How Validation Works

Pydantic validates data when you create a model instance. If something's wrong, it raises a ValidationError with details.

from pydantic import BaseModel, ValidationError

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

try:
    user = User(name="Alice", age="thirty")
except ValidationError as e:
    print(e)

The error message tells you exactly what failed and why.

Pydantic also coerces compatible types:

user = User(name="Alice", age="30")  # String "30" becomes int 30
print(user.age)  # 30 (as integer)

This is helpful when parsing JSON, where numbers often come as strings. Pydantic converts them if possible, or fails with a clear error if not.

You control how strict this coercion is through configuration.

I explain validation modes in my Pydantic course.