Create and Test a Mock API Server in Python Using Flask and Pytest (with GitHub Actions CI)

Have you ever wanted a fast way to mock REST APIs and run automated tests against them — all in Python?

In this guide, we’ll create a mock API server using Flask, write automated tests using Pytest, and run the entire workflow in GitHub Actions CI.

By the end, you’ll have a fully working mock server with 15+ endpoints, a test suite, and a CI pipeline that validates your API on every push. Let’s dive in!

1. Project Structure

Here’s how our project is organized:

python-api-mock-test/
├── app.py                  # Flask mock server
├── requirements.txt        # Project dependencies
├── test_mock_apis.sh       # Optional: curl test script
├── tests/
│   └── test_api.py         # Pytest test suite

2. Create the Flask Mock Server (app.py)

This file will define your API endpoints:

import os
import sys

# Ensure the current directory is in the module search path for CI environments
sys.path.insert(0, os.path.abspath(os.path.dirname(__file__)))

from flask import Flask, jsonify, request

app = Flask(__name__)

@app.before_request
def log_request():
    print(f"==> {request.method} {request.path}")

@app.route('/')
def index():
    return jsonify({
        'message': 'Mock Server Running',
        'endpoints': [
            '/api/user/<user_id>',
            '/api/user [POST, PUT, DELETE]',
            '/api/products',
            '/api/order/<order_id>',
            '/api/order [POST]',
            '/api/login',
            '/api/logout',
            '/api/health',
            '/api/notifications',
            '/api/settings [GET, PUT]',
            '/api/profile [GET, PATCH]',
            '/api/reports',
            '/api/upload [POST]'
        ]
    })

@app.route('/api/user/<int:user_id>', methods=['GET'])
def get_user(user_id):
    return jsonify({'user_id': user_id, 'name': 'User'+str(user_id)})

@app.route('/api/user', methods=['POST'])
def create_user():
    data = request.get_json()
    return jsonify({'message': 'User created', 'user': data}), 201

@app.route('/api/user/<int:user_id>', methods=['PUT'])
def update_user(user_id):
    data = request.get_json()
    return jsonify({'message': 'User updated', 'user_id': user_id, 'updates': data})

@app.route('/api/user/<int:user_id>', methods=['DELETE'])
def delete_user(user_id):
    return jsonify({'message': f'User {user_id} deleted'})

@app.route('/api/products', methods=['GET'])
def get_products():
    return jsonify({'products': ['Product A', 'Product B', 'Product C']})

@app.route('/api/order/<int:order_id>', methods=['GET'])
def get_order(order_id):
    return jsonify({'order_id': order_id, 'status': 'processing'})

@app.route('/api/order', methods=['POST'])
def create_order():
    data = request.get_json()
    return jsonify({'message': 'Order placed', 'order': data}), 201

@app.route('/api/login', methods=['POST'])
def login():
    creds = request.get_json()
    return jsonify({'message': f"Welcome {creds.get('username')}", 'token': 'mock-token-123'})

@app.route('/api/logout', methods=['POST'])
def logout():
    return jsonify({'message': 'User logged out'})

@app.route('/api/health', methods=['GET'])
def health_check():
    return jsonify({'status': 'ok', 'uptime': '100h'})

@app.route('/api/notifications', methods=['GET'])
def get_notifications():
    return jsonify({'notifications': ['Message 1', 'Alert 2', 'Reminder 3']})

@app.route('/api/settings', methods=['GET'])
def get_settings():
    return jsonify({'theme': 'dark', 'language': 'en', 'timezone': 'UTC'})

@app.route('/api/settings', methods=['PUT'])
def update_settings():
    data = request.get_json()
    return jsonify({'message': 'Settings updated', 'settings': data})

@app.route('/api/profile', methods=['GET'])
def get_profile():
    return jsonify({'name': 'John Doe', 'email': 'john@example.com', 'role': 'admin'})

@app.route('/api/profile', methods=['PATCH'])
def update_profile():
    data = request.get_json()
    return jsonify({'message': 'Profile updated', 'updates': data})

@app.route('/api/reports', methods=['GET'])
def get_reports():
    return jsonify({'reports': ['Report1.pdf', 'Report2.xlsx']})

@app.route('/api/upload', methods=['POST'])
def upload_file():
    return jsonify({'message': 'File uploaded successfully', 'file_id': '12345'})

if __name__ == '__main__':
    print("Starting Flask mock server on http://localhost:5000")
    app.run(debug=False, port=5000)

3. Install Dependencies

Create your requirements.txt:

Flask==2.3.2
pytest==8.2.1

Then install them in a virtual environment:

python -m venv venv
source venv/bin/activate
pip install -r requirements.txt

4. Write Tests with Pytest

Create tests/test_api.py:

import pytest
from app import app

@pytest.fixture
def client():
    with app.test_client() as client:
        yield client

def test_get_user(client):
    response = client.get('/api/user/1')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert response.json['user_id'] == 1

def test_create_user(client):
    response = client.post('/api/user', json={'name': 'New User'})
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 201
    assert response.json['user']['name'] == 'New User'

def test_update_user(client):
    response = client.put('/api/user/1', json={'name': 'Updated'})
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert response.json['updates']['name'] == 'Updated'

def test_delete_user(client):
    response = client.delete('/api/user/1')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200

def test_get_products(client):
    response = client.get('/api/products')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert 'products' in response.json

def test_get_order(client):
    response = client.get('/api/order/100')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert response.json['order_id'] == 100

def test_create_order(client):
    response = client.post('/api/order', json={'item': 'Book'})
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 201
    assert response.json['order']['item'] == 'Book'

def test_login(client):
    response = client.post('/api/login', json={'username': 'admin'})
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert 'token' in response.json

def test_logout(client):
    response = client.post('/api/logout')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200

def test_health_check(client):
    response = client.get('/api/health')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert response.json['status'] == 'ok'

def test_get_notifications(client):
    response = client.get('/api/notifications')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert isinstance(response.json['notifications'], list)

def test_get_settings(client):
    response = client.get('/api/settings')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert 'theme' in response.json

def test_update_settings(client):
    response = client.put('/api/settings', json={'theme': 'light'})
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert response.json['settings']['theme'] == 'light'

def test_get_profile(client):
    response = client.get('/api/profile')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert response.json['name'] == 'John Doe'

def test_update_profile(client):
    response = client.patch('/api/profile', json={'name': 'Jane'})
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert response.json['updates']['name'] == 'Jane'

def test_get_reports(client):
    response = client.get('/api/reports')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert isinstance(response.json['reports'], list)

def test_upload_file(client):
    response = client.post('/api/upload')
    print("Response Status:", response.status_code)
    print("Response JSON:", response.get_json())
    assert response.status_code == 200
    assert 'file_id' in response.json

Run tests:

pytest

5. Run Mock API Manually

Start the server:

python app.py

Now access it in your browser or Postman at:

GET http://localhost:5000/api/health or GET http://127.0.0.1:5000/api/health

6. Automate with GitHub Actions CI

Create .github/workflows/test.yml:

name: Flask Mock Server Test

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest

    steps:
      - name: Checkout repo
        uses: actions/checkout@v3

      - name: Set up Python
        uses: actions/setup-python@v4
        with:
          python-version: '3.10'

      - name: Cache pip dependencies
        uses: actions/cache@v3
        with:
          path: ~/.cache/pip
          key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
          restore-keys: |
            ${{ runner.os }}-pip-

      - name: Install dependencies
        working-directory: .
        run: |
          python -m venv venv
          source venv/bin/activate
          export PYTHONPATH=.
          pip install --upgrade pip
          pip install -r requirements.txt

      - name: Start Flask server in background
        working-directory: .
        run: |
          source venv/bin/activate
          export PYTHONPATH=.
          nohup python app.py > flask.log 2>&1 &
          sleep 5

      - name: Run pytest tests
        working-directory: .
        run: |
          source venv/bin/activate
          export PYTHONPATH=.
          pytest tests

      - name: Run curl script (optional)
        working-directory: .
        run: |
          chmod +x test_mock_apis.sh
          ./test_mock_apis.sh

      - name: Show Flask server logs
        if: always()
        working-directory: .
        run: cat flask.log

7. Push and Verify

Push your code to GitHub, and the CI workflow will:

✅ Install dependencies

✅ Start your mock server

✅ Run your test suite

✅ Print logs if anything fails

Bonus: Curl Test Script

You can create test_mock_apis.sh:

#!/bin/bash

BASE_URL="http://127.0.0.1:5000"

declare -a endpoints=(
  "/api/user/1"
  "/api/user"
  "/api/user/1" # PUT
  "/api/user/1" # DELETE
  "/api/products"
  "/api/order/100"
  "/api/order"
  "/api/login"
  "/api/logout"
  "/api/health"
  "/api/notifications"
  "/api/settings"
  "/api/settings" # PUT
  "/api/profile"
  "/api/profile" # PATCH
  "/api/reports"
  "/api/upload"
)

declare -a methods=(
  "GET"
  "POST"
  "PUT"
  "DELETE"
  "GET"
  "GET"
  "POST"
  "POST"
  "POST"
  "GET"
  "GET"
  "GET"
  "PUT"
  "GET"
  "PATCH"
  "GET"
  "POST"
)

declare -a payloads=(
  ""
  '{"name": "New User"}'
  '{"name": "Updated"}'
  ""
  ""
  ""
  '{"item": "Book"}'
  '{"username": "admin"}'
  ""
  ""
  ""
  ""
  '{"theme": "light"}'
  ""
  '{"name": "Jane"}'
  ""
  ""
)

echo "Testing all mock API endpoints..."

for i in "${!endpoints[@]}"; do
  method=${methods[$i]}
  url="$BASE_URL${endpoints[$i]}"
  data=${payloads[$i]}
  echo
  echo "===== $method $url ====="
  if [[ "$method" == "GET" || "$method" == "DELETE" ]]; then
    curl -s -X $method "$url" -H "Content-Type: application/json"
  else
    curl -s -X $method "$url" -H "Content-Type: application/json" -d "$data"
  fi
  echo
done

Make it executable:

chmod +x test_mock_apis.sh
./test_mock_apis.sh

8. .gitignore

Ignore unnecessary files:

# Byte-compiled / optimized / DLL files
__pycache__/
*.py[cod]
*.pyo

# Virtual environment
.venv/
venv/
env/

# Editor settings
.vscode/
.idea/

# Test output and logs
flask.log
*.log
*.coverage
.coverage.*

# Pytest cache
.pytest_cache/

# Mac-specific files
.DS_Store

# Bash history or temp files
*.sh~


.idea

Conclusion

You now have:

• A full Flask-based mock API server
• A Pytest suite to validate behavior
• CI integration with GitHub Actions

This setup is ideal for API prototyping, contract testing, mocking microservices, or onboarding frontend teams.

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