Validate Forms with Pydantic in Python

Published: 2026-01-10

Introduction

Pydantic v2 provides fast, type-safe data validation for Python. It’s the standard choice for FastAPI, CLI tools, and configuration management.

Step 1: Install Pydantic

pip install pydantic

Step 2: Define Models

from pydantic import BaseModel, EmailStr, field_validator

class UserForm(BaseModel):
    email: EmailStr
    password: str
    age: int

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

    @field_validator("age")
    @classmethod
    def validate_age(cls, v):
        if v < 0 or v > 150:
            raise ValueError("Invalid age")
        return v

Step 3: Handle Errors

from pydantic import ValidationError

try:
    user = UserForm(email="[email protected]", password="weak", age=25)
except ValidationError as e:
    for error in e.errors():
        print(f"Field: {error['loc']}, Message: {error['msg']}")

Conclusion

Pydantic v2 offers excellent performance and a clean API for data validation. The field_validator decorator gives you full control over validation logic, and the error system provides detailed feedback for users.