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Alex Spinov
Alex Spinov

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Stop Building APIs from Scratch for Prototypes — Use This Instead

#python #tutorial #webdev #productivity

Every time I start a new frontend project, the same problem hits: the backend isn't ready yet.

I used to build minimal Express/Flask servers just to have something to fetch from. That's at least 30 minutes of boilerplate before writing any frontend code.

Then I built a tool that creates a full REST API from a JSON file in 10 seconds.

The Problem

You're building a React dashboard. You need:

  • GET /users — list of users
  • GET /users/1 — single user
  • POST /users — create user
  • PUT /users/1 — update user
  • DELETE /users/1 — delete user

Plus filtering, pagination, and sorting.

Setting this up with Express: 30-60 minutes.
Setting this up with json-api-mocker: 10 seconds.

How It Works

  1. Create a JSON file with your data: 
{
  "users": [
    {"id": 1, "name": "Alice", "email": "alice@test.com", "role": "admin"},
    {"id": 2, "name": "Bob", "email": "bob@test.com", "role": "user"}
  ],
  "posts": [
    {"id": 1, "userId": 1, "title": "Hello World"},
    {"id": 2, "userId": 2, "title": "Python Tips"}
  ]
}
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  1. Run the mocker: 
pip install flask
python mocker.py --data data.json --port 3000
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  1. You now have full CRUD for every collection: 
# List all users
curl http://localhost:3000/users

# Filter by field
curl http://localhost:3000/users?role=admin

# Pagination
curl http://localhost:3000/users?_page=1&_limit=10

# Sort
curl http://localhost:3000/users?_sort=name&_order=asc

# Full-text search
curl http://localhost:3000/users?q=alice

# Create
curl -X POST http://localhost:3000/users \
  -H "Content-Type: application/json" \
  -d '{"name": "Charlie", "email": "charlie@test.com"}'
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The Code (Under 100 Lines)

The core is surprisingly simple — Flask + a JSON file:

from flask import Flask, jsonify, request, abort
import json

app = Flask(__name__)
DATA = {}

def register_routes(collection):
    def get_all():
        items = DATA.get(collection, [])
        # Apply filters from query params
        for key, val in request.args.items():
            if not key.startswith('_'):
                items = [i for i in items if str(i.get(key, '')) == val]
        return jsonify(items)

    def get_one(item_id):
        item = next((i for i in DATA[collection] if i['id'] == item_id), None)
        return jsonify(item) if item else abort(404)

    def create():
        item = request.get_json()
        item['id'] = max(i['id'] for i in DATA[collection]) + 1
        DATA[collection].append(item)
        return jsonify(item), 201

    app.add_url_rule(f'/{collection}', f'{collection}_list', get_all)
    app.add_url_rule(f'/{collection}/<int:item_id>', f'{collection}_get', get_one)
    app.add_url_rule(f'/{collection}', f'{collection}_create', create, methods=['POST'])

# Load data and register routes
with open('data.json') as f:
    DATA = json.load(f)
for key in DATA:
    register_routes(key)

app.run(port=3000)
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That's it. Every key in your JSON becomes a REST endpoint.

Killer Features

1. Simulate Slow APIs 

python mocker.py --data data.json --delay 500
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Now every response takes 500ms. Perfect for testing loading states and skeleton screens.

2. Read-Only Mode 

python mocker.py --data data.json --readonly
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POST/PUT/DELETE return 405. Great for demo environments.

3. Auto-Persistence

Changes are saved back to the JSON file. Restart the server and your data is still there.

4. CORS Enabled by Default

No more "Access-Control-Allow-Origin" headaches. Works with any frontend framework out of the box.

When to Use This vs. Real Backends

Use Mock API Use Real Backend
Prototyping UI Production
Frontend demos User-facing features
Unit/integration tests Load testing
Hackathons When you need auth/validation
Teaching/tutorials When data integrity matters

Get It 

git clone https://github.com/spinov001-art/json-api-mocker
cd json-api-mocker
pip install flask
python mocker.py --data data.json
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The full version includes pagination, sorting, full-text search, delay simulation, and read-only mode — all in under 150 lines of Python.

What's your go-to tool for mocking APIs during development? I'd love to hear alternatives.

For more developer tools, check out my collection of 140+ open-source projects.

Follow for more Python tooling and API tutorials.

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