Python API Tutorial for Beginners | Build Your First REST API Step by Step

In this beginner-friendly tutorial, you’ll learn how to build a simple REST API using FastAPI and test it using Postman.

We’ll create, read, update, and delete (CRUD) items in memory.

Prerequisites

Before you begin, make sure you have:

• Python 3.10 or newer is installed

• Basic knowledge of JSON and HTTP requests

• Postman installed for API testing

Step 1 — Create Project Folder and Virtual Environment

Open your Terminal and run:

# Create project folder and go inside
mkdir fastapi_api
cd fastapi_api

# Create virtual environment named 'venv'
python3 -m venv venv

# Activate the virtual environment
source venv/bin/activate

After activation, your terminal prompt will show (venv) indicating you’re inside the virtual environment.

Step 2 — Install FastAPI and Uvicorn

With the virtual environment activated, run:

python3 -m pip install fastapi==0.120.0 uvicorn==0.38.0

• FastAPI is the framework to build the API.

• Uvicorn is the ASGI server that runs your app.

Step 3 — Create main.py with API Code

Create a file named main.py inside fastapi_api folder and paste this code:

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
from typing import Optional

app = FastAPI(title="Simple Items API")

# Data model
class Item(BaseModel):
    id: Optional[int] = None
    name: str
    description: Optional[str] = ""

# In-memory storage
items = {}
next_id = 1

# Get all items
@app.get("/items")
def list_items():
    return list(items.values())

# Get single item by ID
@app.get("/items/{item_id}")
def get_item(item_id: int):
    item = items.get(item_id)
    if not item:
        raise HTTPException(status_code=404, detail="Item not found")
    return item

# Create new item
@app.post("/items", status_code=201)
def create_item(item: Item):
    global next_id
    item.id = next_id
    items[next_id] = item.dict()
    next_id += 1
    return items[item.id]

# Update existing item
@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):
    if item_id not in items:
        raise HTTPException(status_code=404, detail="Item not found")
    updated = items[item_id]
    updated["name"] = item.name or updated["name"]
    updated["description"] = item.description or updated.get("description", "")
    items[item_id] = updated
    return updated

# Delete item
@app.delete("/items/{item_id}", status_code=204)
def delete_item(item_id: int):
    if item_id not in items:
        raise HTTPException(status_code=404, detail="Item not found")
    del items[item_id]
    return None

Step 4 — Run FastAPI Server

Run the server with this command (inside the project folder and with your virtual environment activated):

python3 -m uvicorn main:app --reload --port 8000

Step 5 — Access Interactive API Documentation

Open these URLs in your browser:

• Swagger UI: http://127.0.0.1:8000/docs

• ReDoc: http://127.0.0.1:8000/redoc

You can test all endpoints interactively here without Postman.

Step 6 — Test API Endpoints with Postman

Use Postman to test these endpoints:

🟢 Create Item (POST)

• URL: POST http://127.0.0.1:8000/items

• Headers: Content-Type: application/json

• Body (raw JSON):

{
  "name": "Apple",
  "description": "Fresh red apple"
}

• Response:

{
  "id": 1,
  "name": "Apple",
  "description": "Fresh red apple"
}

🔵 Get Item (GET)

• URL: GET http://127.0.0.1:8000/items/1

• Response:

{
  "id": 1,
  "name": "Apple",
  "description": "Fresh red apple"
}

🟠 Update Item (PUT)

• URL: PUT http://127.0.0.1:8000/items/1

• Headers: Content-Type: application/json

• Body (raw JSON):

{
  "name": "Green Apple"
}

• Response:

{
  "id": 1,
  "name": "Green Apple",
  "description": "Fresh red apple"
}

🔴 Delete Item (DELETE)

• URL: DELETE http://127.0.0.1:8000/items/1

• Response:
  Status code 204 No Content (no body)

⚪ Get All Items (GET)

• URL: GET http://127.0.0.1:8000/items

• Response Example:

[
  {
    "id": 1,
    "name": "Green Apple",
    "description": "Fresh red apple"
  }
]

Step 7 — Summary

Framework	Features	Documentation
Flask	Lightweight, beginner-friendly	flask.palletsprojects.com
FastAPI	High-performance, async-ready, built-in docs, validation	fastapi.tiangolo.com