🛡️ Pydantic v2 Deep Dive & Custom Validators: Let's Learn from ZERO!

Almost every API you build needs to validate incoming data, reject invalid values, convert data into Python objects, and control what goes back to the frontend . In FastAPI, Pydantic does all of that. 🚀

No Pydantic knowledge is assumed. We'll connect every concept to a React + FastAPI social-feed backend so you see how it works in a real project, not just as definitions.

1. 🤔 What problem does Pydantic solve?

Your React frontend sends this to your FastAPI backend:

{ "username" : "deepika" , "age" : 22 , "email" : "[email protected]" }

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But what if it accidentally sends this?

{ "username" : 12345 , "age" : "twenty-two" , "email" : "not-an-email" }

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❌ Username is a number instead of a string

❌ Age is text instead of a number

❌ Email is not a valid email address

Should your backend blindly accept it? Absolutely not!…