Pydantic ValidationError: Invalid Data

Why This Happens

Pydantic validates data against model schemas at runtime. When data does not conform, it raises ValidationError with details about every invalid field.

The Problem

from pydantic import BaseModel

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

User(name='Alice', age='not_a_number')

The Fix

from pydantic import BaseModel, ValidationError

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

try:
    user = User(**raw_data)
except ValidationError as e:
    print(e.json())

Step-by-Step Fix

1. 1

   Read error details

   ValidationError lists every invalid field with the reason.

2. 2

   Validate input data

   Clean data before passing to Pydantic.

3. 3

   Use Optional for nullable fields

   Use Optional[type] for fields that may be null.