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.
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