learning python notes 6

Objected Oriented Programming(senior)

1.__slots__

In python, we can dynamically bind method or attribute to an instance. e.g.

class Student(object): pass def setName(self,name): self.name=name # bind attribute to an instance a=Student() a.name='chen' # bind attribute&&method to an instance Student.age=18 from types import MethodType Student.setName=MethodType(setName,Student)

Though these methods provide us with convienence, they make the use of class unsafer. Thus, we need some thing to limit the attribute we can add when we create an instance. And that’s it: slots. When we define a class, we can add an slots attribute to it, which stores the attributes that can be binded to this class. If we bind it to an not stored attribute, explainer will raise an AttributeType error.

2.Using @property

If we pay attention to packaging, then we will find we need to write functions for class to gurantee its safty. However, in python we can use @property to simply our call step. e.g.

#!/usr/bin/env python3 # -*- coding: utf-8 -*- class Student(object): @property def score(self): return self._score @score.setter def score(self,value): if(value>100 or value<0): raise ValueError('score must between 0 to 100') self._score=value a=Student() a.score=99 print(a.score)

3.Multiple Inheritance

Python also supports multiple inheritance. And the way it goes is similar to inheritance in c++. Just add the inherited class in the parenthesis.

class Animal(object): pass class Mammal(Animal): pass class Bird(Animal): pass class runnable(object): pass class flyable(object): pass class Dog(Mammal,flyable): pass

Also, we often use MixIn to represent this is a multi-inheritance.

4.costumizing class

In this secion, we will face with various in-built functions: - init(). - str(). # support us to return a str when having defined a class. - getitems(). # if we want to use index to get element in class, use this. - iter(). # let the class have iteratable attribute which supports for loop. - getattr(). # be called when explainer cannot find attribute in this class. - call(). # let an object be an function that can be called without passing self argument.

Let’s look at an example:

#!/usr/bin/env python3 # -*- coding: utf-8 -*- class fibonaci(object): def __init__(self): self.a,self.b=0,1 def __iter__(self): return self def __next__(self): self.a,self.b=self.b,self.a+self.b if(self.a>1000): raise StopIteration() else: return self.a def __getitem__(self,n): if isinstance(n,slice): start=n.start stop=n.stop if start==None: start=0 a,b=0,1 L=[] for i in range(stop): a,b=b,a+b if i>=start: L.append(a) return L if isinstance(n,int): a,b=0,1 for i in range(n): a,b=b,a+b return a def __getattr__(self,attr): if attr=='age': return 99 else: raise AttributeError('\'student\' object don\'t have \'%s\' attribute'% attr) def __call__(self): print('This is a fibonaci num') a=fibonaci() a() print(a.age) print(a.che)

5.enum

syntax:

from enum import Enum. Month=Enum('month',('Jan','Feb',..'Dec')).

Through this, we define a enum, each instance in it is class type.

6.metaclass

(Leave one page to talk about it specially) This metaclass seems quite diffcult to understand :(