Python Class Inheritance, __init__ and cls

Python Class Inheritance, __init__ and cls - python

The desired output of the code is that I have a class variable Team.stuff which has one entry holding the b instance, and the Player.stuff variable should be empty. Instead I get an error...
class Player:
stuff=[]
def __init__(self):
cls.stuff.append(self)
class Team(Player):
def __init__(self):
super(Team, self).__init__()
b=Team()
ERROR
cls.stuff.append(self)
NameError: global name 'cls' is not defined
I could pass the cls variable in the Team.__init__(), but I'm not sure if that is the "correct" way, and more importantly the Player.__init__() would need a class variable, and I'm not sure on the syntax on how to do that.

class Player(object):
stuff=[]
def __init__(self):
self.stuff.append(self)
class Team(Player):
def __init__(self):
super(Team, self).__init__()
b = Team()
print(Team.stuff)
prints (something like)
[<__main__.Team object at 0xb7519dec>]

Remember that cls is not a keyword in Python. Rather it is a convention for the first argument of a method when that method is supposed to be a class function and not one that is called on a particular instance of something.
If you want stuff to be an attribute of a function, you have to define it like so: self.stuff=[].
And then, when you refer to it in methods, do it with the self keyword as well: self.stuff.append(..).

The reason for this is that cls in your __init__ function is not defined. The argument names self and cls are just naming conventions. In reality, you could name them foo and bar for an instance and a class method respectively, and foo would point at the class instance whilst bar would point a the class itself.
Moreover, self.stuff would be valid as when searching for an attribute, if an object does not have an attribute of such name in it's __dict__ dictionary, the __dict__ of it's class is looked instead. If both are missing, further lookups are done according to mro order.
Notice that, once you set an attribute stuff on an object instance, it will shadow the class definition.
Explained by example:
class MyClassA(object):
stuff = []
class MyClassB(MyClassA):
def __init__(foobar):
# `foobar` points at the instance of `MyClassB` class.
# MyClassA.stuff gets `1` object appended to it.
foobar.stuff.append(1)
# This should print '[1]'
print foobar.stuff
# `MyClassB` object gets a _new_ attribute named `stuff` pointing at a list.
foobar.stuff = []
# This should print '[]'.
print foobar.stuff

Related

AttributeError: type object 'Vehicle' has no attribute 'rect' [duplicate]

This question already has answers here:
What is the purpose of the `self` parameter? Why is it needed?
(26 answers)
Why do we use __init__ in Python classes?
(9 answers)
Closed 6 months ago.
I'm learning the Python programming language and I've came across something I don't fully understand.
In a method like:
def method(self, blah):
def __init__(?):
....
....
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
I think they might be OOP constructs, but I don't know very much.

In this code:
class A(object):
def __init__(self):
self.x = 'Hello'
def method_a(self, foo):
print self.x + ' ' + foo
... the self variable represents the instance of the object itself. Most object-oriented languages pass this as a hidden parameter to the methods defined on an object; Python does not. You have to declare it explicitly. When you create an instance of the A class and call its methods, it will be passed automatically, as in ...
a = A() # We do not pass any argument to the __init__ method
a.method_a('Sailor!') # We only pass a single argument
The __init__ method is roughly what represents a constructor in Python. When you call A() Python creates an object for you, and passes it as the first parameter to the __init__ method. Any additional parameters (e.g., A(24, 'Hello')) will also get passed as arguments--in this case causing an exception to be raised, since the constructor isn't expecting them.

Yep, you are right, these are oop constructs.
__init__ is the constructor for a class. The self parameter refers to the instance of the object (like this in C++).
class Point:
def __init__(self, x, y):
self._x = x
self._y = y
The __init__ method gets called after memory for the object is allocated:
x = Point(1,2)
It is important to use the self parameter inside an object's method if you want to persist the value with the object. If, for instance, you implement the __init__ method like this:
class Point:
def __init__(self, x, y):
_x = x
_y = y
Your x and y parameters would be stored in variables on the stack and would be discarded when the init method goes out of scope. Setting those variables as self._x and self._y sets those variables as members of the Point object (accessible for the lifetime of the object).
N.B. Some clarification of the use of the word "constructor" in this answer. Technically the responsibilities of a "constructor" are split over two methods in Python. Those methods are __new__ (responsible for allocating memory) and __init__ (as discussed here, responsible for initialising the newly created instance).

A brief illustrative example
In the hope it might help a little, here's a simple example I used to understand the difference between a variable declared inside a class, and a variable declared inside an __init__ function:
class MyClass(object):
i = 123
def __init__(self):
self.i = 345
a = MyClass()
print(a.i)
print(MyClass.i)
Output:
345
123

In short:
self as it suggests, refers to itself- the object which has called the method. That is, if you have N objects calling the method, then self.a will refer to a separate instance of the variable for each of the N objects. Imagine N copies of the variable a for each object
__init__ is what is called as a constructor in other OOP languages such as C++/Java. The basic idea is that it is a special method which is automatically called when an object of that Class is created

Class objects support two kinds of operations: attribute references and instantiation
Attribute references use the standard syntax used for all attribute references in Python: obj.name. Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:
class MyClass:
"""A simple example class"""
i = 12345
def f(self):
return 'hello world'
then MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class: "A simple example class".
Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example:
x = MyClass()
The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named __init__(), like this:
def __init__(self):
self.data = []
When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:
x = MyClass()
Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,
class Complex:
def __init__(self, realpart, imagpart):
self.r = realpart
self.i = imagpart
x = Complex(3.0, -4.5)
x.r, x.i
Taken from official documentation which helped me the most in the end.
Here is my example
class Bill():
def __init__(self,apples,figs,dates):
self.apples = apples
self.figs = figs
self.dates = dates
self.bill = apples + figs + dates
print ("Buy",self.apples,"apples", self.figs,"figs
and",self.dates,"dates.
Total fruitty bill is",self.bill," pieces of fruit :)")
When you create instance of class Bill:
purchase = Bill(5,6,7)
You get:
> Buy 5 apples 6 figs and 7 dates. Total fruitty bill is 18 pieces of
> fruit :)

__init__ does act like a constructor. You'll need to pass "self" to any class functions as the first argument if you want them to behave as non-static methods. "self" are instance variables for your class.

Try out this code. Hope it helps many C programmers like me to Learn Py.
#! /usr/bin/python2
class Person:
'''Doc - Inside Class '''
def __init__(self, name):
'''Doc - __init__ Constructor'''
self.n_name = name
def show(self, n1, n2):
'''Doc - Inside Show'''
print self.n_name
print 'Sum = ', (n1 + n2)
def __del__(self):
print 'Destructor Deleting object - ', self.n_name
p=Person('Jay')
p.show(2, 3)
print p.__doc__
print p.__init__.__doc__
print p.show.__doc__
Output:
Jay
Sum = 5
Doc - Inside Class
Doc - __init__ Constructor
Doc - Inside Show
Destructor Deleting object - Jay

Had trouble undestanding this myself. Even after reading the answers here.
To properly understand the __init__ method you need to understand self.
The self Parameter
The arguments accepted by the __init__ method are :
def __init__(self, arg1, arg2):
But we only actually pass it two arguments :
instance = OurClass('arg1', 'arg2')
Where has the extra argument come from ?
When we access attributes of an object we do it by name (or by reference). Here instance is a reference to our new object. We access the printargs method of the instance object using instance.printargs.
In order to access object attributes from within the __init__ method we need a reference to the object.
Whenever a method is called, a reference to the main object is passed as the first argument. By convention you always call this first argument to your methods self.
This means in the __init__ method we can do :
self.arg1 = arg1
self.arg2 = arg2
Here we are setting attributes on the object. You can verify this by doing the following :
instance = OurClass('arg1', 'arg2')
print instance.arg1
arg1
values like this are known as object attributes. Here the __init__ method sets the arg1 and arg2 attributes of the instance.
source: http://www.voidspace.org.uk/python/articles/OOP.shtml#the-init-method

note that self could actually be any valid python identifier. For example, we could just as easily write, from Chris B's example:
class A(object):
def __init__(foo):
foo.x = 'Hello'
def method_a(bar, foo):
print bar.x + ' ' + foo
and it would work exactly the same. It is however recommended to use self because other pythoners will recognize it more easily.

What does self do? What is it meant to be? Is it mandatory?
The first argument of every class method, including init, is always a reference to the current instance of the class. By convention, this argument is always named self. In the init method, self refers to the newly created object; in other class methods, it refers to the instance whose method was called.
Python doesn't force you on using "self". You can give it any name you want. But remember the first argument in a method definition is a reference to the object. Python adds the self argument to the list for you; you do not need to include it when you call the methods.
if you didn't provide self in init method then you will get an error
TypeError: __init___() takes no arguments (1 given)
What does the init method do? Why is it necessary? (etc.)
init is short for initialization. It is a constructor which gets called when you make an instance of the class and it is not necessary. But usually it our practice to write init method for setting default state of the object. If you are not willing to set any state of the object initially then you don't need to write this method.

__init__ is basically a function which will "initialize"/"activate" the properties of the class for a specific object, once created and matched to the corresponding class..
self represents that object which will inherit those properties.

Basically, you need to use the 'self' keyword when using a variable in multiple functions within the same class. As for init, it's used to setup default values incase no other functions from within that class are called.

The 'self' is a reference to the class instance
class foo:
def bar(self):
print "hi"
Now we can create an instance of foo and call the method on it, the self parameter is added by Python in this case:
f = foo()
f.bar()
But it can be passed in as well if the method call isn't in the context of an instance of the class, the code below does the same thing
f = foo()
foo.bar(f)
Interestingly the variable name 'self' is just a convention. The below definition will work exactly the same.. Having said that it is very strong convention which should be followed always, but it does say something about flexible nature of the language
class foo:
def bar(s):
print "hi"

Just a demo for the question.
class MyClass:
def __init__(self):
print('__init__ is the constructor for a class')
def __del__(self):
print('__del__ is the destructor for a class')
def __enter__(self):
print('__enter__ is for context manager')
return self
def __exit__(self, exc_type, exc_value, traceback):
print('__exit__ is for context manager')
def greeting(self):
print('hello python')
if __name__ == '__main__':
with MyClass() as mycls:
mycls.greeting()
$ python3 class.objects_instantiation.py
__init__ is the constructor for a class
__enter__ is for context manager
hello python
__exit__ is for context manager
__del__ is the destructor for a class

In this code:
class Cat:
def __init__(self, name):
self.name = name
def info(self):
print 'I am a cat and I am called', self.name
Here __init__ acts as a constructor for the class and when an object is instantiated, this function is called. self represents the instantiating object.
c = Cat('Kitty')
c.info()
The result of the above statements will be as follows:
I am a cat and I am called Kitty

# Source: Class and Instance Variables
# https://docs.python.org/2/tutorial/classes.html#class-and-instance-variables
class MyClass(object):
# class variable
my_CLS_var = 10
# sets "init'ial" state to objects/instances, use self argument
def __init__(self):
# self usage => instance variable (per object)
self.my_OBJ_var = 15
# also possible, class name is used => init class variable
MyClass.my_CLS_var = 20
def run_example_func():
# PRINTS 10 (class variable)
print MyClass.my_CLS_var
# executes __init__ for obj1 instance
# NOTE: __init__ changes class variable above
obj1 = MyClass()
# PRINTS 15 (instance variable)
print obj1.my_OBJ_var
# PRINTS 20 (class variable, changed value)
print MyClass.my_CLS_var
run_example_func()

Here, the guy has written pretty well and simple: https://www.jeffknupp.com/blog/2014/06/18/improve-your-python-python-classes-and-object-oriented-programming/
Read above link as a reference to this:
self? So what's with that self parameter to all of the Customer
methods? What is it? Why, it's the instance, of course! Put another
way, a method like withdraw defines the instructions for withdrawing
money from some abstract customer's account. Calling
jeff.withdraw(100.0) puts those instructions to use on the jeff
instance.
So when we say def withdraw(self, amount):, we're saying, "here's how
you withdraw money from a Customer object (which we'll call self) and
a dollar figure (which we'll call amount). self is the instance of the
Customer that withdraw is being called on. That's not me making
analogies, either. jeff.withdraw(100.0) is just shorthand for
Customer.withdraw(jeff, 100.0), which is perfectly valid (if not often
seen) code.
init self may make sense for other methods, but what about init? When we call init, we're in the process of creating an object, so how can there already be a self? Python allows us to extend
the self pattern to when objects are constructed as well, even though
it doesn't exactly fit. Just imagine that jeff = Customer('Jeff
Knupp', 1000.0) is the same as calling jeff = Customer(jeff, 'Jeff
Knupp', 1000.0); the jeff that's passed in is also made the result.
This is why when we call init, we initialize objects by saying
things like self.name = name. Remember, since self is the instance,
this is equivalent to saying jeff.name = name, which is the same as
jeff.name = 'Jeff Knupp. Similarly, self.balance = balance is the same
as jeff.balance = 1000.0. After these two lines, we consider the
Customer object "initialized" and ready for use.
Be careful what you __init__
After init has finished, the caller can rightly assume that the
object is ready to use. That is, after jeff = Customer('Jeff Knupp',
1000.0), we can start making deposit and withdraw calls on jeff; jeff is a fully-initialized object.

Python __init__ and self what do they do?
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
The example given is not correct, so let me create a correct example based on it:
class SomeObject(object):
def __init__(self, blah):
self.blah = blah
def method(self):
return self.blah
When we create an instance of the object, the __init__ is called to customize the object after it has been created. That is, when we call SomeObject with 'blah' below (which could be anything), it gets passed to the __init__ function as the argument, blah:
an_object = SomeObject('blah')
The self argument is the instance of SomeObject that will be assigned to an_object.
Later, we might want to call a method on this object:
an_object.method()
Doing the dotted lookup, that is, an_object.method, binds the instance to an instance of the function, and the method (as called above) is now a "bound" method - which means we do not need to explicitly pass the instance to the method call.
The method call gets the instance because it was bound on the dotted lookup, and when called, then executes whatever code it was programmed to perform.
The implicitly passed self argument is called self by convention. We could use any other legal Python name, but you will likely get tarred and feathered by other Python programmers if you change it to something else.
__init__ is a special method, documented in the Python datamodel documentation. It is called immediately after the instance is created (usually via __new__ - although __new__ is not required unless you are subclassing an immutable datatype).

Python/Console returning 'self' is not defined [duplicate]

This question already has answers here:
What is the purpose of the `self` parameter? Why is it needed?
(26 answers)
Why do we use __init__ in Python classes?
(9 answers)
Closed 6 months ago.
I'm learning the Python programming language and I've came across something I don't fully understand.
In a method like:
def method(self, blah):
def __init__(?):
....
....
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
I think they might be OOP constructs, but I don't know very much.

In this code:
class A(object):
def __init__(self):
self.x = 'Hello'
def method_a(self, foo):
print self.x + ' ' + foo
... the self variable represents the instance of the object itself. Most object-oriented languages pass this as a hidden parameter to the methods defined on an object; Python does not. You have to declare it explicitly. When you create an instance of the A class and call its methods, it will be passed automatically, as in ...
a = A() # We do not pass any argument to the __init__ method
a.method_a('Sailor!') # We only pass a single argument
The __init__ method is roughly what represents a constructor in Python. When you call A() Python creates an object for you, and passes it as the first parameter to the __init__ method. Any additional parameters (e.g., A(24, 'Hello')) will also get passed as arguments--in this case causing an exception to be raised, since the constructor isn't expecting them.

Yep, you are right, these are oop constructs.
__init__ is the constructor for a class. The self parameter refers to the instance of the object (like this in C++).
class Point:
def __init__(self, x, y):
self._x = x
self._y = y
The __init__ method gets called after memory for the object is allocated:
x = Point(1,2)
It is important to use the self parameter inside an object's method if you want to persist the value with the object. If, for instance, you implement the __init__ method like this:
class Point:
def __init__(self, x, y):
_x = x
_y = y
Your x and y parameters would be stored in variables on the stack and would be discarded when the init method goes out of scope. Setting those variables as self._x and self._y sets those variables as members of the Point object (accessible for the lifetime of the object).
N.B. Some clarification of the use of the word "constructor" in this answer. Technically the responsibilities of a "constructor" are split over two methods in Python. Those methods are __new__ (responsible for allocating memory) and __init__ (as discussed here, responsible for initialising the newly created instance).

A brief illustrative example
In the hope it might help a little, here's a simple example I used to understand the difference between a variable declared inside a class, and a variable declared inside an __init__ function:
class MyClass(object):
i = 123
def __init__(self):
self.i = 345
a = MyClass()
print(a.i)
print(MyClass.i)
Output:
345
123

In short:
self as it suggests, refers to itself- the object which has called the method. That is, if you have N objects calling the method, then self.a will refer to a separate instance of the variable for each of the N objects. Imagine N copies of the variable a for each object
__init__ is what is called as a constructor in other OOP languages such as C++/Java. The basic idea is that it is a special method which is automatically called when an object of that Class is created

Class objects support two kinds of operations: attribute references and instantiation
Attribute references use the standard syntax used for all attribute references in Python: obj.name. Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:
class MyClass:
"""A simple example class"""
i = 12345
def f(self):
return 'hello world'
then MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class: "A simple example class".
Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example:
x = MyClass()
The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named __init__(), like this:
def __init__(self):
self.data = []
When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:
x = MyClass()
Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,
class Complex:
def __init__(self, realpart, imagpart):
self.r = realpart
self.i = imagpart
x = Complex(3.0, -4.5)
x.r, x.i
Taken from official documentation which helped me the most in the end.
Here is my example
class Bill():
def __init__(self,apples,figs,dates):
self.apples = apples
self.figs = figs
self.dates = dates
self.bill = apples + figs + dates
print ("Buy",self.apples,"apples", self.figs,"figs
and",self.dates,"dates.
Total fruitty bill is",self.bill," pieces of fruit :)")
When you create instance of class Bill:
purchase = Bill(5,6,7)
You get:
> Buy 5 apples 6 figs and 7 dates. Total fruitty bill is 18 pieces of
> fruit :)

__init__ does act like a constructor. You'll need to pass "self" to any class functions as the first argument if you want them to behave as non-static methods. "self" are instance variables for your class.

Try out this code. Hope it helps many C programmers like me to Learn Py.
#! /usr/bin/python2
class Person:
'''Doc - Inside Class '''
def __init__(self, name):
'''Doc - __init__ Constructor'''
self.n_name = name
def show(self, n1, n2):
'''Doc - Inside Show'''
print self.n_name
print 'Sum = ', (n1 + n2)
def __del__(self):
print 'Destructor Deleting object - ', self.n_name
p=Person('Jay')
p.show(2, 3)
print p.__doc__
print p.__init__.__doc__
print p.show.__doc__
Output:
Jay
Sum = 5
Doc - Inside Class
Doc - __init__ Constructor
Doc - Inside Show
Destructor Deleting object - Jay

Had trouble undestanding this myself. Even after reading the answers here.
To properly understand the __init__ method you need to understand self.
The self Parameter
The arguments accepted by the __init__ method are :
def __init__(self, arg1, arg2):
But we only actually pass it two arguments :
instance = OurClass('arg1', 'arg2')
Where has the extra argument come from ?
When we access attributes of an object we do it by name (or by reference). Here instance is a reference to our new object. We access the printargs method of the instance object using instance.printargs.
In order to access object attributes from within the __init__ method we need a reference to the object.
Whenever a method is called, a reference to the main object is passed as the first argument. By convention you always call this first argument to your methods self.
This means in the __init__ method we can do :
self.arg1 = arg1
self.arg2 = arg2
Here we are setting attributes on the object. You can verify this by doing the following :
instance = OurClass('arg1', 'arg2')
print instance.arg1
arg1
values like this are known as object attributes. Here the __init__ method sets the arg1 and arg2 attributes of the instance.
source: http://www.voidspace.org.uk/python/articles/OOP.shtml#the-init-method

note that self could actually be any valid python identifier. For example, we could just as easily write, from Chris B's example:
class A(object):
def __init__(foo):
foo.x = 'Hello'
def method_a(bar, foo):
print bar.x + ' ' + foo
and it would work exactly the same. It is however recommended to use self because other pythoners will recognize it more easily.

What does self do? What is it meant to be? Is it mandatory?
The first argument of every class method, including init, is always a reference to the current instance of the class. By convention, this argument is always named self. In the init method, self refers to the newly created object; in other class methods, it refers to the instance whose method was called.
Python doesn't force you on using "self". You can give it any name you want. But remember the first argument in a method definition is a reference to the object. Python adds the self argument to the list for you; you do not need to include it when you call the methods.
if you didn't provide self in init method then you will get an error
TypeError: __init___() takes no arguments (1 given)
What does the init method do? Why is it necessary? (etc.)
init is short for initialization. It is a constructor which gets called when you make an instance of the class and it is not necessary. But usually it our practice to write init method for setting default state of the object. If you are not willing to set any state of the object initially then you don't need to write this method.

__init__ is basically a function which will "initialize"/"activate" the properties of the class for a specific object, once created and matched to the corresponding class..
self represents that object which will inherit those properties.

Basically, you need to use the 'self' keyword when using a variable in multiple functions within the same class. As for init, it's used to setup default values incase no other functions from within that class are called.

The 'self' is a reference to the class instance
class foo:
def bar(self):
print "hi"
Now we can create an instance of foo and call the method on it, the self parameter is added by Python in this case:
f = foo()
f.bar()
But it can be passed in as well if the method call isn't in the context of an instance of the class, the code below does the same thing
f = foo()
foo.bar(f)
Interestingly the variable name 'self' is just a convention. The below definition will work exactly the same.. Having said that it is very strong convention which should be followed always, but it does say something about flexible nature of the language
class foo:
def bar(s):
print "hi"

Just a demo for the question.
class MyClass:
def __init__(self):
print('__init__ is the constructor for a class')
def __del__(self):
print('__del__ is the destructor for a class')
def __enter__(self):
print('__enter__ is for context manager')
return self
def __exit__(self, exc_type, exc_value, traceback):
print('__exit__ is for context manager')
def greeting(self):
print('hello python')
if __name__ == '__main__':
with MyClass() as mycls:
mycls.greeting()
$ python3 class.objects_instantiation.py
__init__ is the constructor for a class
__enter__ is for context manager
hello python
__exit__ is for context manager
__del__ is the destructor for a class

In this code:
class Cat:
def __init__(self, name):
self.name = name
def info(self):
print 'I am a cat and I am called', self.name
Here __init__ acts as a constructor for the class and when an object is instantiated, this function is called. self represents the instantiating object.
c = Cat('Kitty')
c.info()
The result of the above statements will be as follows:
I am a cat and I am called Kitty

# Source: Class and Instance Variables
# https://docs.python.org/2/tutorial/classes.html#class-and-instance-variables
class MyClass(object):
# class variable
my_CLS_var = 10
# sets "init'ial" state to objects/instances, use self argument
def __init__(self):
# self usage => instance variable (per object)
self.my_OBJ_var = 15
# also possible, class name is used => init class variable
MyClass.my_CLS_var = 20
def run_example_func():
# PRINTS 10 (class variable)
print MyClass.my_CLS_var
# executes __init__ for obj1 instance
# NOTE: __init__ changes class variable above
obj1 = MyClass()
# PRINTS 15 (instance variable)
print obj1.my_OBJ_var
# PRINTS 20 (class variable, changed value)
print MyClass.my_CLS_var
run_example_func()

Python __init__ and self what do they do?
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
The example given is not correct, so let me create a correct example based on it:
class SomeObject(object):
def __init__(self, blah):
self.blah = blah
def method(self):
return self.blah
When we create an instance of the object, the __init__ is called to customize the object after it has been created. That is, when we call SomeObject with 'blah' below (which could be anything), it gets passed to the __init__ function as the argument, blah:
an_object = SomeObject('blah')
The self argument is the instance of SomeObject that will be assigned to an_object.
Later, we might want to call a method on this object:
an_object.method()
Doing the dotted lookup, that is, an_object.method, binds the instance to an instance of the function, and the method (as called above) is now a "bound" method - which means we do not need to explicitly pass the instance to the method call.
The method call gets the instance because it was bound on the dotted lookup, and when called, then executes whatever code it was programmed to perform.
The implicitly passed self argument is called self by convention. We could use any other legal Python name, but you will likely get tarred and feathered by other Python programmers if you change it to something else.
__init__ is a special method, documented in the Python datamodel documentation. It is called immediately after the instance is created (usually via __new__ - although __new__ is not required unless you are subclassing an immutable datatype).

Python official name for an attribute that is not a method

According to my understanding the data members of objects in Python are referred to as 'attributes'.
Attributes that are callable are referred to as an object's 'methods', but I couldn't find a name for non-callable attributes, such as val in the following example:
class C:
def __init__(self):
self.val = 42. # How would this be called?
def self.action():
"""A method."""
print(self.val)
I am sure different people may call val different things like 'field' or 'variable' but I am interested in an official name.

Surprisingly hard to find official information on this topic. After reading this article I do believe it should simply be called Class Variable and Instance Variable.
Attributes, Properties, Methods and Variables
Attribute is the collection name for the three names Property, Method and Variable. The latter two are prefixed by either Class or Instance. A property can only belong to the Class.
class Foo:
a = 1
def __init__(self):
self.b = 2
#property
def c(self):
return 3
#classmethod
def d(cls):
return 4
def e(self):
return 5
Foo.a # Class Attribute: Class Variable
Foo().a # Class Attribute: Class Variable
Foo().b # Instance Attribute: Instance Variable
Foo.c # Class Attribute: Property
Foo.d # Class Attribute: Class Method
Foo().d # Class Attribute: Class Method
Foo.e # Class Attribute: Class Method
Foo().e # Instance Attribute: Instance Method
Sources
Difference between Class and Instance methods
How do I assign a property to an instance in Python?
What's the difference between a Python "property" and "attribute"?
https://docs.python.org/3/tutorial/classes.html#python-scopes-and-namespaces
Diagram made in Creately

I'm not sure if one exists, but I'd suggest just "instance attribute".
Features about this naming:
It excludes methods. Methods are all callable class attributes, so this wording excludes all methods.
It includes callable instance attributes. Consider the following code:
class Container:
def __init__(self, item):
self.item = item
c = Container(x)
c.item # is an "instance attribute"
c.item == x # True
Note that c.item is an "instance attribute" regardless of whether or not it's callable. I think this is behaviour you're after, but I'm not sure.
It excludes non-callable class attributes, e.g.
class SomeClass:
x = 5 # Is not an "instance attribute"
It includes per-instance attributes, e.g.
obj.x = 5
obj.x # Is an "instance attribute"
In the end, all of these features may be positives or negatives depending on specifically what you want. But I don't know specifically what you want, and this is as close as I can get. If you can provide more information, I can give a better suggestion.

What needs to be inside of init.py [duplicate]

This question already has answers here:
What is the purpose of the `self` parameter? Why is it needed?
(26 answers)
Why do we use __init__ in Python classes?
(9 answers)
Closed 6 months ago.
I'm learning the Python programming language and I've came across something I don't fully understand.
In a method like:
def method(self, blah):
def __init__(?):
....
....
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
I think they might be OOP constructs, but I don't know very much.

In this code:
class A(object):
def __init__(self):
self.x = 'Hello'
def method_a(self, foo):
print self.x + ' ' + foo
... the self variable represents the instance of the object itself. Most object-oriented languages pass this as a hidden parameter to the methods defined on an object; Python does not. You have to declare it explicitly. When you create an instance of the A class and call its methods, it will be passed automatically, as in ...
a = A() # We do not pass any argument to the __init__ method
a.method_a('Sailor!') # We only pass a single argument
The __init__ method is roughly what represents a constructor in Python. When you call A() Python creates an object for you, and passes it as the first parameter to the __init__ method. Any additional parameters (e.g., A(24, 'Hello')) will also get passed as arguments--in this case causing an exception to be raised, since the constructor isn't expecting them.

Yep, you are right, these are oop constructs.
__init__ is the constructor for a class. The self parameter refers to the instance of the object (like this in C++).
class Point:
def __init__(self, x, y):
self._x = x
self._y = y
The __init__ method gets called after memory for the object is allocated:
x = Point(1,2)
It is important to use the self parameter inside an object's method if you want to persist the value with the object. If, for instance, you implement the __init__ method like this:
class Point:
def __init__(self, x, y):
_x = x
_y = y
Your x and y parameters would be stored in variables on the stack and would be discarded when the init method goes out of scope. Setting those variables as self._x and self._y sets those variables as members of the Point object (accessible for the lifetime of the object).
N.B. Some clarification of the use of the word "constructor" in this answer. Technically the responsibilities of a "constructor" are split over two methods in Python. Those methods are __new__ (responsible for allocating memory) and __init__ (as discussed here, responsible for initialising the newly created instance).

A brief illustrative example
In the hope it might help a little, here's a simple example I used to understand the difference between a variable declared inside a class, and a variable declared inside an __init__ function:
class MyClass(object):
i = 123
def __init__(self):
self.i = 345
a = MyClass()
print(a.i)
print(MyClass.i)
Output:
345
123

In short:
self as it suggests, refers to itself- the object which has called the method. That is, if you have N objects calling the method, then self.a will refer to a separate instance of the variable for each of the N objects. Imagine N copies of the variable a for each object
__init__ is what is called as a constructor in other OOP languages such as C++/Java. The basic idea is that it is a special method which is automatically called when an object of that Class is created

Class objects support two kinds of operations: attribute references and instantiation
Attribute references use the standard syntax used for all attribute references in Python: obj.name. Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:
class MyClass:
"""A simple example class"""
i = 12345
def f(self):
return 'hello world'
then MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class: "A simple example class".
Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example:
x = MyClass()
The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named __init__(), like this:
def __init__(self):
self.data = []
When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:
x = MyClass()
Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,
class Complex:
def __init__(self, realpart, imagpart):
self.r = realpart
self.i = imagpart
x = Complex(3.0, -4.5)
x.r, x.i
Taken from official documentation which helped me the most in the end.
Here is my example
class Bill():
def __init__(self,apples,figs,dates):
self.apples = apples
self.figs = figs
self.dates = dates
self.bill = apples + figs + dates
print ("Buy",self.apples,"apples", self.figs,"figs
and",self.dates,"dates.
Total fruitty bill is",self.bill," pieces of fruit :)")
When you create instance of class Bill:
purchase = Bill(5,6,7)
You get:
> Buy 5 apples 6 figs and 7 dates. Total fruitty bill is 18 pieces of
> fruit :)

__init__ does act like a constructor. You'll need to pass "self" to any class functions as the first argument if you want them to behave as non-static methods. "self" are instance variables for your class.

Try out this code. Hope it helps many C programmers like me to Learn Py.
#! /usr/bin/python2
class Person:
'''Doc - Inside Class '''
def __init__(self, name):
'''Doc - __init__ Constructor'''
self.n_name = name
def show(self, n1, n2):
'''Doc - Inside Show'''
print self.n_name
print 'Sum = ', (n1 + n2)
def __del__(self):
print 'Destructor Deleting object - ', self.n_name
p=Person('Jay')
p.show(2, 3)
print p.__doc__
print p.__init__.__doc__
print p.show.__doc__
Output:
Jay
Sum = 5
Doc - Inside Class
Doc - __init__ Constructor
Doc - Inside Show
Destructor Deleting object - Jay

Had trouble undestanding this myself. Even after reading the answers here.
To properly understand the __init__ method you need to understand self.
The self Parameter
The arguments accepted by the __init__ method are :
def __init__(self, arg1, arg2):
But we only actually pass it two arguments :
instance = OurClass('arg1', 'arg2')
Where has the extra argument come from ?
When we access attributes of an object we do it by name (or by reference). Here instance is a reference to our new object. We access the printargs method of the instance object using instance.printargs.
In order to access object attributes from within the __init__ method we need a reference to the object.
Whenever a method is called, a reference to the main object is passed as the first argument. By convention you always call this first argument to your methods self.
This means in the __init__ method we can do :
self.arg1 = arg1
self.arg2 = arg2
Here we are setting attributes on the object. You can verify this by doing the following :
instance = OurClass('arg1', 'arg2')
print instance.arg1
arg1
values like this are known as object attributes. Here the __init__ method sets the arg1 and arg2 attributes of the instance.
source: http://www.voidspace.org.uk/python/articles/OOP.shtml#the-init-method

note that self could actually be any valid python identifier. For example, we could just as easily write, from Chris B's example:
class A(object):
def __init__(foo):
foo.x = 'Hello'
def method_a(bar, foo):
print bar.x + ' ' + foo
and it would work exactly the same. It is however recommended to use self because other pythoners will recognize it more easily.

What does self do? What is it meant to be? Is it mandatory?
The first argument of every class method, including init, is always a reference to the current instance of the class. By convention, this argument is always named self. In the init method, self refers to the newly created object; in other class methods, it refers to the instance whose method was called.
Python doesn't force you on using "self". You can give it any name you want. But remember the first argument in a method definition is a reference to the object. Python adds the self argument to the list for you; you do not need to include it when you call the methods.
if you didn't provide self in init method then you will get an error
TypeError: __init___() takes no arguments (1 given)
What does the init method do? Why is it necessary? (etc.)
init is short for initialization. It is a constructor which gets called when you make an instance of the class and it is not necessary. But usually it our practice to write init method for setting default state of the object. If you are not willing to set any state of the object initially then you don't need to write this method.

__init__ is basically a function which will "initialize"/"activate" the properties of the class for a specific object, once created and matched to the corresponding class..
self represents that object which will inherit those properties.

Basically, you need to use the 'self' keyword when using a variable in multiple functions within the same class. As for init, it's used to setup default values incase no other functions from within that class are called.

The 'self' is a reference to the class instance
class foo:
def bar(self):
print "hi"
Now we can create an instance of foo and call the method on it, the self parameter is added by Python in this case:
f = foo()
f.bar()
But it can be passed in as well if the method call isn't in the context of an instance of the class, the code below does the same thing
f = foo()
foo.bar(f)
Interestingly the variable name 'self' is just a convention. The below definition will work exactly the same.. Having said that it is very strong convention which should be followed always, but it does say something about flexible nature of the language
class foo:
def bar(s):
print "hi"

Just a demo for the question.
class MyClass:
def __init__(self):
print('__init__ is the constructor for a class')
def __del__(self):
print('__del__ is the destructor for a class')
def __enter__(self):
print('__enter__ is for context manager')
return self
def __exit__(self, exc_type, exc_value, traceback):
print('__exit__ is for context manager')
def greeting(self):
print('hello python')
if __name__ == '__main__':
with MyClass() as mycls:
mycls.greeting()
$ python3 class.objects_instantiation.py
__init__ is the constructor for a class
__enter__ is for context manager
hello python
__exit__ is for context manager
__del__ is the destructor for a class

In this code:
class Cat:
def __init__(self, name):
self.name = name
def info(self):
print 'I am a cat and I am called', self.name
Here __init__ acts as a constructor for the class and when an object is instantiated, this function is called. self represents the instantiating object.
c = Cat('Kitty')
c.info()
The result of the above statements will be as follows:
I am a cat and I am called Kitty

# Source: Class and Instance Variables
# https://docs.python.org/2/tutorial/classes.html#class-and-instance-variables
class MyClass(object):
# class variable
my_CLS_var = 10
# sets "init'ial" state to objects/instances, use self argument
def __init__(self):
# self usage => instance variable (per object)
self.my_OBJ_var = 15
# also possible, class name is used => init class variable
MyClass.my_CLS_var = 20
def run_example_func():
# PRINTS 10 (class variable)
print MyClass.my_CLS_var
# executes __init__ for obj1 instance
# NOTE: __init__ changes class variable above
obj1 = MyClass()
# PRINTS 15 (instance variable)
print obj1.my_OBJ_var
# PRINTS 20 (class variable, changed value)
print MyClass.my_CLS_var
run_example_func()

Python __init__ and self what do they do?
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
The example given is not correct, so let me create a correct example based on it:
class SomeObject(object):
def __init__(self, blah):
self.blah = blah
def method(self):
return self.blah
When we create an instance of the object, the __init__ is called to customize the object after it has been created. That is, when we call SomeObject with 'blah' below (which could be anything), it gets passed to the __init__ function as the argument, blah:
an_object = SomeObject('blah')
The self argument is the instance of SomeObject that will be assigned to an_object.
Later, we might want to call a method on this object:
an_object.method()
Doing the dotted lookup, that is, an_object.method, binds the instance to an instance of the function, and the method (as called above) is now a "bound" method - which means we do not need to explicitly pass the instance to the method call.
The method call gets the instance because it was bound on the dotted lookup, and when called, then executes whatever code it was programmed to perform.
The implicitly passed self argument is called self by convention. We could use any other legal Python name, but you will likely get tarred and feathered by other Python programmers if you change it to something else.
__init__ is a special method, documented in the Python datamodel documentation. It is called immediately after the instance is created (usually via __new__ - although __new__ is not required unless you are subclassing an immutable datatype).

Difference between methods and attributes in python

I am learning python and doing an exercise about classes. It tells me to add an attribute to my class and a method to my class. I always thought these were the same thing until I read the exercise. What is the difference between the two?

Terminology
Mental model:
A variable stored in an instance or class is called an attribute.
A function stored in an instance or class is called a method.
According to Python's glossary:
attribute: A value associated with an object which is referenced by
name using dotted expressions. For example, if an object o has an
attribute a it would be referenced as o.a
method: A function which is defined inside a class body. If called as an attribute of an instance of that class, the method will get the instance object as its first argument (which is usually called self). See function and nested scope.
Examples
Terminology applied to actual code:
a = 10 # variable
def f(b): # function
return b ** 2
class C:
c = 20 # class attribute
def __init__(self, d): # "dunder" method
self.d = d # instance attribute
def show(self): # method
print(self.c, self.d)
e = C(30)
e.g = 40 # another instance attribute

A method is an attribute, but not all attributes are methods. For example, if we have the class
class MyClass(object):
class_name = 'My Class'
def my_method(self):
print('Hello World!')
This class has two attributes, class_name and my_method. But only my_method is a method. Methods are functions that belong to your object. There are additional hidden attributes present on all classes, but this is what your exercise is likely talking about.

A quick,simplified explanation.
Attribute == characteristics.
Method == operations/ actions.
For example, Let's describe a cat (meow!).
What are the attributes(characteristics) of a cat?
It has different breed, name, color, whether they have spots...etc.
What are methods (actions) of a cat?
It can meow, climb, scratch you, destroy your laptop, etc.
Notice the difference, attributes define characteristics of the cat.
Methods, on the other hand, defines action/operation (verb).
Now, putting the above definition in mind, let's create an object of class 'cat'...meowww
class Cat():
To create attributes, use def init(self, arg1, arg2) - (as shown below).
The 'self' keyword is a reference to a particular instance of a class.
def __init__(self, mybreed, name):
# Attributes
self.breed = mybreed
self.name = name
# Operations/actions --> methods
def kill_mouse(self):
print('Insert some method to kill mouse here')
Notice (above) 'mybreed' is an input argument that the user need to specify, whereas self.breed is an attribute of the instance assigned to 'mybreed' argument. Usually, they're the same (e.g. breed for both, self.breed = breed). Here, it's coded differently to avoid confusion.
And attributes are usually written as 'self.attribute_name' (as shown above).
Now, methods are more like actions, or operations, where you define a function inside the body of a class to perform some operation, for example, killing a mouse. A method could also utilize the attributes that you defined within the object itself.
Another key difference between a method and attribute is how you call it.
For example, let's say we create an instance using the above class we defined.
my_cat = Cat()
To call an attribute, you use
my_cat.name
or
my_cat.breed
For methods, you call it to execute some action. In Python, you call method with an open and close parenthesis, as shown below:
my_cat.kill_mouse()

A method is a function defined in the class. An attribute is an instance variable defined in the class.
Example:
class Example(object):
def __init__(self, name):
self.name = name
def hello(self):
print 'Hi, I am ' + self.name
Here hello is a method, and name is an attribute.

class example:
global a
# a=0
def __init__(self,x,y):
self.fname=x
self.lname=y
def show(self):
return "first name: {} & Last name: {}".format(self.fname,self.lname)
obj1=example('reyan','ishtiaq')
obj2=example('ishtiaq','reyan')
print('method associated with obj1: '+ obj1.show())
print('method associated with obj2: '+ obj2.show())
obj1.a=20
obj2.a=30
print(obj1.a)
print(obj2.a)
output:
method associated with obj1: first name: reyan & Last name: ishtiaq................
method associated with obj2: first name: ishtiaq & Last name: reyan................
20
30

#Below u can see that I made a class called "example" with two class attributes:
variable1 and variable2.
class example():
def init(self, variable1, variable2):
self.variable1 = variable1
self.variable2 = variable1
i did not construct a method inside this class. Notice that variable1 comes first and after comes variable2 inside the init():
#below i created an object "object1" with the example class. I created the example class with two arguments "variable1" and "variable2". "self" does not count), so i have to pass two arguments when calling the example class. I gave two variables "10" and "20".
object1 = example(10,20)
with the code below i just get the value of the first argument, which is 10.
print(object1.variable1)

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