Python -> (8) study notes

Class Class

In Python, all data types are treated as objects, of course, can also be custom objects. Custom object data type is object-oriented class (Class) concept .

Define a simple class

Class definition syntax:

class nameoftheclass(parent_class):
    statement1
    statement2
    statement3

In the class declaration you can write any Python statements, including the definition of the function (called in the class method):

>>> class MyClass(object):
...     """A simple example class"""
...     i = 12345
...     def f(self):
...         return 'hello world'

__init__ method

Instantiate the class to use the function symbols. As long as the class object is a non-parametric function returns a new instance of the class can.

x = MyClass()

More than create a new instance of the class and assigns this object to the local variable x. The instantiation operation creates an empty object .

Many classes like to create objects for an initial state . Therefore a class may define a named __init__()special method:

def __init__(self):
    self.data = []

Class defines a __init__()method, then the class instantiation operator will call the new class instance is created automatically __init__()method. So in this example, you can create a new instance:

x = MyClass()

__init__()The method can have arguments. In fact, the parameters __init__()passed to the class instantiation operator. E.g:

>>> class Complex:
...     def __init__(self, realpart, imagpart):
...         self.r = realpart
...         self.i = imagpart
...
>>> x = Complex(3.0, -4.5)
>>> x.r, x.i
(3.0, -4.5)

Python Inheritance

When a class inherits from another class, it inherits all the features (e.g., variables and methods) of the parent class. This helps to reuse code.

First, create a named Personclass, and then create two derived classes Studentand Teacher. When the two classes from Persontime to inherit the class, in addition to their classes would Personall methods of the class will have its own new methods and purpose of the variable.

#!/usr/bin/env python3

class Person(object):
    """
    返回具有给定名称的 Person 对象
    """

    def __init__(self, name):
        self.name = name

    def get_details(self):
        """
        返回包含人名的字符串
        """
        return self.name


class Student(Person):
    """
    返回 Student 对象，采用 name, branch, year 3 个参数
    """

    def __init__(self, name, branch, year):
        Person.__init__(self, name)
        self.branch = branch
        self.year = year

    def get_details(self):
        """
        返回包含学生具体信息的字符串
        """
        return "{} studies {} and is in {} year.".format(self.name, self.branch, self.year)


class Teacher(Person):
    """
    返回 Teacher 对象，采用字符串列表作为参数
    """
    def __init__(self, name, papers):
        Person.__init__(self, name)
        self.papers = papers

    def get_details(self):
        return "{} teaches {}".format(self.name, ','.join(self.papers))


person1 = Person('Sachin')
student1 = Student('Kushal', 'CSE', 2005)
teacher1 = Teacher('Prashad', ['C', 'C++'])

print(person1.get_details())
print(student1.get_details())
print(teacher1.get_details())

Multiple Inheritance

A class can inherit from more than one class, with all the variables and methods of the parent class, the following syntax:

class MyClass(Parentclass1, Parentclass2,...):
    def __init__(self):
        Parentclass1.__init__(self)
        Parentclass2.__init__(self)
        ...
        ...

Deleting objects

Use keywords delto delete objects:

>>> s = "I love you"
>>> del s
>>> s
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 's' is not defined

Properties reading method

Do not use the property in Python (attributes) reading methods (getters and setters). Properties can be used directly:

>>> class Student(object):
...     def __init__(self, name):
...         self.name = name
...
>>> std = Student("Kushal Das")
>>> print(std.name)
Kushal Das
>>> std.name = "Python"
>>> print(std.name)
Python

@property decorator

To be more precise adjustment of access control attributes, you can use @propertydecorator, @propertydecorator is responsible for the property to become a method call

#!/usr/bin/env python3

class Account(object):
    """账号类,
    amount 是美元金额.
    """
    def __init__(self, rate):
        self.__amt = 0
        self.rate = rate

    @property
    def amount(self):
        """账号余额（美元）"""
        return self.__amt

    @property
    def cny(self):
        """账号余额（人民币）"""
        return self.__amt * self.rate

    @amount.setter
    def amount(self, value):
        if value < 0:
            print("Sorry, no negative amount in the account.")
            return
        self.__amt = value

if __name__ == '__main__':
    acc = Account(rate=6.6) # 基于课程编写时的汇率
    acc.amount = 20
    print("Dollar amount:", acc.amount)
    print("In CNY:", acc.cny)
    acc.amount = -100
    print("Dollar amount:", acc.amount)