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Program with GUTs

The document discusses the importance of writing good unit tests in programming, emphasizing the need for tests to be meaningful and act as documentation for the code. It critiques common misconceptions about Test-Driven Development (TDD) and outlines best practices for creating effective tests, including clarity, specificity, and error handling. Additionally, it addresses various test cases related to leap years as practical examples.

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Program with GUTs Kevlin Henney
@KevlinHenney @kevlin@mastodon.social @kevlin.bsky.social threads.net/@kevlin.henney instagram.com/kevlin.henney about.me/kevlin linkedin.com/in/kevlin kevlinhenney.medium.com kevlin@curbralan.com slideshare.net/Kevlin
So you’re writing unit tests? Great! Are they any good? Kevlin Henney “Program with GUTs” medium.com/97-things/program-with-guts-828e69dd8e15
Do you have GUTs? Kevlin Henney “Program with GUTs” medium.com/97-things/program-with-guts-828e69dd8e15
Very many people say “TDD” when they really mean, “I have good unit tests” (“I have GUTs”?) Alistair Cockburn “The modern programming professional has GUTs”
(“I have GUTs
Or have you landed someone (future you?) with interest- accumulating technical debt in their testbase? Kevlin Henney “Program with GUTs” medium.com/97-things/program-with-guts-828e69dd8e15
What do I mean by good? Kevlin Henney “Program with GUTs” medium.com/97-things/program-with-guts-828e69dd8e15
We think in generalities, but we live in detail. Alfred North Whitehead
mastodon.social/deck/@0xabad1dea@infosec.exchange/112015426639751687
public static bool IsLeapYear(int year) 
[Test] public void Test() 
[Test] public void TestIsLeapYear() 
[Test] public void TestIsLeapYearIsOK() 
[Test] public void TestIsLeapYearIsCorrect() 
[Test] public void TestIsLeapYearWorks() 
[Test] public void TestIsLeapYearWorksAsExpected() 
[Test] public void Test1()  [Test] public void Test2() 
[Test] public void TestLeapYears()  [Test] public void TestNonLeapYears() 
[Test] public void TestLeapYears() { Assert.IsTrue(IsLeapYear(2024)); Assert.IsTrue(IsLeapYear(2000)); } [Test] public void TestNonLeapYears() 
[Test] public void Test2024IsALeapYear()  [Test] public void Test2000IsALeapYear()  [Test] public void Test2023IsNotALeapYear()  [Test] public void Test1900IsNotALeapYear() 
[Test] public void Test2016IsALeapYear()  [Test] public void Test2400IsALeapYear()  [Test] public void Test2022IsNotALeapYear()  [Test] public void Test2100IsNotALeapYear() 
Write Tests for People Gerard Meszaros 97-things-every-x-should-know.gitbooks.io/97-things-every-programmer-should-know/content/en/thing_95
Gerard Meszaros 97-things-every-x-should-know.gitbooks.io/97-things-every-programmer-should-know/content/en/thing_95 So who should you be writing the tests for? For the person trying to understand your code.
Gerard Meszaros 97-things-every-x-should-know.gitbooks.io/97-things-every-programmer-should-know/content/en/thing_95 Good tests act as documentation for the code they are testing.
A year divisible by 4 is a leap year A year divisible by 400 is a leap year A year not divisible by 4 is not a leap year A year divisible by 100 is not a leap year
[Test] public void A_year_divisible_by_4_is_a_leap_year()  [Test] public void A_year_divisible_by_400_is_a_leap_year()  [Test] public void A_year_not_divisible_by_4_is_not_a_leap_year()  [Test] public void A_year_divisible_by_100_is_not_a_leap_year() 
A_year_divisible_by_4_is_a_leap_year A_year_divisible_by_400_is_a_leap_year A_year_not_divisible_by_4_is_not_a_leap_year A_year_divisible_by_100_is_not_a_leap_year
A_year_not_divisible_by_4_is_not_a_leap_year A_year_divisible_by_4_is_a_leap_year A_year_divisible_by_100_is_not_a_leap_year A_year_divisible_by_400_is_a_leap_year
A_year_not_divisible_by_4_is_not_a_leap_year A_year_divisible_by_4_but_not_by_100_is_a_leap_ye A_year_divisible_by_100_but_not_by_400_is_not_a_l A_year_divisible_by_400_is_a_leap_year
A_year_not_divisible_by_4_is_not_a_leap_year A_year_divisible_by_4_but_not_by_100_is_a_leap_year A_year_divisible_by_100_but_not_by_400_is_not_a_leap_year A_year_divisible_by_400_is_a_leap_year
Not leap years Leap years
Not leap years Leap years Not divisible by 4 Divisible by 4 Divisible by 100 Divisible by 400
IsLeapYear_YearNotDivisibleBy4_ReturnsFalse IsLeapYear_YearDivisibleBy4ButNotBy100_ReturnsTrue IsLeapYear_YearDivisibleBy100ButNotBy400_ReturnsFalse IsLeapYear_YearDivisibleBy400_ReturnsTrue
A_year_not_divisible_by_4_should_not_be_a_leap_year A_year_divisible_by_4_but_not_by_100_should_not_be_a_leap_y A_year_divisible_by_100_but_not_by_400_should_not_be_a_leap A_year_divisible_by_400_should_not_be_a_leap_year
A_year_not_divisible_by_4_must_not_be_a_leap_year A_year_divisible_by_4_but_not_by_100_must_not_be_a_leap_yea A_year_divisible_by_100_but_not_by_400_must_not_be_a_leap_y A_year_divisible_by_400_must_not_be_a_leap_year
A_year_not_divisible_by_4_is_not_a_leap_year A_year_divisible_by_4_but_not_by_100_is_a_leap_year A_year_divisible_by_100_but_not_by_400_is_not_a_leap_year A_year_divisible_by_400_is_a_leap_year
Propositions are vehicles for stating how things are or might be.
Thus only indicative sentences which it makes sense to think of as being true or as being false are capable of expressing propositions.
public static bool IsLeapYear(int year) { return year % 4 == 0 && year % 100 != 0 || year % 400 == 0; }
A_year_not_divisible_by_4_is_not_a_leap_year A_year_divisible_by_4_but_not_by_100_is_a_leap_year A_year_divisible_by_100_but_not_by_400_is_not_a_leap_year A_year_divisible_by_400_is_a_leap_year
public static bool IsLeapYear(int year) { return year % 4 == 0; }
A_year_not_divisible_by_4_is_not_a_leap_year A_year_divisible_by_4_but_not_by_100_is_a_leap_year A_year_divisible_by_100_but_not_by_400_is_not_a_leap_year A_year_divisible_by_400_is_a_leap_year
A failing test should tell you exactly what is wrong quickly, without you having to spend a lot of time analyzing the failure. This means... Marit van Dijk “Use Testing to Develop Better Software Faster” medium.com/97-things/use-testing-to-develop-better-software-faster-9dd2616543d3
Each test should test one thing. Marit van Dijk “Use Testing to Develop Better Software Faster” medium.com/97-things/use-testing-to-develop-better-software-faster-9dd2616543d3
Use meaningful, descriptive names. Don’t just describe what the test does either (we can read the code), tell us why it does this. This can help decide whether a test should be updated in line with changed functionality or whether an actual failure that should be fixed has been found. Marit van Dijk “Use Testing to Develop Better Software Faster” medium.com/97-things/use-testing-to-develop-better-software-faster-9dd2616543d3
Never trust a test you haven’t seen fail. Marit van Dijk “Use Testing to Develop Better Software Faster” medium.com/97-things/use-testing-to-develop-better-software-faster-9dd2616543d3
public class Leap_year_spec { public class A_year_is_a_leap_year { [Test] public void if_it_is_divisible_by_4_but_not_by_100()  [Test] public void if_it_is_divisible_by_400()  } public class A_year_is_not_a_leap_year { [Test] public void if_it_is_not_divisible_by_4()  [Test] public void if_it_is_divisible_by_100_but_not_by_400()  } }
namespace Leap_year_spec { public class A_year_is_a_leap_year { [Test] public void if_it_is_divisible_by_4_but_not_by_100()  [Test] public void if_it_is_divisible_by_400()  } public class A_year_is_not_a_leap_year { [Test] public void if_it_is_not_divisible_by_4()  [Test] public void if_it_is_divisible_by_100_but_not_by_400()  } }
namespace Leap_year_spec { public class A_year_is_a_leap_year { [Test] public void if_it_is_divisible_by_4_but_not_by_100()  [Test] public void if_it_is_divisible_by_400()  } public class A_year_is_not_a_leap_year { [Test] public void if_it_is_not_divisible_by_4()  [Test] public void if_it_is_divisible_by_100_but_not_by_400()  } }
Nat Pryce & Steve Freeman Are your tests really driving your development? For tests to drive development they must do more than just test that code performs its required functionality: they must clearly express that required functionality to the reader. That is, they must be clear specifications of the required functionality.
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year { [Test] public void if_it_is_not_divisible_by_4() { Assert.IsFalse(IsLeapYear(2023)); } [Test] public void if_it_is_divisible_by_100_but_not_by_400()  } }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year { [Test] public void if_it_is_not_divisible_by_4() { Assert.IsFalse(IsLeapYear(2023)); Assert.IsFalse(IsLeapYear(2022)); Assert.IsFalse(IsLeapYear(1999)); Assert.IsFalse(IsLeapYear(3)); } [Test] public void if_it_is_divisible_by_100_but_not_by_400()  } }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year { [TestCase(2023)] [TestCase(2022)] [TestCase(1999)] [TestCase(3)] public void if_it_is_not_divisible_by_4(int year) { Assert.IsFalse(IsLeapYear(year)); } [Test] public void if_it_is_divisible_by_100_but_not_by_400()  } }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year { [Test] public void if_it_is_not_divisible_by_4( [Values(2023, 2022, 1999, 3)] int year) { Assert.IsFalse(IsLeapYear(year)); } [Test] public void if_it_is_divisible_by_100_but_not_by_400()  } }
namespace Leap_year_spec { public class A_year_is_a_leap_year { [Test] public void if_it_is_divisible_by_4_but_not_by_100( [Values(2024, 2016, 1984, 4) int year)  [Test] public void if_it_is_divisible_by_400( [Range(400, 4000, 400)] int year)  } public class A_year_is_not_a_leap_year { [Test] public void if_it_is_not_divisible_by_4( [Values(2023, 2022, 1999, 3)] int year)  [Test] public void if_it_is_divisible_by_100_but_not_by_400( [Values(2100, 1900, 1800, 100)] int year)  } }
Simple cases Common cases Error cases Boundary cases
Simple cases Common cases Error cases Boundary cases ?
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class Error_cases  }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class A_year_is_not_supported  }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class A_year_is_not_supported { [Test] public void if_it_is_0() { Assert.Throws<ArgumentOutOfRangeException>(() => IsLeapYear(0)); } } }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class A_year_is_not_supported { [Test] public void if_it_is_0() { Assert.Catch<ArgumentOutOfRangeException>(() => IsLeapYear(0)); } } }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class A_year_is_not_supported { [Test] public void if_it_is_0() { Assert.Catch<ArgumentOutOfRangeException>(() => IsLeapYear(0)); } [Test] public void if_it_is_negative( [Values(-1, -4, -100, -400)] int year) { Assert.Catch<ArgumentOutOfRangeException>(() => IsLeapYear(year)); } } }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class A_year_is_not_supported { [Test] public void if_it_is_0() { Assert.Catch<ArgumentOutOfRangeException>(() => IsLeapYear(0)); } [Test] public void if_it_is_negative( [Values(-1, -4, -100, -400, int.MinValue)] int year) { Assert.Catch<ArgumentOutOfRangeException>(() => IsLeapYear(year)); } } }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class A_year_is_supported  public class A_year_is_not_supported  }
namespace Leap_year_spec { public class A_year_is_a_leap_year  public class A_year_is_not_a_leap_year  public class A_year_is_supported { [Test] public void if_it_is_positive( [Values(1, 10000, int.MaxValue)] int year) { Assert.DoesNotThrow(() => IsLeapYear(year)); } } public class A_year_is_not_supported  }
Leo Tolstoy Anna Karenina All happy families are alike; each unhappy family is unhappy in its own way.
namespace Leap_year_spec  public class A_year_is_a_leap_year  public void if_it_is_divisible_by_4_but_not_by_400()  public void if_it_is_divisible_by_400()  public class A_year_is_not_a_leap_year  public void if_is_not_divisible_by_4()  public void if_it_is_divisible_by_100_but_not_by_400()  public class A_year_is_supported  public void if_it_is_positive()  public class A_year_is_not_supported  public void if_it_is_0()  public void if_it_is_negative() 
namespace Leap_year_spec  public class A_year_is_a_leap_year  public void if_it_is_divisible_by_4_but_not_by_400()  public void if_it_is_divisible_by_400()  public class A_year_is_not_a_leap_year  public void if_is_not_divisible_by_4()  public void if_it_is_divisible_by_100_but_not_by_400()  public class A_year_is_supported  public void if_it_is_positive()  public class A_year_is_not_supported  public void if_it_is_0()  public void if_it_is_negative() 
Producer Consumer Queue Length Capacity Enqueue ⮞ ⮜ Dequeue
public class Queue<T> { ... public Queue(int capacity) ... public int Capacity => ... public int Length => ... public bool Enqueue(T toBack) ... public bool Dequeue(out T fromFront) ... }
Nat Pryce & Steve Freeman Are your tests really driving your development? Tests that are not written with their role as specifications in mind can be very confusing to read. The difficulty in understanding what they are testing can greatly reduce the velocity at which a codebase can be changed.
public class QueueTests { [Test] public void TestConstructor() ... [Test] public void TestCapacity() ... [Test] public void TestLength() ... [Test] public void TestEnqueue() ... [Test] public void TestDequeue() ... }
public class QueueTests { [Test] public void Constructor() ... [Test] public void Capacity() ... [Test] public void Length() ... [Test] public void Enqueue() ... [Test] public void Dequeue() ... }
public class QueueTests { [Test] public void CanBeConstructed() ... [Test] public void HasCapacity() ... [Test] public void HasLength() ... [Test] public void CanBeEnqueuedOn() ... [Test] public void CanBeDequeuedFrom() ... }
public class QueueTests { [Test] public void CanSometimesBeConstructed() ... [Test] public void HasCapacity() ... [Test] public void HasLength() ... [Test] public void CanSometimesBeEnqueuedOn() ... [Test] public void CanSometimesBeDequeuedFrom() ... }
85 twitter.com/venkat_s/status/1633131055635890179
namespace Queue_spec  public class Creating_a_queue  public void leaves_it_empty()  public void preserves_positive_bounding_capacity()  public void fails_with_non_positive_bounding_capacity()  public class Enqueuing_on  public void an_empty_queue_makes_it_longer()  public void a_non_empty_queue_makes_it_longer()  public void a_non_full_queue_up_to_capacity_makes_it_full()  public void a_full_queue_is_ignored()  public class Dequeuing_from  public void an_empty_queue_is_ignored_with_default_value()  public void a_non_empty_queue_gives_values_in_order_enqueued()  public void a_full_queue_makes_it_non_full() 
namespace Queue_spec  public class Creating_a_queue  public void leaves_it_empty()  public void preserves_positive_bounding_capacity()  public void fails_with_non_positive_bounding_capacity()  public class Enqueuing_on  public void an_empty_queue_makes_it_longer()  public void a_non_empty_queue_makes_it_longer()  public void a_non_full_queue_up_to_capacity_makes_it_full()  public void a_full_queue_is_ignored()  public class Dequeuing_from  public void an_empty_queue_is_ignored_with_default_value()  public void a_non_empty_queue_gives_values_in_order_enqueued()  public void a_full_queue_makes_it_non_full() 
public class Enqueuing_on  public void an_empty_queue_makes_it_longer(string value) { var queue = new Queue<string>(2); var enqueued = queue.Enqueue(value); Assert.IsTrue(enqueued); Assert.AreEqual(1, queue.Length); }
public class Enqueuing_on  public void an_empty_queue_makes_it_longer(string value) { var queue = new Queue<string>(2); var enqueued = queue.Enqueue(value); Assert.IsTrue(enqueued); Assert.AreEqual(1, queue.Length); }
Gerard Meszaros 97-things-every-x-should-know.gitbooks.io/97-things-every-programmer-should-know/content/en/thing_95 For each usage scenario, the test(s): ▪ Describe the context, starting point, or preconditions that must be satisfied ▪ Illustrate how the software is invoked ▪ Describe the expected results or postconditions to be verified
public class Enqueuing_on  public void an_empty_queue_makes_it_longer(string value) { // Arrange: var queue = new Queue<string>(2); // Act: var enqueued = queue.Enqueue(value); // Assert: Assert.IsTrue(enqueued); Assert.AreEqual(1, queue.Length); }
public class Enqueuing_on  public void an_empty_queue_makes_it_longer(string value) { // Establish precondition for operation: var queue = new Queue<string>(2); // Perform operation of interest: var enqueued = queue.Enqueue(value); // Confirm postcondition of operation: Assert.IsTrue(enqueued); Assert.AreEqual(1, queue.Length); }
public class Enqueuing_on  public void an_empty_queue_makes_it_longer(string value) { // Given: var queue = new Queue<string>(2); // When: var enqueued = queue.Enqueue(value); // Then: Assert.IsTrue(enqueued); Assert.AreEqual(1, queue.Length); }
Thinking in States In most real-world situations, people’s relaxed attitude to state is not an issue. Unfortunately, however, many programmers are quite vague about state too — and that is a problem. Niclas Nilsson 97-things-every-x-should-know.gitbooks.io/97-things-every-programmer-should-know/content/en/thing_84
Non-Empty Empty Dequeue Enqueue Dequeue [Length > 1] Enqueue Dequeue [Length == 1] new [Capacity > 0]
Non-Full Empty Dequeue Enqueue Dequeue [Length > 1] Enqueue Dequeue [Length == 1] new [Capacity > 0] Full Non-Empty [Length = Capacity] [Length < Capacity]
namespace Queue_spec  public class A_new_queue  public void is_empty()  public void preserves_positive_bounding_capacity()  public void fails_with_non_positive_bounding_capacity()  public class An_empty_queue  public void ignores_dequeuing_with_default_value()  public void becomes_non_empty_when_value_enqueued()  public class A_non_empty_queue  public class that_is_not_full  public void becomes_longer_when_value_enqueued()  public void becomes_full_when_enqueued_up_to_capacity()  public class that_is_full  public void ignores_further_enqueued_values()  public void becomes_non_full_when_dequeued()  public void dequeues_values_in_order_enqueued() 
namespace Queue_spec  public class A_new_queue  public void is_empty()  public void preserves_positive_bounding_capacity()  public void fails_with_non_positive_bounding_capacity()  public class An_empty_queue  public void ignores_dequeuing_with_default_value()  public void becomes_non_empty_when_value_enqueued()  public class A_non_empty_queue  public class that_is_not_full  public void becomes_longer_when_value_enqueued()  public void becomes_full_when_enqueued_up_to_capacity()  public class that_is_full  public void ignores_further_enqueued_values()  public void becomes_non_full_when_dequeued()  public void dequeues_values_in_order_enqueued() 
Given When Then
Given can be used to group tests for operations with respect to common initial state
When can be used to group tests by operation, differentiated by initial state or outcome
Then can be used to group tests by common outcome, regardless of initial state
I hope that’s been useful. You’re off to do revisit some tests? OK, catch you later. Kevlin Henney “Program with GUTs” medium.com/97-things/program-with-guts-828e69dd8e15

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In this document
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The presentation introduces the concept of 'GUTs' (Good Unit Tests), emphasizing the importance of writing effective unit tests.

Quotes from Alistair Cockburn on TDD and unit tests, underscoring the distinction between having tests and having 'good' tests.

Tests for leap year determination are shown, highlighting various assertions in testing practices and the need for detailed test cases.

Gerard Meszaros discusses the role of tests as documentation and clarity in defining test cases related to leap years.

Wisdom on effective testing practices, such as meaningful test names and understanding what types of tests were run.

Testing edge cases like unsupported numbers in leap year logic is discussed, stressing the need for exception handling.

Focus on structuring tests with clear conditions (Given, When, Then) to enhance both development and test comprehension.

Program with GUTs