FastAPI for AI Engineers - Part 4: Stop Bad Data Before It Breaks Your API (Pydantic and Data Validation)

In the previous article, we connected our FastAPI application to a database using SQLite and SQLAlchemy. We also used classes like: class StudentCreate ( BaseModel ): name : str department : str cgpa : float without fully understanding what was happening behind the scenes. Today, we'll fix that. If you haven't read it check it out: FastAPI for AI Engineers - Part 3: Connecting to a database Ananya S Ananya S Ananya S Follow Jun 6 FastAPI for AI Engineers - Part 3: Connecting to a database # ai # fastapi # python # backend 6 reactions Add Comment 6 min read Why Do We Need Data Validation? Imagine you're building a weather application. A user asks: What is the temperature in Chennai? A valid response might be: 35 or 35°C But what if the API returns: Sunny This is clearly wrong. Temperature should be represented as a number. Even if the value itself is inaccurate, we still know that temperature must be numeric. This is where validation becomes important. Validation allows us to define rules about what data is acceptable before it enters our application. For example: Temperature should be numeric Age cannot be negative CGPA should be between 0 and 10 Email addresses should follow a valid format Without validation, applications can receive invalid data and behave unexpectedly. The Problem Without Validation Consider a student registration API. @app.post ( " /student " ) def create_student ( student ): return student A user could send: { "name" : "Ananya" , "cgpa" : "Excellent" } The API would accept it. But a CGPA should be a number, not text. As applications grow, manually checking every field becomes difficult. We need a better solution. Enter Pydantic Pydantic is a Python library used for data validation. FastAPI uses Pydantic extensively behind the scenes. Instead of manually validating data, we define a schema. from pydantic import BaseModel class Student ( BaseModel ): name : str cgpa : float Now FastAPI knows: name must be a string cgpa must be a floating-point nu