Class variables are variables that are defined in a class but not in any method, belong to the class itself and are shared by all instances. ① Class variables are defined at the class level, and all instances share the same data by default; ② Modifying class variables requires the class name operation, otherwise an instance variable with the same name will be created without affecting other instances; ③ Instance variables are defined by self, and each object has an independent copy of the data, which is used to store object-specific status information.
In Python, instance variables and class variables are two confusing but very important concepts. Their core difference lies in their scope of action and storage location: instance variables belong to a specific object, while class variables belong to the class itself.

What are class variables?
A class variable is a variable defined in a class but declared outside of any method. It is shared by all instances of the class. That is to say, no matter how many instances are created, there is only one copy of the class variable.

For example:
class Dog: species = 'Canine' # class variable def __init__(self, name): self.name = name # instance variable
In this example, species
are class variables, and all instances of Dog
will have this property by default, unless you modify species
of an instance separately.

If you change the class variable, all instances that do not explicitly overwrite it will be affected:
Dog.species = 'New Species' print(dog1.species) # Output New Species print(dog2.species) # Output New Species
What are instance variables?
An instance variable is a variable bound to a specific object, and each instance has its own independent data. It is usually defined by self
in the __init__
method.
For example, in the above example, name
is an instance variable:
dog1 = Dog('Buddy') dog2 = Dog('Max') print(dog1.name) #Buddy print(dog2.name) # Max
These two names do not affect each other because they are attributes of different objects.
Common misunderstanding: Use instances to modify class variables?
Sometimes novices mistakenly think that modifying class variables through instances will affect other instances, but this is not the case. Let’s take a look at a situation that is prone to errors:
dog1.species = 'Feline' print(dog1.species) # Feline print(dog2.species) # New Species (or class variable)
At this time, dog1
did not modify the class variable, but added a new instance variable with the same name to itself. This can lead to inconsistent behavior during subsequent visits.
So if you want to actually modify the class variable, you should use the class name to operate:
Dog.species = 'Another Change'
How to choose which one to use?
-
Use class variables :
- Store data shared by all instances
- Save memory (avoid repeated storage of the same value)
- Available as a default configuration or constant
-
Use instance variables :
- Each object needs independent data
- Status-related attributes (such as user name, order ID, etc.)
Basically that's it. Understanding the scope and life cycle of class variables and instance variables can help you write clearer and less buggy Python code.
The above is the detailed content of Python instance vs class variables. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

Polymorphism is a core concept in Python object-oriented programming, referring to "one interface, multiple implementations", allowing for unified processing of different types of objects. 1. Polymorphism is implemented through method rewriting. Subclasses can redefine parent class methods. For example, the spoke() method of Animal class has different implementations in Dog and Cat subclasses. 2. The practical uses of polymorphism include simplifying the code structure and enhancing scalability, such as calling the draw() method uniformly in the graphical drawing program, or handling the common behavior of different characters in game development. 3. Python implementation polymorphism needs to satisfy: the parent class defines a method, and the child class overrides the method, but does not require inheritance of the same parent class. As long as the object implements the same method, this is called the "duck type". 4. Things to note include the maintenance

The "Hello,World!" program is the most basic example written in Python, which is used to demonstrate the basic syntax and verify that the development environment is configured correctly. 1. It is implemented through a line of code print("Hello,World!"), and after running, the specified text will be output on the console; 2. The running steps include installing Python, writing code with a text editor, saving as a .py file, and executing the file in the terminal; 3. Common errors include missing brackets or quotes, misuse of capital Print, not saving as .py format, and running environment errors; 4. Optional tools include local text editor terminal, online editor (such as replit.com)

To generate a random string, you can use Python's random and string module combination. The specific steps are: 1. Import random and string modules; 2. Define character pools such as string.ascii_letters and string.digits; 3. Set the required length; 4. Call random.choices() to generate strings. For example, the code includes importrandom and importstring, set length=10, characters=string.ascii_letters string.digits and execute ''.join(random.c

AlgorithmsinPythonareessentialforefficientproblem-solvinginprogramming.Theyarestep-by-stepproceduresusedtosolvetaskslikesorting,searching,anddatamanipulation.Commontypesincludesortingalgorithmslikequicksort,searchingalgorithmslikebinarysearch,andgrap

ListslicinginPythonextractsaportionofalistusingindices.1.Itusesthesyntaxlist[start:end:step],wherestartisinclusive,endisexclusive,andstepdefinestheinterval.2.Ifstartorendareomitted,Pythondefaultstothebeginningorendofthelist.3.Commonusesincludegetting

A class method is a method defined in Python through the @classmethod decorator. Its first parameter is the class itself (cls), which is used to access or modify the class state. It can be called through a class or instance, which affects the entire class rather than a specific instance; for example, in the Person class, the show_count() method counts the number of objects created; when defining a class method, you need to use the @classmethod decorator and name the first parameter cls, such as the change_var(new_value) method to modify class variables; the class method is different from the instance method (self parameter) and static method (no automatic parameters), and is suitable for factory methods, alternative constructors, and management of class variables. Common uses include:

Python's csv module provides an easy way to read and write CSV files. 1. When reading a CSV file, you can use csv.reader() to read line by line and return each line of data as a string list; if you need to access the data through column names, you can use csv.DictReader() to map each line into a dictionary. 2. When writing to a CSV file, use csv.writer() and call writerow() or writerows() methods to write single or multiple rows of data; if you want to write dictionary data, use csv.DictWriter(), you need to define the column name first and write the header through writeheader(). 3. When handling edge cases, the module automatically handles them

Parameters are placeholders when defining a function, while arguments are specific values ??passed in when calling. 1. Position parameters need to be passed in order, and incorrect order will lead to errors in the result; 2. Keyword parameters are specified by parameter names, which can change the order and improve readability; 3. Default parameter values ??are assigned when defined to avoid duplicate code, but variable objects should be avoided as default values; 4. args and *kwargs can handle uncertain number of parameters and are suitable for general interfaces or decorators, but should be used with caution to maintain readability.
