Path operations (GET, POST, PUT, DELETE) in FastAPI

Introduction

Path operations let your web app respond to different requests like getting data, sending new data, updating, or deleting it. They make your app interactive and useful.

When you want to show information to users (GET).

When users submit new data, like a form (POST).

When you need to change existing data (PUT).

When you want to remove data (DELETE).

Syntax

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/path")
async def read_item():
    return {"message": "GET request"}

@app.post("/path")
async def create_item():
    return {"message": "POST request"}

@app.put("/path")
async def update_item():
    return {"message": "PUT request"}

@app.delete("/path")
async def delete_item():
    return {"message": "DELETE request"}

Each decorator (@app.get, @app.post, etc.) tells FastAPI which HTTP method to listen for.

Functions below decorators handle the request and return a response.

Examples

This GET path takes an item ID from the URL and returns it.

FastAPI

from fastapi import FastAPI

app = FastAPI()

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

This POST path accepts JSON data to create a new item.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

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

@app.post("/items/")
async def create_item(item: Item):
    return {"item_name": item.name, "item_price": item.price}

This PUT path updates an item by its ID.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.put("/items/{item_id}")
async def update_item(item_id: int):
    return {"message": f"Item {item_id} updated"}

This DELETE path removes an item by its ID.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.delete("/items/{item_id}")
async def delete_item(item_id: int):
    return {"message": f"Item {item_id} deleted"}

Sample Program

This example shows a simple in-memory store for items. You can create, read, update, and delete items by their ID using the correct HTTP method.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

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

items_db = {}

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    item = items_db.get(item_id)
    if item:
        return item
    return {"error": "Item not found"}

@app.post("/items/{item_id}")
async def create_item(item_id: int, item: Item):
    if item_id in items_db:
        return {"error": "Item already exists"}
    items_db[item_id] = item.dict()
    return {"message": "Item created", "item": items_db[item_id]}

@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item):
    if item_id not in items_db:
        return {"error": "Item not found"}
    items_db[item_id] = item.dict()
    return {"message": "Item updated", "item": items_db[item_id]}

@app.delete("/items/{item_id}")
async def delete_item(item_id: int):
    if item_id in items_db:
        del items_db[item_id]
        return {"message": "Item deleted"}
    return {"error": "Item not found"}

Important Notes

Use the correct HTTP method to match the action you want (GET to read, POST to create, etc.).

Path parameters like {item_id} let you work with specific items easily.

FastAPI automatically converts and validates data types for you.

Summary

Path operations define how your app responds to different HTTP requests.

GET reads data, POST creates, PUT updates, DELETE removes.

Use path parameters to work with specific resources.

Practice

1. Which HTTP method in FastAPI is typically used to retrieve data without changing anything on the server?

easy

A. DELETE

B. POST

C. PUT

D. GET

Solution

1. Step 1: Understand HTTP methods purpose

   GET is used to read or retrieve data without modifying it.

2. Step 2: Match method to FastAPI operation

   FastAPI uses @app.get() decorator for GET requests to fetch data.

3. Final Answer:

   GET -> Option D

4. Quick Check:

   GET = Read data [OK]

Hint: GET always fetches data without side effects [OK]

Common Mistakes:

• Confusing GET with POST which creates data
• Using PUT for reading data
• Thinking DELETE retrieves data

2. Which of the following is the correct syntax to define a POST path operation in FastAPI to create a new item?

easy

A. @app.post('/items/')\ndef create_item(): pass

B. @app.get('/items/')\ndef create_item(): pass

C. @app.put('/items/')\ndef create_item(): pass

D. @app.delete('/items/')\ndef create_item(): pass

Solution

1. Step 1: Identify the HTTP method for creating data

   POST is used to create new resources in REST APIs.

2. Step 2: Match FastAPI decorator for POST

   FastAPI uses @app.post() decorator to handle POST requests.

3. Final Answer:

   @app.post('/items/')\ndef create_item(): pass -> Option A

4. Quick Check:

   POST = Create data [OK]

Hint: Use @app.post() for creating new data [OK]

Common Mistakes:

• Using @app.get() for creating data
• Confusing PUT with POST
• Using DELETE for creation

3. What will be the response when this FastAPI code is called with a PUT request to update item with id 5?

from fastapi import FastAPI
app = FastAPI()

items = {5: "old value"}

@app.put('/items/{item_id}')
async def update_item(item_id: int):
    items[item_id] = "new value"
    return items[item_id]

medium

A. "new value"

B. "old value"

C. KeyError

D. None

Solution

1. Step 1: Understand the PUT operation code

   The function updates the dictionary value for the given item_id to "new value".

2. Step 2: Check the returned value

   The function returns the updated value, which is "new value".

3. Final Answer:

   "new value" -> Option A

4. Quick Check:

   PUT updates and returns new value [OK]

Hint: PUT replaces existing data and returns updated value [OK]

Common Mistakes:

• Expecting old value after update
• Thinking KeyError occurs without check
• Assuming None is returned

4. Identify the error in this FastAPI DELETE operation code:

from fastapi import FastAPI
app = FastAPI()

items = {1: "apple", 2: "banana"}

@app.delete('/items/{item_id}')
async def delete_item(item_id: int):
    del items[item_id]
    return {"message": "Item deleted"}

medium

A. Missing return statement

B. Wrong HTTP method decorator

C. Deleting item without checking if it exists

D. Function should not be async

Solution

1. Step 1: Analyze the delete operation

   The code deletes an item by key without verifying if the key exists in the dictionary.

2. Step 2: Understand possible runtime error

   If the item_id does not exist, del will raise a KeyError causing a server error.

3. Final Answer:

   Deleting item without checking if it exists -> Option C

4. Quick Check:

   Delete needs existence check [OK]

Hint: Always check if item exists before deleting [OK]

Common Mistakes:

• Assuming item always exists
• Ignoring KeyError on delete
• Thinking async is wrong here

5. You want to create a FastAPI app that supports all four main HTTP methods (GET, POST, PUT, DELETE) for managing a list of users stored in a dictionary. Which approach correctly handles updating a user only if it exists, creating a new user, retrieving a user by ID, and deleting a user safely?

hard

A. Use @app.get to retrieve, @app.post to create, @app.put to update without checking existence, and @app.delete to delete without checking existence

B. Use @app.get to retrieve, @app.post to create, @app.put to update with existence check, and @app.delete to delete with existence check

C. Use only @app.post for all operations to simplify code

D. Use @app.put to create and update, @app.get to delete, @app.post to retrieve

Solution

1. Step 1: Match HTTP methods to CRUD operations

   GET retrieves, POST creates, PUT updates, DELETE removes resources.

2. Step 2: Ensure safe update and delete

   Update and delete should check if the user exists to avoid errors.

3. Final Answer:

   Use @app.get to retrieve, @app.post to create, @app.put to update with existence check, and @app.delete to delete with existence check -> Option B

4. Quick Check:

   Correct HTTP methods with safety checks [OK]

Hint: Match HTTP verbs to actions and check existence before update/delete [OK]

Common Mistakes:

• Skipping existence checks causing errors
• Using wrong HTTP methods for actions
• Trying to do all actions with one method