What are the main principles and methods you use when implementing data validation in an application?

When implementing data validation in an application, I adhere to the following principles:

• Validation at different levels: first on the client side (if there is a UI), then on the server for security.
• Clear and understandable errors: so that the user or developer understands what exactly is wrong.
• Use of standard libraries and frameworks: for example, Pydantic, Marshmallow, or built-in Django/Flask tools.
• Type and format checks: for example, email validation, numeric values, required fields.

Example in Python using Pydantic:

from pydantic import BaseModel, EmailStr, ValidationError

class User(BaseModel):
    name: str
    age: int
    email: EmailStr

try:
    user = User(name='Иван', age=30, email='ivan@example.com')
except ValidationError as e:
    print(e.json())

This approach helps to centrally and reliably validate data.