Deterministic tests in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a deterministic pytest function

What is the output of this pytest test function when run?

PyTest

import pytest

def add(a, b):
    return a + b

def test_add():
    assert add(2, 3) == 5
    assert add(0, 0) == 0
    assert add(-1, 1) == 0

A SyntaxError due to missing colon

B All assertions pass, test result: PASSED

C AssertionError on add(-1, 1) == 0, test result: FAILED

D TypeError because add expects strings

💡 Hint

Check the add function and the assertions carefully.

The add function correctly sums two numbers. All assertions match expected sums, so the test passes.

❓ assertion

intermediate

Choosing the correct assertion for deterministic output

Which assertion correctly tests that a function returns a fixed string 'hello' every time?

PyTest

def greet():
    return 'hello'

A assert greet() == 'Hello'

B assert greet() != 'hello'

C assert greet() == 'hello'

D assert greet() is 'hello'

💡 Hint

Check string equality and case sensitivity.

Option C correctly asserts the function returns the exact string 'hello'. Option C uses 'is' which is not reliable for string equality.

🔧 Debug

advanced

Identify why this pytest test is non-deterministic

Why does this test sometimes fail unpredictably?

PyTest

import random

def get_random_number():
    return random.randint(1, 10)

def test_random_number():
    assert get_random_number() == 5

A The test depends on random output, so it is non-deterministic and may fail.

B The function get_random_number always returns 5, so the test should pass.

C The test has a syntax error in the assert statement.

D The test is missing a pytest decorator.

💡 Hint

Think about what random.randint returns each time.

The function returns a random number between 1 and 10, so the assertion that it equals 5 is only sometimes true, causing flaky test failures.

❓ framework

advanced

Best practice to make a pytest test deterministic when testing time-dependent code

Which approach ensures a pytest test for a function that returns the current time is deterministic?

PyTest

import time

def get_current_time():
    return time.time()

A Mock time.time() to return a fixed timestamp during the test.

B Use assert get_current_time() > 0 to check it returns a positive number.

C Call get_current_time() twice and compare results.

D Add a sleep(1) before calling get_current_time() to stabilize output.

💡 Hint

Think about controlling external dependencies in tests.

Mocking time.time() fixes the returned value, making the test deterministic and repeatable.

🧠 Conceptual

expert

Why are deterministic tests important in continuous integration?

Choose the best reason why deterministic tests are critical in a CI pipeline.

A They require less code because they do not use assertions.

B They run faster than non-deterministic tests because they skip setup steps.

C They allow tests to randomly fail sometimes to catch hidden bugs.

D They ensure tests produce the same results every run, avoiding flaky failures that waste developer time.

💡 Hint

Think about reliability and trust in automated testing.

Deterministic tests give consistent results, so developers can trust test outcomes and fix real issues without chasing random failures.