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Fakes and dependency injection · Software Testing

How do you test code that sends emails, charges cards or depends on the time?

Why you need it

Real emails, real payments and the real clock make tests slow, flaky and sometimes expensive: you don't want every test run to charge a card. The fix is a design habit: let the code receive these things instead of reaching for them. Then a test can hand in a harmless stand-in, and the code is easier to change too.

The intuition

Hard to test:

def shop_status():
    hour = datetime.datetime.now().hour   # reaches out to the real clock

Easy to test, because the time is passed in:

def shop_status(now):
    hour = now.hour

That's dependency injection: give a function what it depends on (the time, a mailer, a database, a payment client) as arguments.

In tests you pass a test double instead of the real thing:

Double	What it does
stub	returns fixed answers ("the time is 09:00")
fake	a small working version (an in-memory database)
mock / spy	records how it was called, so the test can check it

Python's unittest.mock can build these for you, but a ten-line class is often clearer.

Definition

Dependency injection

Passing a function or object the things it depends on, instead of letting it create or fetch them itself.

Test double

A stand-in for a real dependency in tests: a stub, fake or mock.

Examples

Pass the time in

Try it yourself

With now as a parameter, any moment can be tested.

import datetime

def shop_status(now):
    if 9 <= now.hour < 18:
        return "open"
    return "closed"

print(shop_status(datetime.datetime(2026, 1, 5, 8, 59)))
print(shop_status(datetime.datetime(2026, 1, 5, 9, 0)))
print(shop_status(datetime.datetime(2026, 1, 5, 18, 0)))

Output

closed
open
closed

Step by step

L3The function no longer looks at the real clock.
L8A test can pick exactly the boundary minutes: 08:59, 09:00, 18:00.

A fake mailer

Try it yourself

The fake records what would have been sent, so the test can check it.

class FakeMailer:
    def __init__(self):
        self.sent = []

    def send(self, to, subject):
        self.sent.append((to, subject))

def remind_overdue(invoices, today, mailer):
    for inv in invoices:
        if inv["due"] < today and not inv["paid"]:
            mailer.send(inv["email"], f"Invoice {inv['id']} is overdue")

invoices = [
    {"id": 1, "email": "a@example.com", "due": "2026-03-01", "paid": False},
    {"id": 2, "email": "b@example.com", "due": "2026-03-01", "paid": True},
    {"id": 3, "email": "c@example.com", "due": "2026-04-01", "paid": False},
]
mailer = FakeMailer()
remind_overdue(invoices, "2026-03-10", mailer)
print(mailer.sent)

Output

[('a@example.com', 'Invoice 1 is overdue')]

Step by step

L1Same send() as the real mailer, but it only remembers.
L8The code gets the mailer and today's date as arguments.
L10ISO dates (YYYY-MM-DD) compare correctly as text.
L20Only invoice 1 is overdue and unpaid.

Common mistakes

Calling real services in tests

Tests that send real emails or hit a real API fail when the network is slow and can cost money. Inject the service and pass a fake.

Mocking everything

If a test mocks every function, it only checks that the code calls what it calls today. Fake the outside world (time, network, payments), not your own logic.

Tests that depend on today

A test using the real date can pass today and fail next month. Pass a fixed date.

Quick quiz

1. Why pass `now` into a function instead of calling `datetime.now()` inside it?

ATests can choose any moment, so results don't change from day to day
BIt runs faster
Cdatetime.now() is wrong at night

Show the answer

Answer: A

Injecting the time makes the function deterministic: same input, same output.

2. A test needs to check that a welcome email was sent, without sending one. What do you pass in?

AA fake mailer that records messages
BThe real mailer with a test address
CNothing; skip the check

Show the answer

Answer: A

A recording fake (a spy) lets the test inspect what would have been sent.

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

PredictA stub bank
Type exactly what this prints.
ModifyMake it testable
shop_status() reads the real clock, so it can't be tested reliably. Change it to shop_status(now), where now is a datetime, and add one rule from the owner: the shop is closed all day on Sundays (now.weekday() == 6). Don't call datetime.now() any more.
WriteWrite a fake bank
Write a class FakeBank to test checkout with:
- FakeBank() approves every card; FakeBank(declined=["4000"]) declines the cards listed
- charge(card, amount) returns True or False, and records every attempt in self.charges as a tuple (card, amount)

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Terms in this lesson

Term	Chinese	Definition
Dependency injection	依赖注入	Passing in what code depends on, so tests can swap it.
Test double	测试替身	A stub, fake or mock used in place of a real dependency.
Mock	模拟对象	A double that records how it was called.