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How to Use Mock Flip Mutation Testing in Python
Demonstrates how mutation testing tools flip Boolean literals (mock flip) in Python source to verify test suite effectiveness in catching logic changes.
import random
# In mutation testing, a "mock flip" intentionally changes a Boolean
# constant to False (or True) to see if the test suite catches it.
# This is a common "constant mutation" applied to a source file's literals.
def is_even(n: int) -> bool:
"""Return True if n is even. Contains a Boolean literal us…How to Use Stubs, Fakes, Spies, and Mocks in Python Testing
Implement four types of test doubles — stubs, fakes, spies, and mocks — as subclasses of a PaymentGateway interface to replace real dependencies during testing.
class PaymentGateway:
def charge(self, amount):
raise NotImplementedError
class StubPaymentGateway(PaymentGateway):
"""Returns a fixed response without any logic."""
def charge(self, amount):
return {"success": True, "transaction_id": "stub-12345"}
class FakePaymentGateway(PaymentGatewa…Use pytest fixture to mock a database connection in Python
This code shows how to use a pytest fixture and unittest.mock to replace a database connection with a Mock, enabling isolated tests without a real database.
import pytest
import sqlite3
from unittest.mock import Mock
class Database:
def __init__(self, connection):
self.connection = connection
def get_user(self, user_id):
cursor = self.connection.cursor()
cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))
return cursor.…Build a BFF (Backend for Frontend) Mock Aggregator in Python
A minimal HTTP server implementing the BFF pattern that aggregates user data and orders from two mock backends into a single JSON response.
import json
from http.server import BaseHTTPRequestHandler, HTTPServer
from urllib.parse import urlparse
class MockBackendA:
def get_user(self, user_id):
return {"id": user_id, "name": "Alice", "service": "backend-a"}
class MockBackendB:
def get_orders(self, user_id):
return [
{…Facade Pattern in Python with Mock Simplification
This code demonstrates the Facade pattern by hiding complex subsystem interactions behind a simple start/stop interface, and adds a MockFacade for testing failure scenarios.
class SubsystemA:
def operation_a(self):
return "Subsystem A: ready"
class SubsystemB:
def operation_b(self):
return "Subsystem B: ready"
class SubsystemC:
def operation_c(self):
return "Subsystem C: ready"
class Facade:
def __init__(self):
self._a = SubsystemA()
…How to Build an Adapter to Translate External API Responses in Python
Build an adapter class that translates a mock external API's response shape into your internal representation, keeping callers decoupled from the external contract.
import json
from typing import Dict, Any
class ExternalAPI:
"""Mock external service returning a different data shape."""
def get_user(self, user_id: int) -> Dict[str, Any]:
return {
"id": user_id,
"full_name": "Jane Doe",
"email_address": "jane@example.com",
…How to Implement Retry with Exponential Backoff and Jitter in Python
This code demonstrates a retry mechanism with exponential backoff and optional full jitter, using a flaky mock network call for testing.
import random
import time
def retry_with_backoff(func, max_attempts=5, base_delay=0.1, jitter=True):
"""
Retry a function with exponential backoff and optional full jitter.
"""
for attempt in range(max_attempts):
try:
return func()
except Exception as e:
if att…How to Implement a Simple MVVM Binding Mock in Python
A minimal Python implementation of the MVVM pattern, mocking data binding so views auto-update when the view model changes.
class BindingMock:
def __init__(self, view_model):
self.view_model = view_model
self.subscribers = []
def bind(self, property_name, callback):
self.subscribers.append((property_name, callback))
def set(self, property_name, value):
setattr(self.view_model, property_name, va…How to Mock a Timeout per Dependency Call in Python
This code demonstrates how to simulate and test per-call timeouts for external dependencies using Python's unittest.mock and a simple timing wrapper.
```python
import time
from unittest.mock import Mock, patch
def call_dependency(dependency, timeout):
start = time.time()
result = dependency.call()
elapsed = time.time() - start
if elapsed > timeout:
raise TimeoutError(f"Dependency call took {elapsed:.2f}s, exceeding timeout {timeout}s")
…How to Structure a Three-Tier Layered Architecture in Python
A mock three-tier architecture with presentation, business, and data layers that process a user request from input to response.
class PresentationLayer:
def __init__(self, business_layer):
self.business = business_layer
def handle_request(self, user_id):
print(f"[Presentation] Received request for user {user_id}")
data = self.business.process_user(user_id)
print(f"[Presentation] Response: {data}")
…Inbox pattern consumer dedupe mock in Python
Implements a mock inbox consumer that deduplicates incoming messages by ID, with automatic eviction of old seen IDs to prevent unbounded memory growth.
import json
from collections import deque
from dataclasses import dataclass, field
from hashlib import sha256
from typing import Any
@dataclass
class InboxConsumer:
max_seen: int = 1000
seen_ids: set = field(default_factory=set)
seen_history: deque = field(default_factory=deque)
def _mark_seen(self,…Microkernel Plug-in Core Mock in Python
Implements a minimal microkernel plug-in core that registers, unregisters, and executes synchronous or asynchronous plugins via a pluggable manager class.
import json
import abc
import inspect
class MicrokernelCore(abc.ABC):
def __init__(self):
self._plugins = {}
def register(self, name, plugin):
self._plugins[name] = plugin
def unregister(self, name):
return self._plugins.pop(name, None)
def execute(self, name, *args, **kwa…Mock Unit of Work commit and rollback in Python
Verify that a Unit of Work pattern commits on success and rolls back on failure using unittest.mock in Python.
from unittest import mock
class UnitOfWork:
def __init__(self):
self.committed = False
self.rolled_back = False
def commit(self):
self.committed = True
print("Commit executed")
def rollback(self):
self.rolled_back = True
print("Rollback executed")
def b…Object Pool Pattern for Database Connections in Python
Implements a reusable connection pool with acquire/release and context manager support, mocking database connections with idle reuse and exhaustion handling.
import time
from contextlib import contextmanager
from collections import deque
class ConnectionPool:
def __init__(self, size=3, max_idle=5):
self._idle = deque(maxlen=max_idle)
self._active = set()
self.size = size
def _create(self):
return {"created_at": time.time(), "queri…Build a Bulk Array POST Mock Server in Python
Creates an HTTP mock server that accepts POST requests with a JSON array and returns incremental IDs for each item.
import json
from http.server import BaseHTTPRequestHandler, HTTPServer
from urllib.parse import urlparse
class MockHandler(BaseHTTPRequestHandler):
def do_POST(self):
if urlparse(self.path).path != "/bulk":
self.send_response(404)
self.end_headers()
return
cont…Build a Mock REST API with PUT and GET in Python
A minimal mock REST server implementing idempotent PUT for resource replacement and GET for retrieval, built with Python's http.server module.
from http.server import BaseHTTPRequestHandler, HTTPServer
import json
from urllib.parse import urlparse
mock_db = {}
class MockAPIHandler(BaseHTTPRequestHandler):
def do_PUT(self):
parsed = urlparse(self.path)
resource_id = parsed.path.strip("/").split("/")[-1]
content_length = int(self.…How to Build a Batch Operations Multi-Status 207 Mock Server in Python
Build a mock HTTP server that accepts a batch of operations and returns HTTP 207 Multi-Status with per-operation status codes in JSON.
from http.server import BaseHTTPRequestHandler, HTTPServer
import json
class BatchHandler(BaseHTTPRequestHandler):
def do_POST(self):
if self.path != "/batch":
self.send_response(404)
self.end_headers()
return
content_length = int(self.headers.get("Content-Leng…How to Build a Mock REST GET Endpoint Handler in Python
Create a lightweight mock REST GET server in Python using the standard library, with a dict-based route registry that maps paths to handler functions and returns JSON responses with proper HTTP status codes.
from http.server import BaseHTTPRequestHandler, HTTPServer
import json
# Mock API handler registry
def handle_users():
return {"status": "ok", "data": [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]}
def handle_products():
return {"status": "ok", "data": [{"id": 101, "name": "Laptop", "price": 999.99}…How to Build an Idempotency-Key POST Handler in Python
Python HTTP server mock that accepts POST requests and deduplicates them using an Idempotency-Key header, returning the same response for repeated calls.
import hashlib
import json
from http.server import BaseHTTPRequestHandler, HTTPServer
from urllib.parse import urlparse
class MockAPI(BaseHTTPRequestHandler):
responses = {}
def do_POST(self):
length = int(self.headers.get("Content-Length", 0))
body = self.rfile.read(length).decode("utf-8")
…How to Mock OAuth2 Bearer Token Auth Middleware in Python
Create a simple OAuth2 bearer token authentication middleware that verifies signed tokens and enforces scope-based access control.
import hmac
import time
import base64
import json
from functools import wraps
VALID_TOKENS = {"test_token_123": {"user": "alice", "scope": "read:posts"}}
def generate_token(username: str) -> str:
payload = {"user": username, "iat": int(time.time())}
encoded = base64.urlsafe_b64encode(json.dumps(payload).enc…How to Mock X-RateLimit Headers in Python
This code creates a local HTTP server that mimics rate limit headers (X-RateLimit-Limit, Remaining, Reset, Update) and returns 429 responses when the limit is exceeded.
import time
import threading
from http.server import BaseHTTPRequestHandler, HTTPServer
class RateLimitHandler(BaseHTTPRequestHandler):
RATE_LIMIT = 5 # max requests allowed
WINDOW_SECONDS = 60 # per time window
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
…How to Mock a 202 Accepted Long-Running Operation in Python
Build a mock HTTP server that returns a 202 Accepted response immediately and simulates a long-running operation in the background with threading.
import time
import threading
from http.server import HTTPServer, BaseHTTPRequestHandler
class MockHandler(BaseHTTPRequestHandler):
def do_POST(self):
if self.path == "/long-running":
self.send_response(202)
self.send_header("Content-Type", "application/json")
self.end_h…How to Mock a Chunked Encoding Streaming Response in Python
Build a local mock HTTP server with Python's http.server that streams a chunked-encoded response with a 0.5s delay per chunk.
from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
import time
class ChunkedHandler(BaseHTTPRequestHandler):
protocol_version = "HTTP/1.1"
def do_GET(self):
self.send_response(200)
self.send_header("Content-Type", "text/plain")
self.send_header("Transfer-Encoding", "c…How to Mock a Webhook Subscribe Callback URL in Python
Mock a webhook subscribe callback URL using Python's http.server to receive and parse POST requests sent by webhook providers.
import json
from http.server import BaseHTTPRequestHandler, HTTPServer
class WebhookHandler(BaseHTTPRequestHandler):
def do_POST(self):
content_length = int(self.headers.get('Content-Length', 0))
payload = json.loads(self.rfile.read(content_length)) if content_length else {}
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