Classes vs. Functions [closed]

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.

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.

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

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.