Tips for using property in Python

Directory property attribute concrete instance property attribute has two ways decorator way old class new class class attribute way property object and @ property decorator contrast property object class attribute @ property decorator

property Properties

A special property that works like using instance property 1 and can correspond to a method

It is necessary to protect the encapsulation characteristics of classes and let developers use objects. Attributes to manipulate methods, @property 装饰器 You can access the method directly through the method name without adding a pair of () Parentheses.

Let's look at an example of finding the area of a circle

class Circle(object):

    PI = 3.14

    def __init__(self, r):
        # r The radius of a circle 
        self.r = r
        self.__area = self.PI * self.r * self.r
    
    @property
    def area(self):
        return self.__area

    def get_area(self):
        return self.__area


In [2]: c = Circle(10)

In [3]: c.area
Out[3]: 314.0

In [4]: c.get_area()
Out[4]: 314.0

The definition and invocation of property attributes should pay attention to the following points:

When defined, add on top of the instance method @property Decorator; And only one self Parameter Called without parentheses ()

   Instance method: c.get_area()
  property Method of decoration: c.area

Specific examples

For a list page of computer hosts displayed in a shopping mall, it is impossible to display all the contents in the database on the page every time a request is made, but it is displayed locally through the paging function. Therefore, when requesting data from the database, the specified acquisition to be displayed is obtained from the m Articles to articles n The functions of this paging include:

m and n are calculated according to the current page and the total number of data pieces requested by the user Request data in database according to m and n

class Pager(object):
    
    def __init__(self, current_page):
        
        #  The page number currently requested by the user (page 1 Page, p 2 Page ... ) 
        self.current_page = current_page
        
        #  Default display per page 10 Bar data 
        self.per_items = 10 

    @property
    def start(self):
        val = (self.current_page - 1) * self.per_items
        return val

    @property
    def end(self):
        val = self.current_page * self.per_items
        return val

# ipython Test 
In [2]: p = Pager(1)


In [3]: p.start		#  Is the starting value, that is: m
Out[3]: 0

In [4]: p.end		#  Is the end value, that is: n
Out[4]: 10

In [5]: p = Pager(2)

In [6]: p.start
Out[6]: 10

In [7]: p.end
Out[7]: 20

There are two ways for the property attribute

Decorator is: applying decorator in method @property Class attributes are: defined in the class with a value of property Class properties of an object property()

Decorator mode

Applied to the instance method of the class @property Decorator

The classes in Python are 旧式类 And 新式类 , 新式类 Attribute ratio of 旧式类 The attribute of is rich.

Old-style class

Old-style class with 1 kind @property Decorator

   
class Goods:
    
    def __init__(self, name):
        self.name = name
        
    @property
    def price(self):
        return 100
    
# ipython Test 
In [10]: g = Goods(' Watches ')

In [11]: g.price
Out[11]: 100

New class

New class, with 3 kinds @property Decorator

class Goods:
    """
    python3 Default inheritance in object Class 
     With python2 , 3 The result of executing this program is different, because only in the python3 Only in @xxx.setter  @xxx.deleter
    """
    @property
    def price(self):
        print('@property')

    @price.setter
    def price(self, value):
        print('@price.setter')

    @price.deleter
    def price(self):
        print('@price.deleter')

        
# ipython Test 
In [13]: g = Goods()

In [14]: g.price
@property

In [15]: g.price = 100
@price.setter

In [16]: del g.price
@price.deleter

g.price Individual call auto-execution @property Modified price Method and get the return value of the method g.price = 100 Assignment auto-execution @price.setter Modified price Method and set the 100 Parameters assigned to the method del g.price Delete auto-execution @price.deleter Modified price Method

Attention

There is only one way to access attributes in the old class, which corresponds to the @property Method of modification Attributes in the new class are accessed in three ways, and each corresponds to three attributes that are @property , @方法名.setter , self0 Method of modification

Because there are three access methods in the new class, we can define three methods as getting, modifying and deleting the same attribute according to their access characteristics.

# Goods Class @property Application 

class Goods(object):

    def __init__(self, name, price):
        #  Original price 
        self.original_price = price

        #  Discount 
        self.discount = 0.8

    @property
    def price(self):
        #  Actual price  =  Original price  *  Discount 
        new_price = self.original_price * self.discount
        return new_price

    @price.setter
    def price(self, value):
        self.original_price = value

    @price.deleter
    def price(self):
        print(' Delete the original price of goods ')
        del self.original_price

        
# ipython Test 
In [22]: g = Goods(' Millet mobile phone ', 2000)

In [23]: g.price
Out[23]: 1600.0

In [24]: g.price = 3000

In [25]: g.price
Out[25]: 2400.0

In [26]: del g.price
 Delete the original price of goods 

In [27]: g.price
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-27-38ee45b469f2> in <module>
----> 1 g.price

<ipython-input-18-d5ea66eb7ece> in price(self)
     12     def price(self):
     13         #  Actual price  =  Original price  *  Discount 
---> 14         new_price = self.original_price * self.discount
     15         return new_price
     16

AttributeError: 'Goods' object has no attribute 'original_price'

Class attribute mode

Creates a value of property Class properties of the object, when creating the class properties property Property when the, 旧式类 And 新式类 No difference

class Foo:
    
    def get_bar(self):
        return 'get_bar'

    BAR = property(get_bar)
    
    
# ipython  Test 
In [32]: f = Foo()

In [33]: f.BAR
Out[33]: 'get_bar'

f.BAR Automatic call get_bar() Method and get the return value of the method

property() There are 4 parameters in the

The first parameter is the method name, which automatically triggers the execution of the method when calling the object. Property The second parameter is the method name, which calls the object. The property = XXX automatically triggers the execution of the method The third parameter is the method name, which calls the del object. The execution of the method is automatically triggered when the property is called The fourth parameter, a string, calls the object. Attribute. _doc_, which is the description of the attribute

class Foo(object):

    def __init__(self, bar):
        self.bar = bar
    
    def get_bar(self):
        print('get_bar')
        return self.bar

    def set_bar(self, value): 
        """ You must have two parameters """
        print('set bar ' + value)
        self.bar = value

    def del_bar(self):
        print('del bar')
        del self.bar

    BAR = property(get_bar, set_bar, del_bar, "bar description...")

    
# ipython Test 
In [50]: f = Foo('python')

In [51]: f.BAR
get_bar
Out[51]: 'python'

In [52]: f.BAR = 'Java'
set bar Java

In [53]: f.BAR
get_bar
Out[53]: 'Java'

In [54]: del f.BAR
del bar

property object versus @ property decorator

Created as a result of class attributes property Object attributes have three access methods. According to the access characteristics of several attributes, we can define three methods as accessing the same attribute: obtaining, modifying, deleting, and following @property Comparison of decorators.

property Object Class Properties

# Goods Class  property Object class properties   Application 

class Goods(object):

    def __init__(self, name, price):
        #  Original price 
        self.original_price = price

        #  Discount 
        self.discount = 0.8

    def get_price(self):
        #  Actual price  =  Original price  *  Discount 
        new_price = self.original_price * self.discount
        return new_price

    def set_price(self, value):
        self.original_price = value

    def del_price(self):
        print(' Delete the original price of goods ')
        del self.original_price

    PRICE = property(get_price, set_price, del_price, "price description")

    
# ipython Test 
In [59]: g = Goods('Mac Computer ', 9000)

In [60]: g.PRICE
Out[60]: 7200.0

In [61]: g.PRICE = 10000

In [62]: g.PRICE
Out[62]: 8000.0

In [63]: del g.PRICE
 Delete the original price of goods 

@ property Decorator

# Goods Class  @property Decorator   Application 

class Goods(object):

    def __init__(self, name, price):
        #  Original price 
        self.original_price = price

        #  Discount 
        self.discount = 0.8

    @property
    def price(self):
        #  Actual price  =  Original price  *  Discount 
        new_price = self.original_price * self.discount
        return new_price

    @price.setter
    def price(self, value):
        self.original_price = value

    @price.deleter
    def price(self):
        print(' Delete the original price of goods ')
        del self.original_price
        
        
# ipython Test 
In [59]: g = Goods('Mac Computer ', 9000)

In [60]: g.PRICE
Out[60]: 7200.0

In [61]: g.PRICE = 10000

In [62]: g.PRICE
Out[62]: 8000.0

In [63]: del g.PRICE
 Delete the original price of goods 

It can be found that both can be realized, but @property In the old class, only the @property , no @method.setter And @method.deleter And the new class can use both. Therefore, depending on everyone's habits, choose one.

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