Pydantic (Data Validation) in FastAPI

When a client sends data to your API (like a form), you often want it to be structured. FastAPI uses Pydantic models to define this structure.

1. Define a Pydantic Model

A Pydantic model is a Python class that describes what the data should look like:

from fastapi import FastAPI
from typing import List
from pydantic import BaseModel, Field

app = FastAPI()

class Student(BaseModel):
    id: int
    name: str = Field(None, title="Name of the student", max_length=10)
    subjects: List[str] = []

Here:

• id is an integer
• name is a string (max 10 characters)
• subjects is a list of strings, default empty

2. Use POST to Accept the Model

To get data from a client, use the POST method:

@app.post("/students/")
async def student_data(s1: Student):
    return s1

Explanation:

• @app.post("/students/") defines the URL and POST method
• s1: Student tells FastAPI to expect a JSON body matching the Student model
• The function returns the same data back as JSON

3. Test in Swagger UI

1. Start the server: uvicorn main:app --reload
2. Open in browser: http://localhost:8000/docs
3. Find the /students/ route under POST
4. Click Try it out, fill in test data, and click Execute
5. You get a JSON response with the same data you sent

4. Using Individual Fields Instead of a Model

You can accept individual fields using Body:

from fastapi import Body

@app.post("/students")
async def student_data(name: str = Body(...), marks: int = Body(...)):
    return {"name": name, "marks": marks}

- `name` and `marks` are sent in the request body separately

- Works the same as the model, just with individual fields

5. Combining Path, Query, and Body

@app.post("/students/{college}")
async def student_data(college: str, age: int, student: Student):
    retval = {"college": college, "age": age, **student.dict()}
    return retval

Explanation:

• college → path parameter (`/students/XYZ`)
• age → query parameter (`?age=20`)
• student → JSON request body (Pydantic model)
• **student.dict() → converts the Student object to a dictionary

Example request:

POST /students/ABC?age=20
{
  "id": 1,
  "name": "Alice",
  "subjects": ["Math", "Physics"]
}

Response:

{
  "college": "ABC",
  "age": 20,
  "id": 1,
  "name": "Alice",
  "subjects": ["Math", "Physics"]
}

Summary

• Use Pydantic models to structure request body data.
• Use Body(...) for individual fields.
• POST method is used to receive request body.
• You can combine path + query + body in a single endpoint.
• FastAPI automatically validates data and converts JSON to Python objects.