Classes vs Functions

Question

Functions are easy to understand even for someone without any programming experience  but with  a fair math background  On the other hand  classes seem to be more difficult to grasp   Let s say I want to make a class function that calculates the age of a person given his her birthday year and the current year  Should I create a class for this  or a function  Or is the choice dependant to the scenario   P S  I am working on Python  but I guess the question is generic

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Classes  or rather their instances  are for representing things  Classes are used to define the operations supported by a particular class of objects  its instances   If your application needs to keep track of people  then Person is probably a class  the instances of this class represent particular people you are tracking   Functions are for calculating things  They receive inputs and produce an output and or have effects   Classes and functions aren t really alternatives  as they re not for the same things  It doesn t really make sense to consider making a class to  calculate the age of a person given his her birthday year and the current year   You may or may not have classes to represent any of the concepts of Person  Age  Year   and or Birthday  But even if Age is a class  it shouldn t be thought of as calculating a person s age  rather the calculation of a person s age results in an instance of the Age class   If you are modelling people in your application and you have a Person class  it may make sense to make the age calculation be a method of the Person class  A method is basically a function which is defined as part of a class  this is how you  define the operations supported by a particular class of objects  as I mentioned earlier   So you could create a method on your person class for calculating the age of the person  it would probably retrieve the birthday year from the person object and receive the current year as a parameter   But the calculation is still done by a function  just a function that happens to be a method on a class    Or you could simply create a stand-alone function that receives arguments  either a person object from which to retrieve a birth year  or simply the birth year itself   As you note  this is much simpler if you don t already have a class where this method naturally belongs  You should never create a class simply to hold an operation  if that s all there is to the class then the operation should just be a stand-alone function

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Before answering your question   If you do not have a Person class  first you must consider whether you want to create a Person class  Do you plan to reuse the concept of a Person very often  If so  you should create a Person class   You have access to this data in the form of a passed-in variable and you don t care about being messy and sloppy    To answer your question   You have access to their birthyear  so in that case you likely have a Person class with a someperson birthdate field  In that case  you have to ask yourself  is someperson age a value that is reusable   The answer is yes  We often care about age more than the birthdate  so if the birthdate is a field  age should definitely be a derived field   A case where we would not do this  if we were calculating values like someperson chanceIsFemale or someperson positionToDisplayInGrid or other irrelevant values  we would not extend the Person class  you just ask yourself   Would another program care about the fields I am thinking of extending the class with   The answer to that question will determine if you extend the original class  or make a function  or your own class like PersonAnalysisData or something

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i know it is a controversial topic  and likely i get burned now  but here are my thoughts   For myself i figured that it is best to avoid classes as long as possible  If i need a complex datatype I use simple struct  C C     dict  python   JSON  js   or similar  i e  no constructor  no class methods  no operator overloading  no inheritance  etc  When using class  you can get carried away by OOP itself  What Design pattern  what should be private  bla bla   and loose focus on the essential stuff you wanted to code in the first place   If your project grows big and messy  then OOP starts to make sense because some sort of helicopter-view system architecture is needed   function vs class  also depends on the task ahead of you   function   purpose  process data  manipulate data  create result sets   when to use  always code a function if you want to do this     y f x       struct dict json etc  instead of class    purpose  store attr  param   maintain attr  param   reuse attr  param   use attr  param  later  when to use  if you deal with a set of attributes params  preferably not mutable  different languages same thing  struct  C C     JSON  js   dict  python   etc  always prefer simple struct dict json etc over complicated classes  keep it simple     class  if it is a new data type    a simple perspective  is a struct  C   dict  python   json  js   etc  with methods attached  The method should only make sense in combination with the data param stored in the class  my advice  never code complex stuff inside class methods  call an external function instead  warning  do not misuse classes as fake namespace for functions   this happens very often   other use cases  if you want to do a lot of operator overloading then use classes  e g  your own matrix vector multiplication class  ask yourself  is it really a new    data type      Yes    class   No    can you avoid using a class     array vector list  to store a lot of data    purpose  store a lot of homogeneous data of the same data type  e g  time series advice 1  just use what your programming language already have  do not reinvent it  advice 2  if you really want your    class mysupercooldatacontainer     then  overload an existing array vector list etc class  e g     class mycontainer   public std  vector          enum  enum class    i just mention it  advice 1  use enum plus switch-case instead of overcomplicated OOP design patterns advice 2  use finite state machines

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Never create classes  At least the OOP kind of classes in Python being discussed   Consider this simplistic class   class Person object       def   init   self  id  name  city  account balance           self id   id         self name   name         self city   city         self account balance   account balance      def adjust balance self  offset           self account balance    offset   if   name         main         p   Person 123   bob    boston   100 0      p adjust balance 50 0      print  done       format p   dict       vs this namedtuple version   from collections import namedtuple  Person   namedtuple  Person     id    name    city    account balance      def adjust balance person  offset       return person  replace account balance person account balance   offset    if   name         main         p   Person 123   bob    boston   100 0      p   adjust balance p  50 0      print  done       format p     The namedtuple approach is better because    namedtuples have more concise syntax and standard usage  In terms of understanding existing code  namedtuples are basically effortless to understand  Classes are more complex  And classes can get very complex for humans to read  namedtuples are immutable  Managing mutable state adds unnecessary complexity  class inheritance adds complexity  and hides complexity    I can t see a single advantage to using OOP classes  Obviously  if you are used to OOP  or you have to interface with code that requires classes like Django   BTW  most other languages have some record type feature like namedtuples  Scala  for example  has case classes  This logic applies equally there

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Create a function  Functions do specific things  classes are specific things   Classes often have methods  which are functions that are associated with a particular class  and do things associated with the thing that the class is - but if all you want is to do something  a function is all you need   Essentially  a class is a way of grouping functions  as methods  and data  as properties  into a logical unit revolving around a certain kind of thing  If you don t need that grouping  there s no need to make a class

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Like what Amber says in her answer  create a function  In fact when you don t have to make classes if you have something like   class Person object       def   init   self  arg1  arg2           self arg1   arg1         self arg2   arg2      def compute self  other               Example of bad class design  don t care about the result             return self arg1   self arg2   other   Here you just have a function encapsulate in a class  This just make the code less readable and less efficient  In fact the function compute can be written just like this   def compute arg1  arg2  other        return arg1   arg2   other   You should use classes only if you have more than 1 function to it and if keep a internal state  with attributes  has sense  Otherwise  if you want to regroup functions  just create a module in a new  py file   You might look this video  Youtube  about 30min   which explains my point  Jack Diederich shows why classes are evil in that case and why it s such a bad design  especially in things like API  It s quite a long video but it s a must see

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It depends on the scenario  If you only want to compute the age of a person  then use a function since you want to implement a single specific behaviour   But if you want to create an object  that contains the date of birth of a person  and possibly other data   allows to modify it  then computing the age could be one of many operations related to the person and it would be sensible to use a class instead   Classes provide a way to merge together some data and related operations  If you have only one operation on the data then using a function and passing the data as argument you will obtain an equivalent behaviour  with less complex code   Note that a class of the kind   class A object       def   init   self                 initialize     def a single method self                 do stuff   isn t really a class  it is only a  complicated function  A legitimate class should always have at least two methods without counting   init

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I m going to break from the herd on this one and provide an alternate point of view    Never create classes    Reliance on classes has a significant tendency to cause coders to create bloated and slow code  Classes getting passed around  since they re objects  take a lot more computational power than calling a function and passing a string or two  Proper naming conventions on functions can do pretty much everything creating a class can do  and with only a fraction of the overhead and better code readability    That doesn t mean you shouldn t learn to understand classes though  If you re coding with others  people will use them all the time and you ll need to know how to juggle those classes  Writing your code to rely on functions means the code will be smaller  faster  and more readable  I ve seen huge sites written using only functions that were snappy and quick  and I ve seen tiny sites that had minimal functionality that relied heavily on classes and broke constantly   When you have classes extending classes that contain classes as part of their classes  you know you ve lost all semblance of easy maintainability    When it comes down to it  all data you re going to want to pass can easily be handled by the existing datatypes    Classes were created as a mental crutch and provide no actual extra functionality  and the overly-complicated code they have a tendency to create defeats the point of that crutch in the long run

[class] Classes vs. Functions

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