1、 Class definition
The syntax is as follows ：
class ClassName:<statement-1>
……
<statement-N>
After class instantiation , You can use its properties , actually , After creating a class , Can pass The class name accesses its properties .
2、 Class object
Class objects support two operations ： Property references and instantiations . Property references use and Python All attribute references in Use the same standard syntax ：obj.name. After the class object is created , All names in the class namespace Are valid attribute names .
Attributes are variables defined by a class , A method is a function defined by a class
class MyClass:
""" A simple class instance """
i = 12345
def f(self):
return 'hello world'
# Instantiate the class
x = MyClass()
# Access class properties and methods
print("MyClass Attributes of a class i by ：", x.i)
print("MyClass Class method f Output is ：", x.f())The above creates a new class instance and assigns the object to a local variable x,x Empty object . The output of the above program is ： MyClass Attributes of a class i by ： 12345 MyClass Class method f Output is ： hello world
3、__init__()
Class has a name __init__() Special method （ Construction method ）, This method is called automatically when the class is instantiated , Like this ：
def_init_(self):
self.data = [] Class definition __init__() Method , Class instantiation will Automatically call init() Method . Instantiate the class as follows MyClass, Corresponding __init__() Method will be called :
x = MyClass()Of course , init() Method can have parameters , Parameters through init() Passed to the instantiation operation of the class . for example :
class Complex:
def __init__(self, realpart, imagpart):
self.r = realpart
self.i = imagpart
x = Complex(3.0, -4.5)
print(x.r, x.i) # Output results ：3.0 -4.5self Represents an instance of a class , Not class There is only one special difference between a class method and a normal function —— They must have an additional first parameter name , By convention it's The name is self
class Test:
def prt(self):
print(self)
print(self.__class__)
t = Test()
t.prt()<main.Test object at 0x000001F1C3674F40> <class 'main.Test'>
It is obvious from the execution results ,self Represents an instance of a class , Represents the address of the current object , and self.class Point to class . self No python keyword , We'll replace him with Huawei It can also be executed normally
4、 Class method
Inside the class , Use def Keyword to define a method , Different from general function definition , Class method must contain parameters self, And For the first parameter ,self Represents an instance of a class .
# Class definition
class people:
# Define basic attributes
name = ''
age = 0
# Define private properties , Private properties cannot be accessed directly outside the class
__weight = 0
# Define construction method
def __init__(self, n, a, w):
self.name = n
self.age = a
self.__weight = w
def speak(self):
print("%s say : I %d year ." % (self.name, self.age))
# Instantiate the class
p = people('Huawei', 10, 30)
p.speak()Huawei say : I 10 year .
5、 Inherit
Python Class inheritance is also supported , If a language does not support inheritance , Classes have no meaning . Definition of derived class As shown below :
class DerivedClassName(BaseClassName1): <statement-1> . . . <statement-N>
Note the order of base classes in parentheses , If there is the same method name in the base class , When subclass is used, it does not specify , python Search from left to right When the method is not found in the subclass , Find whether the base class contains methods from left to right
BaseClassName（ The base class name in the example ） Must be in the same scope as the derived class definition . Except class , You can also use expressions , Base class definition This is useful in another module :
class DerivedClassName(modname.BaseClassName):
# Class definition
class people:
# Define basic attributes
name = ''
age = 0
# Define private properties , Private properties cannot be accessed directly outside the class
__weight = 0
# Define construction method
def __init__(self, n, a, w):
self.name = n
self.age = a
self.__weight = w
def speak(self):
print("%s say : I %d year ." % (self.name, self.age))
# Single inheritance example
class student(people):
grade = ''
def __init__(self, n, a, w, g):
# Call the constructor of the parent class
people.__init__(self, n, a, w)
self.grade = g
# Override method of parent class
def speak(self):
print("%s say : I %d Year old , I'm reading %d grade " % (self.name, self.age, self.grade))
s = student('Huawei', 10, 60, 3)
s.speak()Huawei say : I 10 Year old , I'm reading 3 grade
6、 Multiple inheritance
Form the following ：
class DerivedClassName(Base1, Base2, Base3): <statement-1> . . . <statement-N>
Note the order of the parent classes in parentheses , If the parent class has the same method name , When subclass is used, it does not specify ,python Search from left to right Cable When the method is not found in the subclass , From left to right, find whether the parent class contains methods
# Class definition
class people:
# Define basic attributes
name = ''
age = 0
# Define private properties , Private properties cannot be accessed directly outside the class
__weight = 0
# Define construction method
def __init__(self, n, a, w):
self.name = n
self.age = a
self.__weight = w
def speak(self):
print("%s say : I %d year ." % (self.name, self.age))
# Single inheritance example
class student(people):
grade = ''
def __init__(self, n, a, w, g):
# Call the constructor of the parent class
people.__init__(self, n, a, w)
self.grade = g
# Override method of parent class
def speak(self):
print("%s say : I %d Year old , I'm reading %d grade " % (self.name, self.age, self.grade))
# Another class , Preparation for multiple inheritance
class speaker():
topic = ''
name = ''
def __init__(self, n, t):
self.name = n
self.topic = t
def speak(self):
print(" My name is %s, I'm a speaker , The theme of my speech is %s" % (self.name, self.topic))
# multiple inheritance
class sample(speaker, student):
a = ''
def __init__(self, n, a, w, g, t):
student.__init__(self, n, a, w, g)
speaker.__init__(self, n, t)
test = sample("Huawei", 25, 80, 4, "Python")
test.speak() # Same method name , By default, the method of the parent class in brackets is called My name is Huawei, I'm a speaker , The theme of my speech is Python
7、 Method rewriting
class Parent: # Define parent class
def myMethod(self):
print(' Call the superclass method ')
class Child(Parent): # Defining subclasses
def myMethod(self):
print(' Call subclass method ')
c = Child() # Subclass instance
c.myMethod() # Subclass call override method
super(Child, c).myMethod() # Call the overridden method of the parent class with the child class object ,super() Function is used to call the parent class ( Superclass ) One way .Call subclass method Call the superclass method
8、 Class （ private ） Properties and methods
Private properties of class
private_attrs： Start with two underscores , Declare the property private , Can't be used outside of the class or accessed directly . Inside the class When used in self.private_attrs
Class method Inside the class , Use def Keyword to define a method , Different from general function definition , Class method must contain parameters self, And first Parameters ,self Represents an instance of a class . self The name of is not meant to be dead , You can also use this, But it's better to use it as agreed self.
Private method of class ：
Example 1 ：
class JustCounter:
__secretCount = 0 # Private variables
publicCount = 0 # Open variables
def count(self):
self.__secretCount += 1
self.publicCount += 1
print(self.__secretCount)
counter = JustCounter()
counter.count()
counter.count()
print(counter.publicCount)
print(counter.__secretCount) # Report errors , Instance cannot access private variables 1 2 2 Traceback (most recent call last): File "E:\ Big data competition \mystudy\ex_821.py", line 15, in <module> print(counter.secretCount) # Report errors , Instance cannot access private variables AttributeError: 'JustCounter' object has no attribute 'secretCount'
Example 2：
class Site:
def __init__(self, name, url):
self.name = name # public
self.__url = url # private
def who(self):
print('name : ', self.name)
print('url : ', self.__url)
def __foo(self): # Private method
print(' This is the private method ')
def foo(self): # Public methods
print(' It's the public way ')
self.__foo()
x = Site(' Extreme value College ', 'http://edu.mathor.com/')
x.who() # Normal output
x.foo() # Normal output
x.__foo() # Report errors Traceback (most recent call last): File "E:\ Big data competition \mystudy\ex_822.py", line 21, in <module> x.foo() # Report errors AttributeError: 'Site' object has no attribute 'foo' name : Extreme value College url : Extreme vocational education - Let everyone find a satisfactory job It's the public way This is the private method
9、 Class
10、 operator overloading
class Vector:
def __init__(self, a, b):
self.a = a
self.b = b
def __str__(self):
return 'Vector (%d, %d)' % (self.a, self.b)
def __add__(self, other):
return Vector(self.a + other.a, self.b + other.b)
v1 = Vector(2, 10)
v2 = Vector(5, -2)
print(v1 + v2)Vector (7, 8)
