Can someone provide a simple explanation of methods vs. functions in OOP context?
A function is a piece of code that is called by name. It can be passed data to operate on (i.e. the parameters) and can optionally return data (the return value). All data that is passed to a function is explicitly passed.
A method is a piece of code that is called by a name that is associated with an object. In most respects it is identical to a function except for two key differences:
A method is implicitly passed the object on which it was called.
A method is able to operate on data that is contained within the class (remembering that an object is an instance of a class - the class is the definition, the object is an instance of that data).
(this is a simplified explanation, ignoring issues of scope etc.)
A method is on an object or is static in class.
A function is independent of any object (and outside of any class).
For Java and C#, there are only methods.
For C, there are only functions.
For C++ and Python it would depend on whether or not you're in a class.
But in basic English:
Function: Standalone feature or functionality.
Method: One way of doing something, which has different approaches or methods, but related to the same aspect (aka class).
'method' is the object-oriented word for 'function'. That's pretty much all there is to it (ie., no real difference).
Unfortunately, I think a lot of the answers here are perpetuating or advancing the idea that there's some complex, meaningful difference.
Really - there isn't all that much to it, just different words for the same thing.
[late addition]
In fact, as Brian Neal pointed out in a comment to this question, the C++ standard never uses the term 'method' when refering to member functions. Some people may take that as an indication that C++ isn't really an object-oriented language; however, I prefer to take it as an indication that a pretty smart group of people didn't think there was a particularly strong reason to use a different term.
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.
Simple way to remember:
Function → Free (Free means it can be anywhere, no need to be in an object or class)
Method → Member (A member of an object or class)
A very general definition of the main difference between a Function and a Method:
Functions are defined outside of classes, while Methods are defined inside of and part of classes.
The idea behind Object Oriented paradigm is to "treat" 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).
Let's say a function is a block of code (usually with its own scope, and sometimes with its own closure) that may receive some arguments and may also return a result.
A method is a function that is owned by an object (in some object oriented systems, it is more correct to say it is owned by a class). Being "owned" by a object/class means that you refer to the method through the object/class; for example, in Java if you want to invoke a method "open()" owned by an object "door" you need to write "door.open()".
Usually methods also gain some extra attributes describing their behaviour within the object/class, for example: visibility (related to the object oriented concept of encapsulation) which defines from which objects (or classes) the method can be invoked.
In many object oriented languages, all "functions" belong to some object (or class) and so in these languages there are no functions that are not methods.
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)
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.
Historically, there may have been a subtle difference with a "method" being something which does not return a value, and a "function" one which does.Each language has its own lexicon of terms with special meaning.
In "C", the word "function" means a program routine.
In Java, the term "function" does not have any special meaning. Whereas "method" means one of the routines that forms the implementation of a class.
In C# that would translate as:
public void DoSomething() {} // method
public int DoSomethingAndReturnMeANumber(){} // function
But really, I re-iterate that there is really no difference in the 2 concepts.
If you use the term "function" in informal discussions about Java, people will assume you meant "method" and carry on. Don't use it in proper documents or presentations about Java, or you will look silly.
Function or a method is a named callable piece of code which performs some operations and optionally returns a value.
In C language the term function is used. Java & C# people would say it a method (and a function in this case is defined within a class/object).
A C++ programmer might call it a function or sometimes method (depending on if they are writing procedural style c++ code or are doing object oriented way of C++, also a C/C++ only programmer would likely call it a function because term 'method' is less often used in C/C++ literature).
You use a function by just calling it's name like,
result = mySum(num1, num2);
You would call a method by referencing its object first like,
result = MyCalc.mySum(num1,num2);
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.
In OO languages such as Object Pascal or C++, a "method" is a function associated with an object. So, for example, a "Dog" object might have a "bark" function and this would be considered a "Method". In contrast, the "StrLen" function stands alone (it provides the length of a string provided as an argument). It is thus just a "function." Javascript is technically Object Oriented as well but faces many limitations compared to a full-blown language like C++, C# or Pascal. Nonetheless, the distinction should still hold.
A couple of additional facts: C# is fully object oriented so you cannot create standalone "functions." In C# every function is bound to an object and is thus, technically, a "method." The kicker is that few people in C# refer to them as "methods" - they just use the term "functions" because there isn't any real distinction to be made.
Finally - just so any Pascal gurus don't jump on me here - Pascal also differentiates between "functions" (which return a value) and "procedures" which do not. C# does not make this distinction explicitly although you can, of course, choose to return a value or not.
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.)
for me:
the function of a method and a function is the same if I agree that:
a function may return a value
may expect parameters
Just like any piece of code you may have objects you put in and you may have an object that comes as a result. During doing that they might change the state of an object but that would not change their basic functioning for me.
There might be a definition differencing in calling functions of objects or other codes. But isn't that something for a verbal differenciations and that's why people interchange them? The mentions example of computation I would be careful with. because I hire employes to do my calculations:
new Employer().calculateSum( 8, 8 );
By doing it that way I can rely on an employer being responsible for calculations. If he wants more money I free him and let the carbage collector's function of disposing unused employees do the rest and get a new employee.
Even arguing that a method is an objects function and a function is unconnected computation will not help me. The function descriptor itself and ideally the function's documentation will tell me what it needs and what it may return. The rest, like manipulating some object's state is not really transparent to me. I do expect both functions and methods to deliver and manipulate what they claim to without needing to know in detail how they do it.
Even a pure computational function might change the console's state or append to a logfile.
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.
To a first order approximation, a method (in C++ style OO) is another word for a member function, that is a function that is part of a class.
In languages like C/C++ you can have functions which are not members of a class; you don't call a function not associated with a class a method.
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.
Function is the concept mainly belonging to Procedure oriented programming where a function is an an entity which can process data and returns you value
Method is the concept of Object Oriented programming where a method is a member of a class which mostly does processing on the class members.
I am not an expert, but this is what I know:
Function is C language term, it refers to a piece of code and the function name will be the identifier to use this function.
Method is the OO term, typically it has a this pointer in the function parameter. You can not invoke this piece of code like C, you need to use object to invoke it.
The invoke methods are also different. Here invoke meaning to find the address of this piece of code. C/C++, the linking time will use the function symbol to locate.
Objecive-C is different. Invoke meaning a C function to use data structure to find the address. It means everything is known at run time.
TL;DR
A Function is a piece of code to run.
A Method is a Function inside an Object.
Example of a function:
function sum(){
console.log("sum")l
}
Example of a Method:
const obj = {
a:1,
b:2,
sum(){
}
}
So thats why we say that a "this" keyword inside a Function is not very useful unless we use it with call, apply or bind .. because call, apply, bind will call that function as a method inside object ==> basically it converts function to method
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.
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.
Function - A function in an independent piece of code which includes some logic and must be called independently and are defined outside of class.
Method - A method is an independent piece of code which is called in reference to some object and are be defined inside the class.
General answer is:
method has object context (this, or class instance reference),
function has none context (null, or global, or static).
But answer to question is dependent on terminology of language you use.
In JavaScript (ES 6) you are free to customising function context (this) for any you desire, which is normally must be link to the (this) object instance context.
In Java world you always hear that "only OOP classes/objects, no functions", but if you watch in detailes to static methods in Java, they are really in global/null context (or context of classes, whithout instancing), so just functions whithout object. Java teachers could told you, that functions were rudiment of C in C++ and dropped in Java, but they told you it for simplification of history and avoiding unnecessary questions of newbies. If you see at Java after 7 version, you can find many elements of pure function programming (even not from C, but from older 1988 Lisp) for simplifying parallel computing, and it is not OOP classes style.
In C++ and D world things are stronger, and you have separated functions and objects with methods and fields. But in practice, you again see functions without this and methods whith this (with object context).
In FreePascal/Lazarus and Borland Pascal/Delphi things about separation terms of functions and objects (variables and fields) are usually similar to C++.
Objective-C comes from C world, so you must separate C functions and Objective-C objects with methods addon.
C# is very similar to Java, but has many C++ advantages.
In C++, sometimes, method is used to reflect the notion of member function of a class. However, recently I found a statement in the book «The C++ Programming Language 4th Edition», on page 586 "Derived Classes"
A virtual function is sometimes called a method.
This is a little bit confusing, but he said sometimes, so it roughly makes sense, C++ creator tends to see methods as functions can be invoked on objects and can behave polymorphic.
In Python 2, it was possible to convert arbitrary callables to methods of a class. Importantly, if the callable was a CPython built-in implemented in C, you could use this to make methods of user-defined classes that were C layer themselves, invoking no byte code when called.
This is occasionally useful if you're relying on the GIL to provide "lock-free" synchronization; since the GIL can only be swapped out between op codes, if all the steps in a particular part of your code can be pushed to C, you can make it behave atomically.
In Python 2, you could do something like this:
import types
from operator import attrgetter
class Foo(object):
... This class maintains a member named length storing the length...
def __len__(self):
return self.length # We don't want this, because we're trying to push all work to C
# Instead, we explicitly make an unbound method that uses attrgetter to achieve
# the same result as above __len__, but without no byte code invoked to satisfy it
Foo.__len__ = types.MethodType(attrgetter('length'), None, Foo)
In Python 3, there is no longer an unbound method type, and types.MethodType only takes two arguments and creates only bound methods (which is not useful for Python special methods like __len__, __hash__, etc., since special methods are often looked up directly on the type, not the instance).
Is there some way of accomplishing this in Py3 that I'm missing?
Things I've looked at:
functools.partialmethod (appears to not have a C implementation, so it fails the requirements, and between the Python implementation and being much more general purpose than I need, it's slow, taking about 5 us in my tests, vs. ~200-300 ns for direct Python definitions or attrgetter in Py2, a roughly 20x increase in overhead)
Trying to make attrgetter or the like follow the non-data descriptor protocol (not possible AFAICT, can't monkey-patch in a __get__ or the like)
Trying to find a way to subclass attrgetter to give it a __get__, but of course, the __get__ needs to be delegated to C layer somehow, and now we're back where we started
(Specific to attrgetter use case) Using __slots__ to make the member a descriptor in the first place, then trying to somehow convert from the resulting descriptor for the data into something that skips the final step of binding and acquiring the real value to something that makes it callable so the real value retrieval is deferred
I can't swear I didn't miss something for any of those options though. Anyone have any solutions? Total hackery is allowed; I recognize I'm doing pathological things here. Ideally it would be flexible (to let you make something that behaves like an unbound method out of a class, a Python built-in function like hex, len, etc., or any other callable object not defined at the Python layer). Importantly, it needs to attach to the class, not each instance (both to reduce per-instance overhead, and to work correctly for dunder special methods, which bypass instance lookup in most cases).
Found a (probably CPython only) solution to this recently. It's a little ugly, being a ctypes hack to directly invoke CPython APIs, but it works, and gets the desired performance:
import ctypes
from operator import attrgetter
make_instance_method = ctypes.pythonapi.PyInstanceMethod_New
make_instance_method.argtypes = (ctypes.py_object,)
make_instance_method.restype = ctypes.py_object
class Foo:
# ... This class maintains a member named length storing the length...
# Defines a __len__ method that, at the C level, fetches self.length
__len__ = make_instance_method(attrgetter('length'))
It's an improvement over the Python 2 version in one way, since, as it doesn't need the class to be defined to make an unbound method for it, you can define it in the class body by simple assignment (where the Python 2 version must explicitly reference Foo twice in Foo.__len__ = types.MethodType(attrgetter('length'), None, Foo), and only after class Foo has finished being defined).
On the other hand, it doesn't actually provide a performance benefit on CPython 3.7 AFAICT, at least not for the simple case here where it's replacing def __len__(self): return self.length; in fact, for __len__ accessed via len(instance) on an instance of Foo, ipython %%timeit microbenchmarks show len(instance) is ~10% slower when __len__ is defined via __len__ = make_instance_method(attrgetter('length')), . This is likely an artifact of attrgetter itself having slightly higher overhead due to CPython not having moved it to the "FastCall" protocol (called "Vectorcall" in 3.8 when it was made semi-public for provisional third-party use), while user-defined functions already benefit from it in 3.7, as well as having to dynamically choose whether to perform dotted or undotted attribute lookup and single or multiple attribute lookup each time (which Vectorcall might be able to avoid by choosing a __call__ implementation appropriate to the gets being performed at construction time) adds more overhead that the plain method avoids. It should win for more complicated cases (say, if the attribute to be retrieved is a nested attribute like self.contained.length), since attrgetter's overhead is largely fixed, while nested attribute lookup in Python means more byte code, but right now, it's not useful very often.
If they ever get around to optimizing operator.attrgetter for Vectorcall, I'll rebenchmark and update this answer.
I would like to do something like the following:
class Foo(object):
def __init__(self):
self.member = 10
pass
def factory(foo):
foo = Foo()
aTestFoo = None
factory(aTestFoo)
print aTestFoo.member
However it crashes with AttributeError: 'NoneType' object has no attribute 'member':
the object aTestFoo has not been modified inside the call of the function factory.
What is the pythonic way of performing that ? Is it a pattern to avoid ? If it is a current mistake, how is it called ?
In C++, in the function prototype, I would have added a reference to the pointer to be created in the factory... but maybe this is not the kind of things I should think about in Python.
In C#, there's the key word ref that allows to modify the reference itself, really close to the C++ way. I don't know in Java... and I do wonder in Python.
Python does not have pass by reference. One of the few things it shares with Java, by the way. Some people describe argument passing in Python as call by value (and define the values as references, where reference means not what it means in C++), some people describe it as pass by reference with reasoning I find quite questionable (they re-define it to use to what Python calls "reference", and end up with something which has nothing to do with what has been known as pass by reference for decades), others go for terms which are not as widely used and abused (popular examples are "{pass,call} by {object,sharing}"). See Call By Object on effbot.org for a rather extensive discussion on the defintions of the various terms, on history, and on the flaws in some of the arguments for the terms pass by reference and pass by value.
The short story, without naming it, goes like this:
Every variable, object attribute, collection item, etc. refers to an object.
Assignment, argument passing, etc. create another variable, object attribute, collection item, etc. which refers to the same object but has no knowledge which other variables, object attributes, collection items, etc. refer to that object.
Any variable, object attribute, collection item, etc. can be used to modify an object, and any other variable, object attribute, collection item, etc. can be used to observe that modification.
No variable, object attribute, collection item, etc. refers to another variable, object attribute, collection items, etc. and thus you can't emulate pass by reference (in the C++ sense) except by treating a mutable object/collection as your "namespace". This is excessively ugly, so don't use it when there's a much easier alternative (such as a return value, or exceptions, or multiple return values via iterable unpacking).
You may consider this like using pointers, but not pointers to pointers (but sometimes pointers to structures containing pointers) in C. And then passing those pointers by value. But don't read too much into this simile. Python's data model is significantly different from C's.
You are making a mistake here because in Python
"We call the argument passing technique _call by sharing_,
because the argument objects are shared between the
caller and the called routine. This technique does not
correspond to most traditional argument passing techniques
(it is similar to argument passing in LISP). In particular it
is not call by value because mutations of arguments per-
formed by the called routine will be visible to the caller.
And it is not call by reference because access is not given
to the variables of the caller, but merely to certain objects."
in Python, the variables in the formal argument list are bound to the
actual argument objects. the objects are shared between caller
and callee; there are no "fresh locations" or extra "stores" involved.
(which, of course, is why the CLU folks called this mechanism "call-
by-sharing".)
and btw, Python functions doesn't run in an extended environment, either. function bodies have very limited access to the surrounding environment.
The Assignment Statements section of the Python docs might be interesting.
The = statement in Python acts differently depending on the situation, but in the case you present, it just binds the new object to a new local variable:
def factory(foo):
# This makes a new instance of Foo,
# and binds it to a local variable `foo`,
foo = Foo()
# This binds `None` to a top-level variable `aTestFoo`
aTestFoo = None
# Call `factory` with first argument of `None`
factory(aTestFoo)
print aTestFoo.member
Although it can potentially be more confusing than helpful, the dis module can show you the byte-code representation of a function, which can reveal how Python works internally. Here is the disassembly of `factory:
>>> dis.dis(factory)
4 0 LOAD_GLOBAL 0 (Foo)
3 CALL_FUNCTION 0
6 STORE_FAST 0 (foo)
9 LOAD_CONST 0 (None)
12 RETURN_VALUE
What that says is, Python loads the global Foo class by name (0), and calls it (3, instantiation and calling are very similar), then stores the result in a local variable (6, see STORE_FAST). Then it loads the default return value None (9) and returns it (12)
What is the pythonic way of performing that ? Is it a pattern to avoid ? If it is a current mistake, how is it called ?
Factory functions are rarely necessary in Python. In the occasional case where they are necessary, you would just return the new instance from your factory (instead of trying to assign it to a passed-in variable):
class Foo(object):
def __init__(self):
self.member = 10
pass
def factory():
return Foo()
aTestFoo = factory()
print aTestFoo.member
Your factory method doesn't return anything - and by default it will have a return value of None. You assign aTestFoo to None, but never re-assign it - which is where your actual error is coming from.
Fixing these issues:
class Foo(object):
def __init__(self):
self.member = 10
pass
def factory(obj):
return obj()
aTestFoo = factory(Foo)
print aTestFoo.member
This should do what I think you are after, although such patterns are not that typical in Python (ie, factory methods).