Table Of ContentAdam Johnson
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Contents
1 Introduction 1
1.1 WhoIsThisBookFor?. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 ABriefTourofThisBook . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Toolbox 9
2.1 TestFramework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 CustomtestManagementCommand . . . . . . . . . . . . . . . . . . . 13
2.3 TestRunner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.4 TestSettings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.5 CustomTestCaseClasses . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.6 Third-PartyPackages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3 Measure! 27
3.1 Built-inOutput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 ShellTimingCommands . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3 MeasureIndividualTests . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4 Pro(cid:713)le . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4 EasyWins 46
4.1 UseaFasterPasswordHasher . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 AlwaysRebuildtheTestDatabaseifItExists . . . . . . . . . . . . . . 47
4.3 DisableDatabaseSerialization . . . . . . . . . . . . . . . . . . . . . . . 49
4.4 DisableInstrumentationPackagesduringTests . . . . . . . . . . . . . 50
4.5 UseanIn-MemoryFileStorageBackend . . . . . . . . . . . . . . . . . 52
4.6 UseanIn-MemoryCacheBackend . . . . . . . . . . . . . . . . . . . . 53
4.7 UseIn-MemoryBackendsforyourTaskQueues . . . . . . . . . . . . . 57
4.8 SkipSlowTestsLocally . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.9 PreventOutput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
i
4.10 PreventLogging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.11 ReducepytestTestCollection . . . . . . . . . . . . . . . . . . . . . . . 69
4.12 PreventWhiteNoiseFromScanningAllStaticFiles . . . . . . . . . . . 70
5 Upgrades 71
5.1 UpgradeDjango . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.2 UpgradePython . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3 UpgradeYourDatabaseServer . . . . . . . . . . . . . . . . . . . . . . . 77
6 Parallelize 79
6.1 WhatIsParallelTesting? . . . . . . . . . . . . . . . . . . . . . . . . . . 80
6.2 HowtoMoveToParallelTesting . . . . . . . . . . . . . . . . . . . . . . 80
6.3 CheckYourTestsAreIsolated . . . . . . . . . . . . . . . . . . . . . . . 81
6.4 ActivateParallelization . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.5 DealwithSharedResources . . . . . . . . . . . . . . . . . . . . . . . . 91
6.6 SplitUpLargeTestGroups . . . . . . . . . . . . . . . . . . . . . . . . . 97
7 Migrations 100
7.1 ReusetheTestDatabaseBetweenRuns . . . . . . . . . . . . . . . . . 101
7.2 SquashYourMigrations . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.3 Don’t DisableMigrationsinTests . . . . . . . . . . . . . . . . . . . . . 105
8 DatabaseCon(cid:713)guration 110
8.1 UseIn-MemoryStorage . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
8.2 Don’t SwapYourDatabasetoSQLiteinTests . . . . . . . . . . . . . . 115
9 CICon(cid:713)guration 117
9.1 Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
9.2 ScaleUp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
9.3 ParallelizeAcrossMultipleCIServers . . . . . . . . . . . . . . . . . . . 121
10 TestStructure 124
10.1 AAA:Arrange-Act-Assert . . . . . . . . . . . . . . . . . . . . . . . . . . 125
10.2 WriteMostlyUnitTestsNotIntegrationTests . . . . . . . . . . . . . . 129
10.3 UsetheRightTestCaseClass . . . . . . . . . . . . . . . . . . . . . . . 141
10.4 TestCaseTransactionBlockers. . . . . . . . . . . . . . . . . . . . . . . 144
11 TestData 151
11.1 AvoidFixtureFiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
11.2 AvoidCommonDatainCustomTestCaseClasses . . . . . . . . . . . . 153
11.3 UseFactories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
11.4 UsesetUpTestData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
11.5 OptimizeDatabaseQueries. . . . . . . . . . . . . . . . . . . . . . . . . 162
ii
11.6 AdvancedTestCaseFeatures . . . . . . . . . . . . . . . . . . . . . . . 163
12 TargetedMocking 166
12.1 TheFiveKindsofMockObjects . . . . . . . . . . . . . . . . . . . . . . 166
12.2 TheDangerWithMocking . . . . . . . . . . . . . . . . . . . . . . . . . 168
12.3 unittest.mock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
12.4 MockSettings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
12.5 MockOutput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
12.6 MockInput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
12.7 MockHTTPRequests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
12.8 MockTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
13 Outroduction 192
13.1 FurtherReading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
13.2 ThankYou . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
iii
Chapter 1
Introduction
The measure of success most often is speed. Doing things better is synony-
mouswithdoingthingsfastersothatwecandoevenmorethingsef(cid:713)ciently
andeffectively. Inbuyingintothispremise,weenteraspiralofacceleration
thatwecanneverhopetomaster.
—DianaScharfHuntandPamHait
Welcome,dearreader.
Oh,tests.
Tests are great. Tests show us where bugs are… at least some of them. Tests help us to
notaddnewbugs.
But…tests. Testsarealsoafrequentsourceoffrustration. Whenyou(cid:713)rststartedwriting
tests, you probably found them an extra nuisance to write. You probably still think the
same,atleastsometimes.
Evenifyoulovewritingtests,runningthemcanbeboring. Slowtestscanmakeyoulose
concentration,juggletasks,andendupgettinglessstuffdone. Theycansuckallthefun
1
SpeedUpYourDjangoTests,Release2021-07-08
outofprogramming. Andprogrammingismeant tobefun!
There’s a phase shift when you speed up your tests, restoring that joy. If your tests take
1000 seconds and you reduce them to 600 seconds, you’ll feel it. Your whole team will
feel it. The same happens if you can reduce a test run from 30 seconds down to 10
seconds.
And there’s no limit to “fast enough”. Reducing test run time is the easiest and safest
waytoincreaseyourspeedofdelivery. Andyourorganizationwantstodeliverasfastas
possible,togetvaluablefeaturesand(cid:713)xesinfrontofusers.
Fasttestsalsotendtobemoreaccuratetests. They’rewelltargeted,sotheyonlyfailwhen
afeatureisbroken. Andwithlessoverhead,youcanexercisefeaturesmorecompletely.
ThecontentisbasedonmyeightyearsofexperiencewithtestsonDjangoprojects. I’ve
spedupmanyprojects’testsuites,improvedDjango’sowntestingframework,andcreated
severalpytestplugins.
In your hands, or at least on your screen, is my best guide for speeding up tests on your
project. I’ve tried to cover most situations, so you should be able to (cid:713)nd something
relevanttoyourproject.
Mayitreduceyourtestspeedandyourtestpain. Enjoy!
—Adam
P.S.Pleasesendmeanyandallfeedbackthroughthecontactdetailsonmysite1.
1.1 Who Is This Book For?
IfyouareaDjangodeveloperworkingwithtests,thisisforyou. Nomatteryourlevel.
Myaimistomakethebookreadablebyajuniordeveloperwhohasbeengiventhevague
task of “improving the tests”, but also to include material that even long time Django
users may not have come across. I’ve tried to be opinionated enough to be actionable,
butprovideadvicethat’sapplicabletoallprojects.
1https://adamj.eu/contact/
2 Chapter1. Introduction
SpeedUpYourDjangoTests,Release2021-07-08
1.2 A Brief Tour of This Book
The(cid:713)rsttwochapters,Toolbox andMeasure!,explainthevarioustoolsyouhaveathand
formodifyingandmeasuringyourtestsrespectively.
Thenextsixchapterscoverwaysofspeedingupyourtestswithoutthebotherofrewrit-
ing your test code (much). Easy Wins covers a number of smaller, non-invasive changes
thatcanspeedupyourtests. Upgradesthenre-iteratestheimportanceofkeepingthings
upgraded, especially with respect to test performance. Parallelize covers using paral-
lelizedtesting,andthechallengesyoumightfaceaddingit. Migrationscoverstacticsfor
improving your work(cid:714)ow with database migrations. Finally, Database Con(cid:713)guration and
CICon(cid:713)gurationcoverwaysofspeedingupyourtestsincon(cid:713)gurationforthoseenviron-
ments.
The threechapters that follow coverhow towrite fast tests. Test Structure and TestData
go into depth on writing faster tests that use less data. And Targeted Mocking shows
some focussed techniques for replacing components during testing that can help you
writebetter,fastertests.
Finally,theOutroductiongivessomeclosingremarksandlinkstofurtherreading.
1.2.1 In a rush?
Togetthe80%ofimprovementsthattake20%oftheeffort:
• RecordyourcurrenttestruntimewithashelltimingcommandasinMeasure!.
• SkimEasyWinsforanychangesthatareapplicabletoyourprojectandmakethem.
When necessary, follow the references back to Toolbox to understand the pieces
you’rechanging.
• Ifyoudon’tyetrunyourtestsinparallel,readParallelize.
• LearnhowtoreuseyourdatabaseandsquashyourmigrationsinMigrations.
• DoasmanyoftheupgradesdescribedinUpgradesaspossible.
• Compareyourtestruntime!
If after this, your tests are still too slow, pro(cid:713)le them with one of the tools described in
thesecondhalfofMeasure!. Thenwhenyouknowwheretofocus,readthelaterchapters
todeterminewaysofspeedinguptheslowesttests.
1.2. ABriefTourofThisBook 3
SpeedUpYourDjangoTests,Release2021-07-08
1.3 Examples
Thisbookhasalotofcodeexamples. Ihopethatisnotasurprise!
Here’ssomeinformationontheirlayout.
1.3.1 Commands
Commandstoruninyourshelllooklikethis:
$ python --version
The$representstheprompt-don’ttypethat. I’musingmacOSbuthopetohaveremoved
any macOS or *nix bias. Commands should work cross-platform, but where that’s not
possibleI’veaddedaseparateWindowsexamplewithaPowerShellprompt:
> python --version
IfI’vewrittensomethingincompatiblewithyourplatform(mostlikelyWindows),please
letmeknow.
1.3.2 Versions
ExampleshavebeenpreparedwithPython3.9andDjango3.2. I’monlyusingtheof(cid:713)cial
Pythondistribution,CPython,sinceDjangooritsdependenciesoftendon’tworkonother
Pythoninterpreters.
Python code is formatted with Black2. (Like Django itself will be one day3.) Import
statementsaresortedandgroupedwithisort4. AndallcodeisalsolintedwithFlake85.
Throughout,Irefertoinstallationcommandswith:
$ python -m pip install Django
I’musingpython -m pipasperpipdevelopers’recommendations-seemyblogposton
thesubject6.
2https://black.readthedocs.io/en/stable/
3https://github.com/django/deps/blob/main/accepted/0008-black.rst
4https://pypi.org/project/isort/
5https://flake8.pycqa.org/en/latest/index.html
6https://adamj.eu/tech/2020/02/25/use-python-m-pip-everywhere/
4 Chapter1. Introduction
SpeedUpYourDjangoTests,Release2021-07-08
Please read these commands as “install using your dependency management solution”.
ThereareseveraltoolsforthisinthePythonecosystem,andIcan’tcoverthemall.
Ifyouaren’tusinganydependencymanagementtool(beyondpip),pip-compile7isagreat
startasit’sasmalllayerontopofpip. It’salsomypreference.
1.3.3 Django Projects
The example projects I’ve used are based on my “simple-core” startproject template8.
Thishasdirectorystructurelikethis:
$ tree
.
├── example
│ ├── __init__.py
│ ├── core
│ │ ├── __init__.py
│ │ ├── models.py
│ │ └── tests
│ │ ├── __init__.py
│ │ └── test_models.py
│ ├── settings.py
│ └── urls.py
└── manage.py
3 directories, 8 files
This is slightly different to Django’s normal startproject template. The whole project
livesinsideasinglePythonpackage,example. example.coreisanapp,insidewhichthere
are models and tests. This is counter to the Django default of having apps in modules
thatarepeerstothe“projectapp”.
Ipreferthislayoutbecauseitmakesitclearwhenyou’reimportingsomethingbelonging
to your project. Owned names always start with example (or whatever your project is
called).
7https://pypi.org/project/pip-tools/
8https://github.com/adamchainz/django-startproject-templates
1.3. Examples 5
