ObjectCalisthenics9stepstobetterOOcode

Agenda

Learnhowtomakeourcodemore:

readablereusabletestablemaintainable

Raiseyouhandifyouknowoneofthefollowing:

DRYKISSSOLIDYAGNIZenofPython

Calisthenics

Cal•is•then•ics-/ˌkaləsˈTHeniks/

"Calisthenicsareexercisesconsistingofavarietyofgrossmotormovements;often

rhythmicalandgenerallywithoutequipmentorapparatus."

Wikipedia

ObjectCalisthenics

JeffBay

WrittenforJava

Whybother?

Codeisreadmorethanit'swritten

Authorunknown

Youneedtowritecodethatminimizesthetimeitwould

takesomeoneelsetounderstandit-evenifthat

someoneelseisyou

ArtofReadableCodebyDustinBoswell,TrevorFoucher

Rule#1

Onlyonelevelofindentationpermethod

classBoard(object):def__init__(self,data):#Level0self.buf=""foriinrange(10):#Level1forjinrange(10):#Level2self.buf+=data[i][j]

classBoard(object):def__init__(self,data):self.buf=""self.collect_rows(data)defcollect_rows(self,data):foriinrange(10):self.collect_row(data[i])defcollect_row(self,row):forjinrange(10):self.buf+=row[j]

products=self.get_products_by_user(...)ifproductsisNone:products=self.get_products_by_media(...)ifproductsisNone:products=self.get_products_by_domain(...)ifproductsisNone:products=self.get_any_products(...):ifproductsisNone:raiseException('Accessdenied')else:...else:...else:...else:...

Chainofcommand

Benefits

SingleresponsibilityBetternamingShortermethodsReusablemethods

Rule#2

Donotuseelsekeyword

ifoptions.get_categories()isNone:...eliflen(options.get_categories())==1:...elifSPECIAL_CATEGORYinoptions.get_categories():...elifoptions.get_categories()andoptions.get_query():...elifoptions.get_content_type():...

deflogin(self,request):ifrequest.user.is_authenticated():returnredirect("homepage")else:messages.add_message(request,messages.INFO,'Badcredentials')returnredirect("login")

deflogin(self,request):ifrequest.user.is_authenticated():returnredirect("homepage")

messages.add_message(request,messages.INFO,'Badcredentials')returnredirect("login")

deffunction(param):ifparamisnotNone:value=paramelse:value="default"

returnvalue

deffunction(param):value="default"ifparamisnotNone:value=param

returnvalue

Extractcode

Defaultvalue

Polymorphism

Strategypattern

Statepattern

Examples

https://github.com/gennad/Design-Patterns-in-Python

https://www.quora.com/Is-it-true-that-a-good-programmer-uses-fewer-if-conditions-than-an-amateur

BenefitsAvoidscodeduplicationLowercomplexityReadability

Rule#3

Wrapprimitivetypesifithasbehaviour

ValueObjectinDDD

classValidator(object):defcheck_date(self,year,month,day):pass

#10thofDecemberor12thofOctober?validator=Validator()validator.check_date(2016,10,12)

classValidator(object):defcheck_date(self,year:Year,month:Month,day:Day)->bool:pass

#Functioncallleavesnodoubt.validator.check_date(Year(2016),Month(10),Day(12))

BenefitsEncapsulationTypehintingAttractssimilarbehaviour

Rule#4

Onlyonedotperline

OK:Fluentinterface

classPoem(object):def__init__(self,content):self.content=content

defindent(self,spaces):self.content=""*spaces+self.content

returnself

defsuffix(self,content):self.content=self.content+"-"+content

returnself

Poem("RoadNotTravelled").indent(4)\.suffix("RobertFrost")\.content

NotOK:getterchain

classCartService(object):defget_token(self):token=self.get_service('auth')\.auth_user('user','password')\.get_result()\.get_token()

returntoken

#1.WhatifNoneisreturnedinsteadofobject?#2.Howaboutexceptionshandling?

classField(object):def__init__(self):self.current=Piece()

classPiece(object):def__init__(self):self.representation=""

classBoard(object):defboard_representation(self,board):buf=''forfieldinboard:buf+=field.current.representation

returnbuf

classField(object):def__init__(self):self.current=Piece()defadd_to(self,buffer):returnself.current.add_to(buffer)

classPiece(object):def__init__(self):self.representation=""defadd_to(self,buffer):returnbuffer+self.representation

classBoard(object):defboard_representation(self,board):buf=''forfieldinboard:buf=field.add_to(buf)

returnbuf

BenefitsEncapsulationDemeter'slawOpen/ClosedPrinciple

Rule#5

Donotabbreviate

Whyabbreviate?

Toomanyresponsibilities

Nametoolong?

Split&extract

Duplicatedcode?

Refactor!

classOrder(object):defship_order(self):pass

order=Order()order.ship_order()

//vs

classOrder(object):defship(self):pass

order=Order()order.ship()

acc=0//accumulator?accuracy?

pos=100//position?pointofsale?positive?

auth=None//authentication?authorization?both?

BenefitsClearintentionsIndicateunderlyingproblems

Rule#6

Keepyourclassessmall

Whatissmallclass?15-20linespermethod50linesperclass10classespermodule

BenefitsSingleResponsibilitySmallermodules

Rule#7

Nomorethan2instancevariableperclass

Classshouldhandlesinglevariablestate

Insomecasesitmightbetwovariables

classCartService(object):def__init__(self):self.logger=Logger()self.cart=CartCollection()self.translationService=TranslationService()self.auth_service=AuthService()self.user_service=UserService()

BenefitsHighcohesionEncapsulationFewerdependencies

Rule#8

Firstclasscollections

collectionsmodule

BenefitsSingleResponsibility

Rule#9

Donotusesetters/getters

Accessorsarefine

Don'tmakedecisionsoutsideofclass

Letclassdoit'sjob

Tell,don'task

classGame(object):def__init__(self):self.score=0

defset_score(self,score):self.score=score

defget_score(self):returnself.score

#UsageENEMY_DESTROYED_SCORE=10game=Game()game.set_score(game.get_score()+ENEMY_DESTROYED_SCORE)

classGame(object):def__init__(self):self.score=0defadd_score(self,score):self.score+=score

#UsageENEMY_DESTROYED_SCORE=10game=Game()game.add_score(ENEMY_DESTROYED_SCORE)

BenefitsOpen/ClosedPrinciple

Catch'emall!

Catch'emall!1. Onlyonelevelofindentationpermethod,2. Donotuseelsekeyword,3. Wrapprimitivetypesifithasbehavior,4. Onlyonedotperline,5. Don’tabbreviate,6. Keepyourentitiessmall,7. Nomorethantwoinstancevariableperclass,8. FirstClassCollections,9. Donotuseaccessors

Catch'emall!1. Onlyonelevelofindentationpermethod,2. Donotuseelsekeyword,3. Wrapprimitivetypesifithasbehavior,4. Onlyonedotperline,5. Don’tabbreviate,6. Keepyourentitiessmall,7. Nomorethantwoinstancevariableperclass,8. FirstClassCollections,9. Donotuseaccessors10. ???11. PROFIT!

Homework

Createnewprojectupto1000lineslong

Applypresentedrulesasstrictlyaspossible

Drawyourownconculsions

Customizetheserules

Finalthoughts

Thesearenotbestpractices

Thesearejustguidelines

Usewithcaution!

Questions?

Links

https://en.wikipedia.org/wiki/Calisthenicshttps://www.cs.helsinki.fi/u/luontola/tdd-2009/ext/ObjectCalisthenics.pdfhttps://pragprog.com/book/twa/thoughtworks-anthologyhttps://github.com/gennad/Design-Patterns-in-Pythonhttps://en.wikipedia.org/wiki/Law_of_Demeterhttps://www.quora.com/Is-it-true-that-a-good-programmer-uses-fewer-if-conditions-than-an-amateur

Thankyou!