What is pydantic? Meaning and example in Python

What is pydantic?

Pydantic: A widely used Python library that validates and converts data according to type hints, defining models as classes. It is popular in web APIs and for parsing LLM structured outputs into checked objects.

Level: intermediateTopic: Python advanced

Pydantic: an example

class User(BaseModel): age: int; User(age='25') converts the string to 25, while age='abc' raises a validation error.

Pydantic in Python

A short example you can run (Python 3.11). The output below is what it really prints.

from pydantic import BaseModel, ValidationError

class Invoice(BaseModel):
    number: str
    amount: float
    gst_rate: float = 18.0

print(Invoice.model_validate({"number": "INV-7", "amount": "1500"}))
try:
    Invoice.model_validate({"number": "INV-8", "amount": "fifteen hundred"})
except ValidationError as e:
    print(e.errors()[0]["loc"], e.errors()[0]["type"])

Output

number='INV-7' amount=1500.0 gst_rate=18.0
('amount',) float_parsing

Related terms

More Python advanced terms

Where you learn it

Pydantic is part of the AI Engineering Program, which teaches it with notes, a hands-on lab and an exam. The first lesson is free.