Testability for Developers

Alexander Tarnowski 1

Swenug 2014-04-23

Testablity for developers

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Why testability and testing for developers?

Unit testingMaintenanceContinuous integrationAutomated acceptance testing

Alexander Tarnowski

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But…

Alexander Tarnowski

Ongoing project

http://leanpub.com/developer_testing

alexander.tarnowski@crisp.se

alexander_tar

blog.crisp.se/author/alexandertarnowskiBlog

www

www.techbookreader.comBlog agilamyter.com

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Testability costs moneyM

isco

ncep

tion

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RequirementsSpecificati

onArchitectural design

Detaileddesign

Coding

Unit testing

Integration

testing

Systemtesting

Acceptance

testing

Mis

conc

epti

onTesting is a criticizing activity

that can build quality in

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Automated Tools

ManualAutomated& manual

Q1Q2 Q3

Q4

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Defining testability

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The Black Box

If this were our system…

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Defining testabilityDefinition 1: Testability = decoupling

Origin: The TDD community

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Defining testabilityDefinition 2: Testability = Controllability Observability

Origin: The testing community

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Defining testabilityDefinition 3: Testability = ControllabilityObservabilityOrthogonalityDeployability

Origin: the ”Developer Testing” book

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Know the driving forces behind testabilityThey are:ControllabilityObservabilityOrthogonalityDeployability

Make concious decisions to achieve them

Programmer wisdom

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Fundamental testing techniques

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Why developers should know about fundamental testing

techniquesPass tester testsIncorporate them in their own tests

Alexander TarnowskiImage: Stuart Miles/FreeDigitalPhotos.net

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Equivalence partitioning

0-17 18-23 24-60 60-100 101+

0

0.2

0.4

0.6

0.8

11.2

1.4

1.6

1.8

WomenMen

Insurance premiums

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Typical equivalence classes

0-10, 11-23, 24-100

Ranges

0, 3, 6, 9, 12, 15

Defined by functions

e.g. f(x) = x mod 3

Sharing a propertye.g. red and blue cars Induction

proofs

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Partition data into equivalence classes

Cover each partition with at least one test case

Programmer wisdom

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Boundary values

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Ages 24-60

22 23 24 25 26 58 59 60 61 62

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Common edge cases

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Numbers0, -1

Integer.MAX_VALUE/int.MaxValuen-1, n+1, m-1, m+1

Strings”” – empty string

nullMax input length

DatesPrecision

Days per monthDifferent locales

Leap years

Collections{} – empty collection

nullIndexing → iteration

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Boundary values and equivalence partitions drive the number of tests

Remember to check boundary valuesRemember to check common edge

cases

Programmer wisdom

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Use parameterized tests to capture boundary values

[TestCase(18, Gender.Male, Result = 1.75)][TestCase(23, Gender.Male, Result = 1.75)][TestCase(24, Gender.Male, Result = 1.0)]// ...public double VerifyPremiumFactor(int age, Gender gender){ return new PremiumRuleEngine().GetPremiumFactor(age, gender);}

Programmer wisdom

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State transition testing

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Start

Reset

Press min+sec

Setting timePress min

Press min

Press sec

Counting down

Press min+sec

Press start/stop

Press start/stop

Press min+sec

Press min+sec

Counting up

Press start/stop

Press min+sec

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Use for clarifying functionality and finding missing or incorrect transitions

Basis for model-based testing

Programmer wisdom

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Decision tables

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Age 18-23 18-23 24-59 24-59 60+ 60+

Gender Male Female

Male Female Male Female

Premium factor 1.0 N N N Y N N

Premium factor 1.05 N N Y N N N

Premium factor 1.25 N N N N N Y

Premium factor 1.35 N N N N Y N

Premium factor 1.65 N Y N N N N

Premium factor 1.75 Y N N N N N

Condition

Condition alternative

Action Action entry

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Decision tables

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Age 18-23 18-23 24-59 24-59 60+ 60+

Gender Male Female

Male Female Male Female

Premium factor 1.75 1.65 1.05 1 1.35 1.25

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Decision tables translate directly into parameterized/data-driven tests!

Programmer wisdom

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Anti-testability constructs

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Temporal coupling & state

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var car = new Car();10 LOCcar.Initialize();while(itsRaining) { 50 LOC}if (sky.StarsProperlyAligned()) { car.SetGear(1);}30 LOCcar.StartEngine();car.ReleaseClutch();

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Avoid partially initialized objectsKeep temporal coupling in mind

when designing messages and protocols

Limit state by keeping program elements small and short-lived

Programmer wisdom

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Indirect inputDirect input (e.g. pure functions):int DoubleValue(int i) { return i * 2;}

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DateTime.Now.Minute;new Calculator.Add(10, 10);using (StreamReader sr = new StreamReader(“Magic.txt")){ int.Parse(sr.ReadToEnd()); }

Indirect inputint DoMagic(int i) { return i +}

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Indirect input is natural and we shouldn’t fear it

To cope with indirect input we need control points and stubs

Programmer wisdom

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Indirect output

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Direct output (i.e. observable through the public API):int DoubleValue(int i) { return i * 2;}

Indirect outputbool DoMagic(int i) { <insert code here> return true;}

Console.Out.WriteLine(i * 42);RemoteEndpoint.Invoke(42, ”magic”, i);EventLog.WriteEntry(”MagicApp”, ”value:” + i);

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Indirect output is natural and we shouldn’t fear it

To cope with indirect output we need observation points and mock objects

Programmer wisdom

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Domain-to-range ratio

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Input values(domain, D)

Output values

(range, R)

Correct computation

Faulty computation

DRR = |D| / |R|

f(0) 0

f(1-9) 1

f(11-99) 2

f(100-999) 3

f(1000-9999) 4

….

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Compressionbool IsValidAge(int age)

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false

~4*109

true

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Use high DRR as a warning flagBe aware of information lossAvoid using too large data typesIntroduce abstractions (OO/DDD)

Programmer wisdom

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Dependencies

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Relations between objects

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public class ListWrapper{ private IList<int> wrapped; public int WrappedListSize { get { return wrapped.Count; } } public ListWrapper() { wrapped = new List<int> { 1, 2, 3 }; }} Indirect input

Legacy

code 101

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Pass in the dependency

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public class ListWrapper{ private IList<int> wrapped; public int WrappedListSize { get { return wrapped.Count; } } public ListWrapper(IList<int> data) { wrapped = data; }}

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Use factory method

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public class ListWrapper{ private IList<int> wrapped; public int WrappedListSize { get { return wrapped.Count; } } public ListWrapper() { wrapped = CreateWrappedList(); } protected virtual IList<int> CreateWrappedList() { return new List<int> { 1, 2, 3 }; }}

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Provide external factory or builder

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public class ListWrapper{ private IList<int> wrapped; public int WrappedListSize { get { return wrapped.Count; } } public ListWrapper(IntegerListBuilder builder) { wrapped = builder.Build(); } }

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public class IntegerListBuilder{ private int startingAt = 1; private int endingWith = 10; public IntegerListBuilder StartingAt(int startingAt) { this.startingAt = startingAt; return this; } public IntegerListBuilder EndingWith(int endingWith) { this.endingWith = endingWith; return this; } public IList<int> Build() { return Enumerable.Range(startingAt, endingWith).ToList(); }}

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Making code testable is about making dependencies explicit

Generic strategies for doing so:Passing them in Using factory methodsUsing factories or builders

Programmer wisdom

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System resource dependencies

FilesSystem clockSockets, etc

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SolutionsSolution #1: Your own abstractionpublic interface ITimeSource{ DateTime Now { get; }}

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Solution #2: Fakesusing (ShimsContext.Create()) { System.Fakes.ShimDateTime.NowGet = () => { return new DateTime(fixedYear, 1, 1); };

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There isn’t a single problem in computer science that cannot be solved by adding another layer of indirection. This is certainly true of system resoruce dependencies.

Use Fakes if there’s no other way. But first try without.

Programmer wisdom

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Dependencies between layers

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PresentationBusiness

Data access

Theoretical dependenciesActual

dependencies

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Clean up messy cross-layer dependencies

Use Dependency Inversion; depend on abstractions

Use Dependency Injection

Programmer wisdom

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Duplication

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?

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Copy and pastepublic class CustomerRepository{ public void Create(Customer customer) { if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue) { throw new ArgumentException("Incomplete customer"); } … }

public void Update(Customer customer) { if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue) { throw new ArgumentException("Incomplete customer"); } … }

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Block copy and paste

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public class CustomerRepository{ public void Create(Customer customer) { if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue) { throw new ArgumentException("Incomplete customer"); }

if (DateTime.Now.Year - customer.YearOfBirth.Value < 18) { throw new ArgumentOutOfRangeException("Underage customer"); } … }

public void Update(Customer customer) { if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue) { throw new ArgumentException("Incomplete customer"); }

if (DateTime.Now.Year - customer.YearOfBirth.Value < 18) { throw new ArgumentOutOfRangeException("Underage customer"); } … }

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Constructor copy and paste

public NetworkInterface(IPAddress ipAddress, NetMask netMask, IPAddress broadcast, IPAddress defaultRoute){ this.ipAddress = ipAddress; this.netMask = netMask; this.broadcast = broadcast; this.defaultRoute = defaultRoute;}

public NetworkInterface(IPAddress ipV6Address, NetMaskIpV6 ipV6NetMask, IPAddress ipV6DefaultRoute){ this.ipV6Address = ipV6Address; this.ipV6NetMask = ipV6NetMask; this.ipV6DefaultRoute = ipV6DefaultRoute;}

public NetworkInterface(IPAddress ipAddress, NetMask netMask, IPAddress broadcast, IPAddress defaultRoute,IPAddress ipV6Address, NetMaskIpV6 ipV6NetMask, IPAddress ipV6DefaultRoute)

{ this.ipAddress = ipAddress; this.netMask = netMask; this.broadcast = broadcast; this.defaultRoute = defaultRoute; this.ipV6Address = ipV6Address; this.ipV6NetMask = ipV6NetMask; this.ipV6DefaultRoute = ipV6DefaultRoute;}

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The same method in different contexts

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public static long DiffTime(DateTime t1, DateTime t2){ if (t1.Date != t2.Date) { throw new ArgumentException("Incomparable dates"); } return (t2.Hour - t1.Hour) * 60;}

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Divergent duplicated methods in different contexts

public static long DiffTime(DateTime t1, DateTime t2){ if (t1.Date != t2.Date) { throw new ArgumentException("Incomparable dates"); } return (t2.Hour * 60 + t2.Minute) - (t1.Hour * 60 + t1.Minute);}

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in 7/8 places…

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Code that’s been mechanically copied and pasted increases the size of the code base and is annoying…

… but the true danger lies in convergence

Programmer wisdom

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Different methods or classes doing similar things

Ignorance FearLaziness ChoiceConflict

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Different ways of doing similar things

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Module Alpha uses… Module Bravo uses…

Logging framework Apple Logging framework Banana

Hand-written SQL An O/R mapper

A date/time library Time computations implemented ”by hand”

Client-side validation Server-side validation

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Competing domain models

Alexander TarnowskiImage: vectorolie,Danilo Rizzuti/FreeDigitalPhotos.net

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Understand that mental duplication makes testing, understanding, and sometimes even recruiting harder

Have a long-term strategy for dealing with mental duplication

Duplication makes things ”hard”. If it’s hard then…

Programmer wisdom

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Test-driven development

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Refactor

Green

Red

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Properties of test-driven code

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Code driven

by tests

Executable in

isolation

Small

Does one thing

Indirect creation

Pure function

s separeted from side

effects

• Drives design, not correctness• Negative tests are not used• There are two kinds of TDD

Classic TDD London school

TDD and testability

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Supplement tests that drive design with tests that drive correctness!

Programmer wisdom

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Summary

Alexander Tarnowski

Testability is a skill

Developers do supporting technical testing

The Black Box

Testability: Controllability Observability Orthogonality Deployability

Testing techniques

Anti-testability constructs Temporal coupling Indirect input Indirect output High DRR

Duplication: Mechanical Mental TDD drives design,

not correctness!

Dependencies: Pass in Factory methods Factories/Builders Fakes