Calculate attributes with subclass parameters-CodePudding

I'm working in small Python program with several classes and subclasses. The main point is that I need to calculate the value of the main class with the attributes of the subclasses.

class Product:
    class Extra_1:
        value = 5

base_value = 25
final_value = base_value   Extra1.value

The expected output for base_value it would be 30.

However I need to call this class from other file through an import, when I do that I'm not able to get the expected output (50) Instead of that I get 30. It seems like Python its not calculating the value with the formula.

import myprogram

myprogram.Product.Extra.value = 25
print(myprogram.Product.final_value)    #Output = 30

I tried to create a function to calculate the final_value and assigning the return as value but I still have the same problem.

CodePudding user response：

class Product:
    class Extra_1:
        value = 5

creates a class Product with an inner class Extra_1. Technically a class is also an object, and a code like

base_value = 25
final_value = base_value   Product.Extra_1.value

references an attribute of an object Product and an attribute of a an object Product.Extra_1. But usually classes are templates to create new objects. A code

x = Product()
y = Product()

creates (instantiates) two new objects of class Product. To achieve what you want, you can define a special method named __init__ (instantiation automatically invokes this method) and redesign your program to use instantiation. See https://docs.python.org/3/tutorial/classes.html

CodePudding user response：

Python Class-Level Variables

There are some things I don't understand. First of all, are Extra, Extra1 and Extra_1 the same subclass? Assuming this and that the last two lines of the first code are inside of the Product class, then this is the expected behavior. Remember that those are class level variables, and, as is, they are evaluated just once. No matter what you do with base_value after the second line got interpreted. And this is true also if you make an instance of the Product class, the class-level variable final_value will not change unless you change it directly.

So, if you still want to use the static subclass attribute base_value and the static class attribute value to calculate your final value (I don't know why) you have to make at least final_value an instance level attribute:

class Product:
    class Extra_1:
        value = 5
    
    def __init__(self):
        self.final_value = Product.base_value   Product.Extra_1.value
    
    base_value = 25

This way, every new instance of Product will calculate its final_value according with the actual value of base_value and value.

import myprogram

#First Product instance will calculate its result with the original values
p1 = myprogram.Product()

#Changing the original value
myprogram.Product.Extra_1.value = 25

#After the modification, creating a new instance
p2 = myprogram.Product()

#This will print the original 30
print(p1.final_value)

#This, in the other hand, will print 50
print(p2.final_value)

Python documentation Class and Instance Variables