How does Python know the instance variable without defining __init__?

How does Python know the instance variable without defining __init__? - python

Let us consider the following example:
class X:
def run(self):
print("An example.")
X().run()
The output is:
> An example.
But when we omit the reference to the instance:
class X:
def run():
print("An example.")
X().run()
The output is:
TypeError: run() takes 0 positional arguments but 1 was given
When we instantiate the class, __ new __ gets called and the instance is created, ok. But how it requires an instance without defining __ init __? (I'm surprised because, I've always written __ init __ thinking that it was responsible for defining the convention / self name for referencing the variable). I'm confused.

When you call instance.run() if implicitly calls run(instance) which is why you're receiving the error:
TypeError: run() takes 0 positional arguments but 1 was given
That's also the reason why instance methods should have self as the first argument.
Second, you're using the old way of declaring a class - class X:
The new way1 is class X(object): but regardless if you're using the new/old annotation, the call X() will return an instance of the class, even in case you didn't define __init__.
And third, if you want to make it a class method you can either do what Pynchia suggested in the comment above (annotate the method with #staticmethod) or declare the method as a class method by specifying that the first argument is cls and annotating it as a class-method:
class X:
#classmethod
def run(cls):
print "a"
X.run() # prints a
1. According to Mark's comment below, in Python 3 we return to the "old way" of declaring a class. Good to know - thanks Mark!

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 AttributeError: Object has no attribute in Unittest

I have 2 scripts, 1st is All_Methods, and another is All_Testcases, as I am using unittest framework, so here we go.
All_Methods is like:
class All_Services():
def abc(self):
x =1
def bca(self):
print "My Name is Taimoor"
self.abc()
def cba(self):
self.bca()
and on another script which is All_TestCases is like this:
from All_Methods import All_Services as service
class All_TestCases(unittest.TestCase):
def test_1_running_method(self)
service.cba(self)
Exception showing is:
AttributeError: 'All_TestCases' object has no attribute 'bca'
Kindly someone tell me, what I am missing here?
Thanks.

You are not using classes in the usual way when you pass in self to methods that you call on the class. Common is to call the methods on instances of the class and getting the self argument implicitly.
When you call Method.running_query_Athena(self) self is an instance of All_TestCases which does not have the method connecting_Athena.
Did you mean for All_TestCases to derive from All_Methods?
Why is All_Methods a class at all?

Use proper indentation since python is solely based on the basis of how the code is indented.
Please, Please use proper naming conventions; as advised under PEP 8.
You're trying to access an instance method without an instance.
Try the following:
class MyClass:
def my_instance_method(self):
return True
#classmethod
def my_class_method(cls):
return True
#staticmethod
def my_static_method():
return True
This won't work:
>> MyClass.my_instance_method()
TypeError: my_instance_method() missing 1 required positional argument: 'self'
but these will since they are not bound to a class instance being created.
MyClass.my_class_method()
MyClass.my_static_method()
An instance method requires that you instantiate the Class; meaning you use:
MyClass().my_instance_method()
Since you seem to want to set response_id on the class instance; using the self argument which denotes the class instance to get the response_id. - I suggest that you use an instance method and instantiate the class as shown above (note the () after the class name)
Kindly do fix your formatting in the question.

There are quite a few things wrong with the code in the example, but putting that aside.
The error is caused by passing an instance of class A as the self argument to a (non-static) method of class B.
Python will attempt to call this method on the instance of class A, resulting in the missing attribute error.
Here is a simplified example of the problem:
class A:
def is_ham(self):
# Python secretly does `self.is_ham()` here,
# because `self` is the current instance of Class A.
# Unless you explicitly pass `self` when calling the method.
return True
class B:
def is_it_ham(self):
# Note, `self` is an instance of class B here.
return A.is_ham(self)
spam = B()
spam.is_it_ham()

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).

Learning Python the Hard Way ex40, i dont understand the init part [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).

Develop Reference

Python is a programming language that lets you work quickly and integrate systems more effectively.