为什么要在具有功能的模块上使用python类？

Question

Im teaching myself python (3.x) and I'm trying to understand the use case for classes. I'm starting to understand what they actually do, but I'm struggling to understand why you would use a class as opposed to creating a module with functions.

For example, how does:

class cls1:
    def func1(arguments...):
    #do some stuff


obj1 = cls1()
obj2 = cls1()

obj1.func1(arg1,arg2...)
obj2.func1(arg1,arg2...)

Differ from:

#module.py contents
    def func1(arguments...):
        #do some stuff

import module

x = module.func1(arg1,arg2...)
y = module.func1(arg1,arg2...)

This is probably very simple but I just can't get my head around it.

So far, I've had quite a bit of success writing python programs, but they have all been pretty procedural, and only importing basic module functions. Classes are my next biggest hurdle.

Answer 1

You use class if you need multiple instance of it, and you want that instances don't interfere each other.

Module behaves like a singleton class, so you can have only one instance of them.

EDIT: for example if you have a module called example.py:

    x = 0
    def incr():
           global x
           x = x + 1

    def getX():
           return x

if you try to import these module twice:

    import example as ex1
    import example as ex2

    ex1.incr()
    ex1.getX()
    1
    ex2.getX()
    1

This is why the module is imported only one time, so ex1 and ex2 points to the same object.

Answer 2

As long as you're only using pure functions (functions that only works on their arguments, always return the same result for the same arguments set, don't depend on any global/shared state and don't change anything - neither their arguments nor any global/shared state - IOW functions that don't have any side effects), then classes are indeed of a rather limited use. But that's functional programming , and while Python can technically be used in a functional style, it's possibly not the best choice here.

As soon has you have to share state between functions, and specially if some of these functions are supposed to change this shared state, you do have a use for OO concepts. There are mainly two ways to share state between functions: passing the state from function to function, or using globals.

The second solution - global state - is known to be troublesome, first because it makes understanding of the program flow (hence debugging) harder, but also because it prevents your code from being reentrant , which is a definitive no-no for quite a lot of now common use cases (multithreaded execution, most server-side web application code etc). Actually it makes your code practically unusable or near-unusable for anything except short simple one-shot scripts...

The second solution most often implies using half-informal complex datastructures (dicts with a given set of keys, often holding other dicts, lists, lists of dicts, sets etc), correctly initialising them and passing them from function to function - and of course have a set of functions that works on a given datastructure. IOW you are actually defining new complex datatypes (a data structure and a set of operations on that data structure), only using the lowest level tools the language provide.

Classes are actually a way to define such a data type at a higher level, grouping together the data and operations. They also offer a lot more, specially polymorphism , which makes for more generic, extensible code, and also easier unit testing.

Answer 3

Consider you have a file or a database with products, and each product has product id, price, availability, discount, published at web status, and more values. And you have second file with thousands of products that contain new prices and availability and discount. You want to update the values and keep control on how many products will be change and other stats. You can do it with Procedural programming and Functional programming but you will find yourself trying to discover tricks to make it work and most likely you will be lost in many different lists and sets.

On the other hand with Object-oriented programming you can create a class Product with instance variables the product-id, the old price, the old availability, the old discount, the old published status and some instance variables for the new values (new price, new availability, new discount, new published status). Than all you have to do is to read the first file/database and for every product to create a new instance of the class Product. Than you can read the second file and find the new values for your product objects. In the end every product of the first file/database will be an object and will be labeled and carry the old values and the new values. It is easier this way to track the changes, make statistics and update your database.

One more example. If you use tkinter, you can create a class for a top level window and every time you want to appear an information window or an about window (with custom color background and dimensions) you can simply create a new instance of this class.

For simple things classes are not needed. But for more complex things classes sometimes can make the solution easier.

Answer 4

I think the best answer is that it depends on what your indented object is supposed to be/do. But in general, there are some differences between a class and an imported module which will give each of them different features in the current module. Which the most important thing is that class has been defined to be objects, this means that they have a lot of options to act like an object which modules don't have. For example some special attributes like __getattr__ , __setattr__ , __iter__ , etc. And the ability to create a lot of instances and even controlling the way that they are created. But for modules, the documentation describes their use-case perfectly:

If you quit from the Python interpreter and enter it again, the definitions you have made (functions and variables) are lost. Therefore, if you want to write a somewhat longer program, you are better off using a text editor to prepare the input for the interpreter and running it with that file as input instead. This is known as creating a script. As your program gets longer, you may want to split it into several files for easier maintenance. You may also want to use a handy function that you've written in several programs without copying its definition into each program.

To support this, Python has a way to put definitions in a file and use them in a script or in an interactive instance of the interpreter. Such a file is called a module; definitions from a module can be imported into other modules or into the main module (the collection of variables that you have access to in a script executed at the top level and in calculator mode).