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How to Write a Contract Test with Mock in Python
Use unittest.mock to verify a consumer's expectations match the provider's response shape in a Python contract test.
from unittest.mock import Mock
# Contract test: verify consumer expects data shape that provider delivers.
# We mock the provider and assert the consumer's calls match the agreed contract.
def fetch_user(provider_client, user_id):
"""Consumer code: expects provider to return {'id', 'name', 'email'}."""
respo…How to Write a Fast Smoke Test for a Critical Path in Python
A quick smoke test that validates the /health critical path executes fast enough, raising errors on wrong paths or slow responses.
import time
def smoke_test(path):
if path != "/health":
raise ValueError("Critical path expected /health")
start = time.perf_counter()
# Simulate the critical health check work
time.sleep(0.01)
elapsed = time.perf_counter() - start
if elapsed > 0.05:
raise RuntimeError("Health …How to Write a pytest Test Function with assert Equal in Python
Define simple pytest test functions that use assert to verify result equality and run them with pytest.main.
import pytest
def add(a, b):
return a + b
def test_add_positive_numbers():
result = add(2, 3)
assert result == 5
def test_add_negative_numbers():
result = add(-2, -3)
assert result == -5
def test_add_mixed_numbers():
result = add(2, -3)
assert result == -1
if __name__ == "__main__":
…How to Write pytest Test Function Assert Equal in Python
Write three pytest test functions that assert the result of an add() function equals an expected numeric value.
import pytest
def add(a, b):
return a + b
def test_add_positive_numbers():
assert add(2, 3) == 5
def test_add_negative_numbers():
assert add(-1, -2) == -3
def test_add_mixed_numbers():
assert add(5, -3) == 2
if __name__ == "__main__":
pytest.main([__file__, "-v"])How to freeze time in Python tests with freezegun
Use the freezegun decorator to freeze datetime.now() at a fixed timestamp so tests that depend on current time run deterministically.
from datetime import datetime
from freezegun import freeze_time
@freeze_time("2024-01-15 12:30:00")
def test_frozen_time():
now = datetime.now()
return now
if __name__ == "__main__":
result = test_frozen_time()
print(result)How to mark known bugs with pytest xfail in Python
Use @pytest.mark.xfail to mark tests that are expected to fail due to known bugs, with optional strict mode to control pass/fail behavior.
import pytest
def divide(a: int, b: int) -> float:
if b == 0:
raise ZeroDivisionError("Cannot divide by zero")
return a / b
@pytest.mark.xfail(reason="Known bug: division returns int instead of float", strict=False)
def test_divide_integer_division():
result = divide(10, 4)
assert isinstanc…How to use Optional type hint in Python
Use the Optional type hint to indicate a parameter can be a string or None, with an example function that handles both cases.
from typing import Optional
def greet(name: Optional[str]) -> str:
if name is None:
return "Hello, anonymous!"
else:
return f"Hello, {name}!"
if __name__ == "__main__":
print(greet("Alice"))
print(greet(None))How to use unittest mock side_effect with a sequence in Python
Demonstrates using Mock.side_effect with a list to return different values per call and raise an exception at a specific call in unittest.
import unittest
from unittest.mock import Mock
class TestMockSideEffectSequence(unittest.TestCase):
def test_side_effect_sequence(self):
mock = Mock()
mock.side_effect = [1, 2, 3, Exception("boom")]
self.assertEqual(mock(), 1)
self.assertEqual(mock(), 2)
self.asser…Interface Segregation with Fake Test Implementations in Python
Defines segregated abstract interfaces (Printer, Scanner) and uses a FakePrinter to record calls for unit testing without real resources.
from abc import ABC, abstractmethod
class Printer(ABC):
@abstractmethod
def print_document(self, doc: str) -> str:
pass
class Scanner(ABC):
@abstractmethod
def scan_document(self) -> str:
pass
class MultiFunctionPrinter(Printer, Scanner):
def print_document(self, doc: str) -> …Mock datetime with time-machine in Python
Use the time-machine library to travel to a fixed datetime when running tests or scripts, mocking datetime.utcnow().
from time_machine import travel
from datetime import datetime
@travel("2020-01-01 10:30:00")
def check_date():
return datetime.utcnow()
if __name__ == "__main__":
print(check_date())Mock datetime.now to freeze time in Python
Use unittest.mock.patch to replace datetime.now with a fixed value so your code always sees the same time during tests.
from datetime import datetime
from unittest.mock import patch
def current_message():
now = datetime.now()
return f"Current time: {now:%Y-%m-%d %H:%M:%S}"
if __name__ == "__main__":
with patch("__main__.datetime") as mock_dt:
mock_dt.now.return_value = datetime(2024, 3, 15, 10, 30, 0)
prin…NamedTuple typed record in Python
Define a lightweight immutable record with type hints using typing.NamedTuple; access fields by name and unpack like a tuple.
from typing import NamedTuple
class Point(NamedTuple):
x: float
y: float
label: str = "origin"
if __name__ == "__main__":
p = Point(3.5, -2.0, "A")
print(p)
print(f"x={p.x}, y={p.y}, label={p.label}")
print("is tuple:", isinstance(p, tuple))
q = Point(1.0, 1.0)
print(q)
# …Table-Driven Tests in Python (unittest)
Run a single unittest test against many input cases using a list of tuples and subTest.
import unittest
def add(a, b):
return a + b
class TestAddFunction(unittest.TestCase):
def test_add_with_table(self):
cases = [
(1, 2, 3),
(-1, 1, 0),
(0, 0, 0),
(2, -3, -1),
]
for x, y, expected in cases:
with self.subTest(x…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 [
{…Builder pattern for mocking complex objects in Python
Use a fluent Builder to construct realistic mock objects with defaults, enabling readable test data setup.
class User:
def __init__(self):
self.name = "default"
self.age = 0
self.email = "unknown@example.com"
self.address = "unknown"
def __repr__(self):
return f"User(name={self.name!r}, age={self.age}, email={self.email!r}, address={self.address!r})"
class UserBuilder:
…Circuit Breaker Pattern in Python: Closed, Open, and Half-Open States
Implement a circuit breaker with closed, open, and half-open states to prevent repeated calls to failing services and allow recovery after a timeout.
class CircuitBreaker:
def __init__(self, failure_threshold=3, timeout_seconds=5):
self.failure_threshold = failure_threshold
self.timeout_seconds = timeout_seconds
self.state = "closed"
self.failure_count = 0
self.last_failure_time = None
def record_success(self):
…Create a Data Helper Class in Python
A reusable DataHelper class that saves and loads JSON and CSV files from a configurable base directory, with automatic header detection for CSV.
import json
import csv
from pathlib import Path
class DataHelper:
def __init__(self, base_path="."):
self.base_path = Path(base_path)
self.base_path.mkdir(exist_ok=True)
def save_json(self, data, filename):
path = self.base_path / filename
with open(path, "w") as f:
…Domain Driven Design Aggregate Root Example in Python
Model an Order as an aggregate root with invariants enforced through methods, demonstrating DDD principles in Python.
from __future__ import annotations
from dataclasses import dataclass
from typing import List, Optional
from uuid import uuid4
class Money:
def __init__(self, amount: float, currency: str = "USD"):
self.amount = amount
self.currency = currency
def __add__(self, other: Money) -> Money:
…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 Aggregate Mock API Routes by Method in Python
Groups mock API routes by path and method, collecting response bodies and counts into a nested dictionary structure.
from collections import defaultdict
def aggregate_mock_routes(routes):
"""Aggregate mock API routes by method and aggregate their response bodies."""
aggregated = defaultdict(lambda: defaultdict(list))
for route in routes:
method = route["method"]
path = route["path"]
response = …How to Apply the Clean Architecture Dependency Rule in Python
Demonstrates the dependency rule with a Protocol repository, a use case, and a presenter wired together at a composition root.
from dataclasses import dataclass
from typing import List, Protocol
class Repository(Protocol):
def get_items(self) -> List[str]:
...
@dataclass
class InMemoryRepository:
items: List[str]
def get_items(self) -> List[str]:
return self.items
class UseCase:
"""Application layer depends…How to Build a Health Check System with Instance Up and Down Status in Python
Track instance health by marking them up or down and simulating health checks with a mock class in Python.
from datetime import datetime
import random
class HealthChecker:
def __init__(self):
self.status = {}
def mark_up(self, instance_id):
self.status[instance_id] = {
"state": "up",
"last_check": datetime.now().isoformat(),
"healthy": True
}
…How to Build a Pipe and Filter Text Processing Chain in Python
A functional pipe-and-filter chain that transforms text through uppercase, whitespace normalization, number removal, stopword filtering, and file export.
import re
import sys
def pipe_filter_chain(stream):
def uppercase(text):
return text.upper()
def strip_whitespace(text):
return " ".join(text.split())
def remove_numbers(text):
return re.sub(r"\d+", "", text)
def remove_stopwords(text, stopwords={"the", "and", "of", "in"}):…Browse by section
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