Path operations (GET, POST, PUT, DELETE) in FastAPI - Step-by-Step Execution

Concept Flow - Path operations (GET, POST, PUT, DELETE)

Client sends HTTP request

↓

FastAPI receives request

↓

Match request method and path

↓

GET

↓

Run GET handler

↓

Send response back to client

The server listens for HTTP requests, matches the method and path, runs the correct handler, and sends back a response.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/{id}")
def read_item(id: int):
    return {"id": id}

@app.post("/items")
def create_item(item: dict):
    return item

Defines GET and POST path operations for /items with FastAPI.

Execution Table

Step	Request Method	Request Path	Matched Handler	Handler Action	Response
1	GET	/items/5	read_item	Return {'id': 5}	{"id": 5}
2	POST	/items	create_item	Return posted item {'name': 'book'}	{"name": "book"}
3	PUT	/items/5	No match	Return 404 Not Found	404 Not Found
4	DELETE	/items/5	No match	Return 404 Not Found	404 Not Found

💡 Requests with no matching method and path return 404 Not Found.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
request_method	None	GET	POST	PUT	DELETE
request_path	None	/items/5	/items	/items/5	/items/5
matched_handler	None	read_item	create_item	None	None
response	None	{"id": 5}	{"name": "book"}	404 Not Found	404 Not Found

Key Moments - 2 Insights

Why does a PUT request to /items/5 return 404 even though the path looks similar to GET?

Because no handler is defined for PUT method on /items/{id}, only GET and POST exist. See execution_table rows 3 and 4.

How does FastAPI know which function to run for a request?

It matches both the HTTP method (GET, POST, etc.) and the path pattern. Only exact method and path matches run the handler. See execution_table column 'Matched Handler'.

Visual Quiz - 3 Questions

Look at the execution table, what response does the GET request to /items/5 return?

A {"id": 5}

B {"name": "book"}

C 404 Not Found

D Empty response

💡 Hint

Check row 1 in the execution_table under the Response column.

At which step does the request method 'PUT' fail to find a matching handler?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Matched Handler' column for 'PUT' in execution_table.

If we add a @app.put("/items/{id}") handler, how would the response at Step 3 change?

A It would still return 404 Not Found

B It would return the PUT handler's response

C It would return the GET handler's response

D It would cause a server error

💡 Hint

Adding a matching handler for PUT changes matched_handler from None to that handler.

Concept Snapshot

FastAPI path operations handle HTTP methods: GET, POST, PUT, DELETE.
Define handlers with decorators @app.get(), @app.post(), etc.
Server matches method and path to run correct handler.
No match returns 404 Not Found.
Handlers return JSON responses by default.

Full Transcript

This visual shows how FastAPI handles HTTP requests by matching the method and path to the correct function. When a client sends a GET request to /items/5, FastAPI runs the read_item function and returns the item id. A POST to /items runs create_item and returns the posted data. PUT and DELETE requests without handlers return 404 errors. Variables like request_method and matched_handler change as requests are processed. Understanding this flow helps beginners see how web APIs respond differently based on HTTP methods.