FastAPI – PUT Method

This document demonstrates how to use the HTTP PUT method in FastAPI to update an existing resource. It builds on previous GET and POST examples and completes a basic CRUD-style workflow.

Example Application

Create or update main.py with the following content:

from fastapi import FastAPI

app = FastAPI()

users = [
    {"name": "abbas", "age": 20},
    {"name": "mmd", "age": 37},
    {"name": "asghar", "age": 19},
]


@app.get("/")
def home_page():
    return {"msg": "API is working"}


@app.get("/users")
def show_users():
    return users


@app.post("/create_user")
def create_user(name: str, age: int):
    new_user = {"name": name, "age": age}
    users.append(new_user)
    return {"msg": f"user {name} with age {age} created"}


@app.put("/update_user/{target_name}")
def update_user(target_name: str, age: int):
    for user in users:
        if user["name"] == target_name:
            user["age"] = age
            return {"msg": f"user {target_name} updated"}
    return {"msg": "user not found"}

Code Overview

PUT Endpoint Definition

@app.put("/update_user/{target_name}")
def update_user(target_name: str, age: int):

Registers an HTTP PUT endpoint
target_name is a path parameter
age is a query parameter
Used to update an existing user’s data

Update Logic

for user in users:
    if user["name"] == target_name:
        user["age"] = age

Iterates over the in-memory users list
Matches user by name
Updates the age field in place

Success Response

{
  "msg": "user abbas updated"
}

Returned when the target user exists and is updated successfully.

Failure Response

{
  "msg": "user not found"
}

Returned when no matching user exists.

Example Requests

Update User Age (PUT)

curl -X PUT "http://localhost:8000/update_user/abbas?age=25"

Verify Update

GET /users

Updated response:

[
  {"name": "abbas", "age": 25},
  {"name": "mmd", "age": 37},
  {"name": "asghar", "age": 19}
]

Running the Application

Start the service using uvicorn:

uvicorn main:app --reload

HTTP Method Summary

Method	Endpoint	Purpose
GET	`/`	Health check
GET	`/users`	Retrieve all users
POST	`/create_user`	Create a new user
PUT	`/update_user/{name}`	Update an existing user

Best Practices

Use PUT for full updates and PATCH for partial updates
Return proper HTTP status codes (404, 200, 201)
Avoid using in-memory data stores in production
Use Pydantic models for request bodies instead of query parameters
Add input validation and error handling
Separate routes, services, and models as the project grows
Make PUT operations idempotent

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