Python tricks

297 Pages·2018·1.487 MB·English

by  Bader, Dan

Python Tricks: The Book
Dan Bader
For online informationand ordering ofthis and other books byDan
Bader,pleasevisitrealpython.com. Formoreinformation,pleasecon-
[email protected].
Copyright©DanBader(realpython.com),2016–2019
ISBN:9781775093305(paperback)
ISBN:9781775093312(electronic)
CoverdesignbyAnjaPircherDesign(anjapircher.com)
“Python” and the Python logos are trademarks or registered trade-
marks of the Python Software Foundation, used by Dan Bader with
permissionfromtheFoundation.
Thankyoufordownloadingthisebook. Thisebookislicensedforyour
personalenjoymentonly. Thisebookmaynotbere-soldorgivenaway
tootherpeople. Ifyouwouldliketosharethisbookwithanotherper-
son, please purchase an additional copy for each recipient. If you’re
reading this book and did not purchase it, or it was not purchased
foryouruseonly,thenpleasereturntorealpython.com/pytricks-book
andpurchaseyourowncopy. Thankyouforrespectingthehardwork
behindthisbook.
Updated 2019-09-05 I would like to thank Michael Howitz,
Johnathan Willitts, Julian Orbach, Johnny Giorgis, Bob White,
Daniel Meyer, Michael Stueben, Smital Desai, Andreas Kreisig,
David Perkins, Jay Prakash Singh, Ben Felder, Steve Schmerler,
MalcolmTelfer,andBoxiangSunfortheirexcellentfeedback.
WhatPythonistasSayAboutPythonTricks: TheBook
“I love, love, love the book. It’s like having a seasoned tutor explain-
ing,well,tricks! I’mlearningPythononthejobandI’mcomingfrom
Powershell, which I learned on the job—so lots of new, great stu(cid:1086).
WheneverIgetstuckinPython(usuallywithFlaskblueprintsorIfeel
likemycodecouldbemorePythonic)Ipostquestionsinourinternal
Pythonchatroom.
I’m often amazed at some of the answers co-workers give me. Dict
comprehensions, lambdas, and generators often pepper their feed-
back. Iamalwaysimpressedandyet(cid:1088)abbergastedathowpowerful
Python is when you know these tricks and can implement them
correctly.
YourbookwasexactlywhatIwantedtohelpgetmefromabewildered
Powershellscriptertosomeonewhoknowshowandwhentousethese
Pythonic‘tricks’everyonehasbeentalkingabout.
Assomeonewhodoesn’thavemydegreeinCSit’snicetohavethetext
toexplainthingsthatothersmighthavelearnedwhentheywereclas-
sicallyeducated. Iamreallyenjoyingthebookandamsubscribedto
theemailsaswell,whichishowIfoundoutaboutthebook.”
—DanielMeyer,Sr.DesktopAdministratoratTeslaInc.
“I (cid:1087)rst heard about your book from a co-worker who wanted to
trick me with your example of how dictionaries are built. I was
almost100%sureaboutthereasonwhytheendproductwasamuch
smaller/simpler dictionary but I must confess that I did not expect
theoutcome:)
HeshowedmethebookviavideoconferencingandIsortofskimmed
throughitashe(cid:1088)ippedthepagesforme,andIwasimmediatelycuri-
oustoreadmore.
ThatsameafternoonIpurchasedmyowncopyandproceededtoread
yourexplanationforthewaydictionariesarecreatedinPythonand
laterthatday,asImetadi(cid:1086)erentco-workerforco(cid:1086)ee,Iusedthesame
trickonhim:)
Hethensprungadi(cid:1086)erentquestiononthesameprinciple,andbecause
ofthewayyouexplainedthingsinyourbook,Iwasabletonotsimply
guess the result but to correctly answer what the outcome would be.
Thatmeansthatyoudidagreatjobatexplainingthings:)
IamnotnewinPythonandsomeoftheconceptsinsomeofthechap-
tersarenotnewtome, butImustsaythatIdogetsomethingoutof
every chapter so far, so kudos for writing a very nice book and for
doing a fantastic job at explaining concepts behind the tricks! I will
certainlyletmyfriendsandco-workersknowaboutyourbook.”
—OgMaciel,PythonDeveloperatRedHat
“I really enjoyed reading Dan’s book. He explains important Python
is ==
aspectswithclearexamples(usingtwotwincatstoexplain vs
forexample).
Itisnotjustcodesamples,itdiscussesrelevantimplementationdetails
comprehensibly. Whatreallymattersthoughisthatthisbookmakes
youwritebetterPythoncode!
ThebookisactuallyresponsibleforrecentnewgoodPythonhabitsI
picked up, for example: using custom exceptions and ABC’s (I found
Dan’sblogsearchingforabstractclasses.) Thesenewlearningsalone
areworththeprice.”
—BobBelderbos,EngineeratOracle&Co-FounderofPyBites
Contents
Contents 6
Foreword 9
1 Introduction 11
1.1 What’saPythonTrick? . . . . . . . . . . . . . . . . 11
1.2 WhatThisBookWillDoforYou . . . . . . . . . . . 13
1.3 HowtoReadThisBook . . . . . . . . . . . . . . . . 14
2 PatternsforCleanerPython 15
2.1 CoveringYourA**WithAssertions . . . . . . . . . . 16
2.2 ComplacentCommaPlacement . . . . . . . . . . . . 25
with
2.3 ContextManagersandthe Statement . . . . . . 29
2.4 Underscores,Dunders,andMore . . . . . . . . . . . 36
2.5 AShockingTruthAboutStringFormatting . . . . . . 48
2.6 “TheZenofPython”EasterEgg . . . . . . . . . . . . 56
3 E(cid:1087)ectiveFunctions 57
3.1 Python’sFunctionsAreFirst-Class . . . . . . . . . . 58
3.2 LambdasAreSingle-ExpressionFunctions . . . . . . 68
3.3 ThePowerofDecorators . . . . . . . . . . . . . . . 73
*args **kwargs
3.4 FunWith and . . . . . . . . . . . . 86
3.5 FunctionArgumentUnpacking . . . . . . . . . . . . 91
3.6 NothingtoReturnHere . . . . . . . . . . . . . . . . 94
6
Contents
4 Classes&OOP 97
4.1 ObjectComparisons: “is”vs“==” . . . . . . . . . . . 98
__repr__
4.2 StringConversion(EveryClassNeedsa ) . 101
4.3 DefiningYourOwnExceptionClasses . . . . . . . . 111
4.4 CloningObjectsforFunandProfit . . . . . . . . . . 116
4.5 AbstractBaseClassesKeepInheritanceinCheck . . . 124
4.6 WhatNamedtuplesAreGoodFor . . . . . . . . . . . 128
4.7 ClassvsInstanceVariablePitfalls . . . . . . . . . . . 136
4.8 Instance,Class,andStaticMethodsDemystified . . . 142
5 CommonDataStructuresinPython 152
5.1 Dictionaries,Maps,andHashtables . . . . . . . . . 155
5.2 ArrayDataStructures . . . . . . . . . . . . . . . . . 162
5.3 Records,Structs,andDataTransferObjects . . . . . 172
5.4 SetsandMultisets . . . . . . . . . . . . . . . . . . . 184
5.5 Stacks(LIFOs) . . . . . . . . . . . . . . . . . . . . 188
5.6 Queues(FIFOs) . . . . . . . . . . . . . . . . . . . . 194
5.7 PriorityQueues . . . . . . . . . . . . . . . . . . . . 200
6 Looping&Iteration 204
6.1 WritingPythonicLoops . . . . . . . . . . . . . . . . 205
6.2 ComprehendingComprehensions . . . . . . . . . . 209
6.3 ListSlicingTricksandtheSushiOperator . . . . . . 213
6.4 BeautifulIterators . . . . . . . . . . . . . . . . . . . 217
6.5 GeneratorsAreSimplifiedIterators . . . . . . . . . . 230
6.6 GeneratorExpressions . . . . . . . . . . . . . . . . 237
6.7 IteratorChains . . . . . . . . . . . . . . . . . . . . 244
7 DictionaryTricks 248
7.1 DictionaryDefaultValues . . . . . . . . . . . . . . . 249
7.2 SortingDictionariesforFunandProfit . . . . . . . . 253
7.3 EmulatingSwitch/CaseStatementsWithDicts . . . . 257
7.4 TheCraziestDictExpressionintheWest . . . . . . . 262
7.5 SoManyWaystoMergeDictionaries . . . . . . . . . 269
7.6 DictionaryPretty-Printing . . . . . . . . . . . . . . 272
7
Contents
8 PythonicProductivityTechniques 275
8.1 ExploringPythonModulesandObjects . . . . . . . . 276
8.2 IsolatingProjectDependenciesWithVirtualenv . . . 280
8.3 PeekingBehindtheBytecodeCurtain . . . . . . . . . 285
9 ClosingThoughts 290
9.1 FreeWeeklyTipsforPythonDevelopers . . . . . . . 292
9.2 RealPythonVideoCourseLibrary . . . . . . . . . . 293
9.3 PythonistaCafe: ACommunityforPythonDevelopers 294
8
Foreword
It’sbeenalmosttenyearssinceIfirstgotacquaintedwithPythonasa
programminglanguage. WhenIfirstlearnedPythonmanyyearsago,
itwaswithalittlereluctance. Ihadbeenprogramminginadifferent
language before, and all of the sudden at work, I was assigned to a
differentteamwhereeveryoneusedPython. Thatwasthebeginning
ofmyownPythonjourney.
WhenIwasfirstintroducedtoPython,Iwastoldthatitwasgoingto
beeasy,thatIshouldbeabletopickitupquickly. WhenIaskedmy
colleagues for resources for learning Python, all they gave me was a
link to Python’s official documentation. Reading the documentation
wasconfusingatfirst,anditreallytookmeawhilebeforeIevenfelt
comfortable navigating through it. Often I found myself needing to
lookforanswersinStackOverflow.
Comingfromadifferentprogramminglanguage,Iwasn’tlookingfor
just any resource for learning how to program or what classes and
objectsare. Iwaslookingforspecificresourcesthatwouldteachme
thefeaturesofPython,whatsetsitapart,andhowwritinginPython
isdifferentthanwritingcodeinanotherlanguage.
Itreallyhastakenmemanyyearstofullyappreciatethislanguage. As
IreadDan’sbook,IkeptthinkingthatIwishedIhadaccesstoabook
likethiswhenIstartedlearningPythonmanyyearsago.
Forexample,oneofthemanyuniquePythonfeaturesthatsurprised
me at first were list comprehensions. As Dan mentions in the book,
atellofsomeonewhojustcametoPythonfromadifferentlanguage
9