oop in python - What's the difference between a method and a function?





15 Answers

A method is on an object.
A function is independent of an object.

For Java, there are only methods.
For C, there are only functions.

For C++ it would depend on whether or not you're in a class.

with example javascript

Can someone provide a simple explanation of methods vs. functions in OOP context?




In general: methods are functions that belong to a class, functions can be on any other scope of the code so you could state that all methods are functions, but not all functions are methods:

Take the following python example:

class Door:
  def open(self):
    print 'hello stranger'

def knock_door:
  a_door = Door()
  Door.open(a_door)

knock_door()

The example given shows you a class called "Door" which has a method or action called "open", it is called a method because it was declared inside a class. There is another portion of code with "def" just below which defines a function, it is a function because it is not declared inside a class, this function calls the method we defined inside our class as you can see and finally the function is being called by itself.

As you can see you can call a function anywhere but if you want to call a method either you have to pass a new object of the same type as the class the method is declared (Class.method(object)) or you have to invoke the method inside the object (object.Method()), at least in python.

Think of methods as things only one entity can do, so if you have a Dog class it would make sense to have a bark function only inside that class and that would be a method, if you have also a Person class it could make sense to write a function "feed" for that doesn't belong to any class since both humans and dogs can be fed and you could call that a function since it does not belong to any class in particular.




If you feel like reading here is "My introduction to OO methods"

The idea behind Object Oriented paradigm is to "threat" the software is composed of .. well "objects". Objects in real world have properties, for instance if you have an Employee, the employee has a name, an employee id, a position, he belongs to a department etc. etc.

The object also know how to deal with its attributes and perform some operations on them. Let say if we want to know what an employee is doing right now we would ask him.

employe whatAreYouDoing.

That "whatAreYouDoing" is a "message" sent to the object. The object knows how to answer to that questions, it is said it has a "method" to resolve the question.

So, the way objects have to expose its behavior are called methods. Methods thus are the artifact object have to "do" something.

Other possible methods are

employee whatIsYourName
employee whatIsYourDepartmentsName

etc.

Functions in the other hand are ways a programming language has to compute some data, for instance you might have the function addValues( 8 , 8 ) that returns 16

// pseudo-code
function addValues( int x, int y )  return x + y 
// call it 
result = addValues( 8,8 )
print result // output is 16...

Since first popular programming languages ( such as fortran, c, pascal ) didn't cover the OO paradigm, they only call to these artifacts "functions".

for instance the previous function in C would be:

int addValues( int x, int y ) 
{
   return x + y;
}

It is not "natural" to say an object has a "function" to perform some action, because functions are more related to mathematical stuff while an Employee has little mathematic on it, but you can have methods that do exactly the same as functions, for instance in Java this would be the equivalent addValues function.

public static int addValues( int x, int y ) {
    return x + y;
}

Looks familiar? That´s because Java have its roots on C++ and C++ on C.

At the end is just a concept, in implementation they might look the same, but in the OO documentation these are called method.

Here´s an example of the previously Employee object in Java.

public class Employee {

    Department department;
    String name;

    public String whatsYourName(){
        return this.name;
    }
    public String whatsYourDeparmentsName(){
         return this.department.name();
    }
    public String whatAreYouDoing(){
        return "nothing";
    } 
    // Ignore the following, only set here for completness
    public Employee( String name ) {
        this.name = name;
    }

}

// Usage sample.
Employee employee = new Employee( "John" ); // Creates an employee called John

// If I want to display what is this employee doing I could use its methods.
// to know it.
String name = employee.whatIsYourName():
String doingWhat = employee.whatAreYouDoint();

// Print the info to the console.

 System.out.printf("Employee %s is doing: %s", name, doingWhat );

Output:
Employee John is doing nothing.

The difference then, is on the "domain" where it is applied.

AppleScript have the idea of "natural language" matphor , that at some point OO had. For instance Smalltalk. I hope it may be reasonable easier for you to understand methods in objects after reading this.

NOTE: The code is not to be compiled, just to serve as an example. Feel free to modify the post and add Python example.




In OO world, the two are commonly used to mean the same thing.

From a pure Math and CS perspective, a function will always return the same result when called with the same arguments ( f(x,y) = (x + y) ). A method on the other hand, is typically associated with an instance of a class. Again though, most modern OO languages no longer use the term "function" for the most part. Many static methods can be quite like functions, as they typically have no state (not always true).




A function is a mathematical concept. For example:

f(x,y) = sin(x) + cos(y)

says that function f() will return the sin of the first parameter added to the cosine of the second parameter. It's just math. As it happens sin() and cos() are also functions. A function has another property: all calls to a function with the same parameters, should return the same result.

A method, on the other hand, is a function that is related to an object in an object-oriented language. It has one implicit parameter: the object being acted upon (and it's state).

So, if you have an object Z with a method g(x), you might see the following:

Z.g(x) = sin(x) + cos(Z.y)

In this case, the parameter x is passed in, the same as in the function example earlier. However, the parameter to cos() is a value that lives inside the object Z. Z and the data that lives inside it (Z.y) are implicit parameters to Z's g() method.




Methods are functions of classes. In normal jargon, people interchange method and function all over. Basically you can think of them as the same thing (not sure if global functions are called methods).

http://en.wikipedia.org/wiki/Method_(computer_science)




Methods on a class act on the instance of the class, called the object.

class Example
{
   public int data = 0; // Each instance of Example holds its internal data. This is a "field", or "member variable".

   public void UpdateData() // .. and manipulates it (This is a method by the way)
   {
      data = data + 1;
   }

   public void PrintData() // This is also a method
   {
      Console.WriteLine(data);
   }
}

class Program
{
   public static void Main()
   {
       Example exampleObject1 = new Example();
       Example exampleObject2 = new Example();

       exampleObject1.UpdateData();
       exampleObject1.UpdateData();

       exampleObject2.UpdateData();

       exampleObject1.PrintData(); // Prints "2"
       exampleObject2.PrintData(); // Prints "1"
   }
}



Since you mentioned Python, the following might be a useful illustration of the relationship between methods and objects in most modern object-oriented languages. In a nutshell what they call a "method" is just a function that gets passed an extra argument (as other answers have pointed out), but Python makes that more explicit than most languages.

# perfectly normal function
def hello(greetee):
  print "Hello", greetee

# generalise a bit (still a function though)
def greet(greeting, greetee):
  print greeting, greetee

# hide the greeting behind a layer of abstraction (still a function!)
def greet_with_greeter(greeter, greetee):
  print greeter.greeting, greetee

# very simple class we can pass to greet_with_greeter
class Greeter(object):
  def __init__(self, greeting):
    self.greeting = greeting

  # while we're at it, here's a method that uses self.greeting...
  def greet(self, greetee):
    print self.greeting, greetee

# save an object of class Greeter for later
hello_greeter = Greeter("Hello")

# now all of the following print the same message
hello("World")
greet("Hello", "World")
greet_with_greeter(hello_greeter, "World")
hello_greeter.greet("World")

Now compare the function greet_with_greeter and the method greet: the only difference is the name of the first parameter (in the function I called it "greeter", in the method I called it "self"). So I can use the greet method in exactly the same way as I use the greet_with_greeter function (using the "dot" syntax to get at it, since I defined it inside a class):

Greeter.greet(hello_greeter, "World")

So I've effectively turned a method into a function. Can I turn a function into a method? Well, as Python lets you mess with classes after they're defined, let's try:

Greeter.greet2 = greet_with_greeter
hello_greeter.greet2("World")

Yes, the function greet_with_greeter is now also known as the method greet2. This shows the only real difference between a method and a function: when you call a method "on" an object by calling object.method(args), the language magically turns it into method(object, args).

(OO purists might argue a method is something different from a function, and if you get into advanced Python or Ruby - or Smalltalk! - you will start to see their point. Also some languages give methods special access to bits of an object. But the main conceptual difference is still the hidden extra parameter.)




Function is a set of logic that can be used to manipulate data. While, Method is function that is used to manipulate the data of the object where it belongs. So technically, if you have a function that is not completely related to your class but was declared in the class, its not a method; It's called a bad design.




From my understanding a method is any operation which can be performed on a class. It is a general term used in programming.

In many languages methods are represented by functions and subroutines. The main distinction that most languages use for these is that functions may return a value back to the caller and a subroutine may not. However many modern languages only have functions, but these can optionally not return any value.

For example, lets say you want to describe a cat and you would like that to be able to yawn. You would create a Cat class, with a Yawn method, which would most likely be a function without any return value.




IMHO people just wanted to invent new word for easier communication between programmers when they wanted to refer to functions inside objects.

If you are saying methods you mean functions inside the class. If you are saying functions you mean simply functions outside the class.

The truth is that both words are used to describe functions. Even if you used it wrongly nothing wrong happens. Both words describe well what you want to achieve in your code.

Function is a code that has to play a role (a function) of doing something. Method is a method to resolve the problem.

It does the same thing. It is the same thing. If you want to be super precise and go along with the convention you can call methods as the functions inside objects.




Let's not over complicate what should be a very simple answer. Methods and functions are the same thing. You call a function a function when it is outside of a class, and you call a function a method when it is written inside a class.




I know many others have already answered, but I found following is a simple, yet effective single line answer. Though it doesn't look a lot better than others answers here, but if you read it carefully, it has everything you need to know about the method vs function.

A method is a function that has a defined receiver, in OOP terms, a method is a function on an instance of an object.




Difference Between Methods and Functions

From reading this doc on Microsoft

Members that contain executable code are collectively known as the function members of a class. The preceding section describes methods, which are the primary kind of function members. This section describes the other kinds of function members supported by C#: constructors, properties, indexers, events, operators, and finalizers.

So methods are the subset of the functions. Every method is a function but not every function is a method, for example, a constructor can't be said as a method but it is a function.




A class is the collection of some data and function optionally with a constructor.

While you creating an instance (copy,replication) of that particular class the constructor initialize the class and return an object.

Now the class become object (without constructor) & Functions are known as method in the object context.

So basically

Class <==new==>Object

Function <==new==>Method

In java the it is generally told as that the constructor name same as class name but in real that constructor is like instance block and static block but with having a user define return type(i.e. Class type)

While the class can have an static block,instance block,constructor, function The object generally have only data & method.




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