Custom request validation in FastAPI - Step-by-Step Execution

Concept Flow - Custom request validation

Receive HTTP Request

↓

Parse JSON Body

↓

Run Custom Validation Function

↓

Process Request

↓

Send Response

The server receives a request, parses the data, runs your custom validation, then either processes or rejects the request.

Execution Sample

FastAPI

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float

@app.post("/items/")
def create_item(item: Item):
    if item.price <= 0:
        raise HTTPException(status_code=422, detail="Price must be positive")
    return item

This code defines a POST endpoint that checks if the price is positive and rejects the request if not.

Execution Table

Step	Action	Input Data	Validation Check	Result	Response
1	Receive request	{"name": "Book", "price": 10}	N/A	Data parsed	Continue
2	Run validation	price=10	price > 0?	True	Process request
3	Return response	Item(name='Book', price=10)	N/A	Success	200 OK with item data
4	Receive request	{"name": "Pen", "price": -5}	N/A	Data parsed	Continue
5	Run validation	price=-5	price > 0?	False	Reject request
6	Return error	N/A	N/A	Validation failed	422 Unprocessable Entity with error detail

💡 Execution stops after sending success or error response based on validation result.

Variable Tracker

Variable	Start	After Step 2 (valid)	After Step 5 (invalid)
item.name	N/A	"Book"	"Pen"
item.price	N/A	10	-5
validation_passed	N/A	True	False

Key Moments - 2 Insights

Why does the server return a 422 error instead of 200 when price is negative?

Because the custom validation check (step 5) fails (price > 0 is False), the code raises HTTPException causing FastAPI to return 422 as shown in step 6.

What happens if the request JSON is missing the price field?

Pydantic automatically raises a validation error before your custom check, so the request never reaches the custom validation step.

Visual Quiz - 3 Questions

Look at the execution table, what is the validation result at step 2?

A Error

B True

C False

D None

💡 Hint

Check the 'Validation Check' and 'Result' columns at step 2 in the execution_table.

At which step does the server reject the request due to invalid price?

A Step 3

B Step 5

C Step 6

D Step 4

💡 Hint

Look for the step where the response is '422 Unprocessable Entity' in the execution_table.

If the price was zero, how would the validation result change at step 5?

A It would be False

B It would be True

C It would cause a syntax error

D It would skip validation

💡 Hint

Validation requires price > 0, so zero fails the check as shown in step 5 logic.

Concept Snapshot

Custom request validation in FastAPI:
- Define Pydantic model for request data
- Use endpoint function to receive model instance
- Add if-checks for custom rules
- Raise HTTPException(status_code=422) on failure
- FastAPI returns error response automatically
- Valid requests proceed and return data

Full Transcript

In FastAPI, custom request validation means checking incoming data beyond basic type checks. When a request arrives, FastAPI parses the JSON body into a Pydantic model. Then your code runs extra checks, like ensuring a price is positive. If the check fails, you raise an HTTPException with status 422. FastAPI sends this error back to the client. If the check passes, the request is processed normally and a success response is sent. This flow helps keep your API data clean and reliable.