What is a class in python

What is a class in python?

can be regarded as a synonym for category or type. All objects belong to a certain class and are called instances of the class.

For example: Bird is an instance of "bird". This is just a general (abstract) class with many subclasses: the bird you see might belong to the subclass "lark". Think of "birds" as the collection of all birds, of which "larks" are a subset. When the class to which an object belongs is a subset of the class to which another object belongs, the former is called a subclass of the latter, so "larks" are subclasses of "birds" and "birds" are "larks" The superclass of "bird"

Defining a subclass is just a process of defining more methods.

Create a class

>>> class Person:    def setName(self,name):
        self.name=name    def getName(self):        return self.name    def greet(self):        print "Hello,world! I'm %s" % self.name        
>>> foo=Person()>>> bar=Person()>>> foo.setName('Nsds')>>> bar.setName('Ysdy')>>> foo.greet()
Hello,world! I'm Nsds>>> bar.greet()
Hello,world! I'm Ysdy

In calling setName of foo When using the greet function, foo automatically passes itself into the function as the first parameter, so it is named self. Without self, member methods cannot access the object itself on which they want to operate its properties.

Properties can be accessed externally:

>>> foo.name'Nsds'>>> bar.name='Yoda'>>> bar.greet()
Hello,world! I'm Yoda

Properties, functions, methods

The self parameter is actually the difference between methods and functions. Methods bind their first parameter to the corresponding instance, so this parameter does not need to be provided. So you can bind the properties to a normal function, so there will be no special self parameters:

(Characteristics are variables inside the object. The state of the object is described by its properties. The object's methods You can change its characteristics and access the characteristics directly from outside the object)

>>> class Class:    def method(self):        print 'I have a self!'

        >>> def function():    print "I don't...">>> s=Class()>>> s.method()
I have a self!>>> s.method=function>>> s.method()
I don't...

The variable birdsong reference is bound to the method bird.sing, or access to the self parameter (still bound to the same instance of the class)

>>> class Bird:
    song='Squaawk'
    def sing(self):        print self.song        
>>> bird=Bird()>>> bird.sing()
Squaawk>>> birdsong=bird.sing>>> birdsong()
Squaawk

Adding double underscores before the name can make the method or feature private (inaccessible from the outside)

>>> class Secretive:    def __inaccessible(self):        print "Bet you can't see me..."
    def accessible(self):        print "The secret message is:"
        self.__inaccessible()        
>>> s=Secretive()>>> s.__inacessible()

Traceback (most recent call last):
  File "<pyshell#182>", line 1, in <module>
    s.__inacessible()
AttributeError: &#39;Secretive&#39; object has no attribute &#39;__inacessible&#39;>>> s.accessible()
The secret message is:
Bet you can&#39;t see me...

In the internal definition of the class, all names starting with double underscores are Is "translated" into the form preceded by a single underscore and the class name

>>> Secretive._Secretive__inaccessible<unbound method Secretive.__inaccessible>
>>> s._Secretive__inaccessible()
Bet you can&#39;t see me...

The namespace of the class

When defining a class, all code located in the class statement is in a special namespace Execute --- the namespace of the class. This namespace is accessible to all members of the class.

The definition of a class is actually the execution code block

>>> =+=1

        
>>> m1=>>>>>>1
>>> m1.members=2
>>>2
>>> m2=>>>>>>2
>>>>>>3
>>>2
>>>

The new members value is written to the characteristics of m1, shielding the variables within the class scope

Super class

>>> class Filter:    def init(self):
        self.blocked=[]    def filter(self,sequence):        return [x for x in sequence if x not in self.blocked]    
>>> class SPAMFilter(Filter):    def init(self):
        self.blocked=['SPAM']        
>>> f=Filter()>>> f.init()>>> f.filter([1,2,3])
[1, 2, 3]>>> s=SPAMFilter()>>> s.init()>>> s.filter(['SPAM','SPAM','egg','name','ff'])
['egg', 'name', 'ff']

Inheritance, super class

>>> class Filter:    def init(self):
        self.blockes=[]    def filter(self,sequence):        return [x for x in sequence if x not in self.blocked]    
>>> class S(Filter):    def init(self):
        self.blocked=['s']        
>>> f=Filter()>>> f.init()>>> f.filter([1,2,3])

Multiple superclasses

Methods in the class that is inherited first will override methods in the class that is inherited later

>>> class C():    def calculate(self,expression):
        self.value=eval(expression)        
>>> class Talker():    def talk(self):        print 'Hi,my value is',self.value        
>>> class TalkingCalculator(C,Talker):    pass>>> tc=TalkingCalculator()>>> tc.calculate('1+2*3')>>> tc.talk()
Hi,my value is 7

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