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Classes and Instance

This includes the following topics:

• Creating and instantiating.
• Class vs Instance variables
• Inhertiance
• Static methods and Class methods

Allows for logical grouping and reusing of data and functions . And overall its can be easier to built upon this.

Terminology note: Data = attribute and Function = methods.

pass statment tell python; nothing to do here.

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Class & its instance

A class is a blueprint for its instances.

class some_name:
    pass

inst_one = some_name()

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Init Method

Its the first method that is used to intialize class variables. It is the constructor.

Please read the comments carefully ```python class some_class:

def __init__(self, some_argument):
    # self = instance of the class
    # assign specific value to instance
    # good practice is to name the class var as arguments.
    self.some_argument = some_argument

def some_func(self):
    # TODO something with class vars.

inst_one = some_class(some_argument)

>_Running some function of class for given instance._
```python
# METHOD 1
inst_one.some_func() #here inst_one = self variable.

# METHOD 2
some_class.some_func(inst_one) # passing instance as argument.

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Instance Variables vs Class variable

Instance variables contain data uniquely associated with the given instance of some class, whereas Class variables are common amongst all the instances and generally defined outside of “init” method!!.

Instance Variable example:

inst_one = some_class()
inst_one.uniq_var = "some val" #this is uniq to this instance.

Class variable example:

class some_class:

    random_var = "some Val" # THis is a CLASS Variable

    # <init method goes here>

There are two methods to access class variable within a class:
Method 1: accessing through instance variable

    def some_func(self):
        self.random_var

Method 2: accessing through class variable

    def some_func(self):
        some_class.random_var

NOTE: Some key points to keep in mind are:

• When an attirbute value is required, it first checked in Instance, the Class, and lastly inherited class.

• An instance variable does NOT contain class variable, thus accessing class variable through them, will require them accesing class namespace themselves.

• If we try to modify class variable throught instance, it will lead to that variable’s modified version being added to instance’s namespace.

A namespace is a dict maintain by python to keep the name assigned uniquely to each variable. It can be accessed using:

# Accessing namespace of an entity: a class or instance.
some_entity.__dict__

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Class methods & Static methods

Class methods:

These are generally used to manipulate class variables.

To convert regular method to class method; use decorator @classmethod and pass the first argument as class: cls keyword being used, since “class” is already a keyword.

Example:

@classmethod
def somefunc(cls,*args):

Class method use cases:

• Returning class/ manipulating class varibables.
• Alternative constructor for a class.

Example:

class some_class:
    class_var = []
    class_info_var = []
    
    def __init__(self,var1):
        self.var1 = var1
    
    @classmethod
    def get_classinfo(cls, <arg,if any>):
        return cls.class_info_var
        
    @classmethod
    def other_const(cls, some_var):
        var1 = after_processing(some_var)
        return cls(var1)

Static methods:

Unlike class or regular methods which pass “cls” or “self” as first arguments, these dont pass any arguments. But these are included in class, since they have some logical connection to it.

Generally, if a method is not using any instance or class variable, then it should be made a stati method.

This created using decorator: @staticmethod. Example:

@staticmethod
def some_name(var):
    #TODO something with var

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Inheritance

Reuseability of code.

Example:

class derived_class(parent_class_name):

Method resolution order

This is order in which python looks up for methods and attributes. This can be viewed using help function:
python print(help(class_name))

Using “super”:

This is used to class method of parent class. Especiall constructor, from dervied class.

Example:

class par:
    def __init__(self,var1):
        #TODO

class derive(par):
    def __init__(self,var1,var2):
        super().__init__(var1)
        # below line also does the same thing
        par.__init__(self,var1)
        self.var2 = var2

Builtin function for help

isinstance(insta, Class_name) function tells us if given instance object of a class.

issubclass(Class1, Class2) ,gives us if class1 is dervied class of class2.