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Testing & modern typing

pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.

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Dependency Injection in Python for Testability

Inject a config dependency into a service so you can swap a real environment-based config for a fake one in tests.

import os


class Config:
    """Simple config loader that can be easily faked in tests."""
    def get(self, key, default=None):
        return os.environ.get(key, default)


class UserService:
    def __init__(self, config):
        self.config = config

    def get_timeout(self):
        return int(self.config.get(…

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Testing & modern typing easy

How to Mock return_value with MagicMock in Python unittest

Use unittest.mock.MagicMock to replace a dependency and set return_value to control what a mocked method returns during unit tests.

unittest mock magicmock

Python

import unittest
from unittest.mock import MagicMock


class PaymentGateway:
    def charge(self, amount):
        raise NotImplementedError


class OrderService:
    def __init__(self, gateway):
        self.gateway = gateway

    def process_order(self, amount):
        return self.gateway.charge(amount)


class Test…

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Testing & modern typing medium

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.

testing mocks stubs

Python

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…

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