python static class variable example
Aug 01, 2025 am 06:33 AM
Static class variables are variables that belong to the class itself and are shared by all instances, and are defined outside the methods in the class. 1. Class variables are accessed and modified by class names, such as Dog.species; 2. All instances share class variables, and the results are the same for printing dog1.species, dog2.species and Dog.species; 3. Modifying class variables through classes will affect all instances; 4. If the instance modifys class variables (such as dog1.species = "xxx"), create an instance variable with the same name and no longer share class variables; 5. Common uses include counters, such as Person.count, record the number of instances, incrementing each time they are instantiated. Class variables should always be manipulated with class names to avoid occlusion problems, and the summary is complete.
In Python, static class variables (also called class variables) are variables that belong to the class itself, not an instance. All instances of this class share this variable. The following is a simple example to illustrate how to define and use static class variables.
? What are static class variables?
Static class variables are variables defined in a class but outside of any method. It is shared by all instances.
class Dog:
# Static class variables (class variables)
species = "Canis lupus"
def __init__(self, name):
self.name = name # instance variable # create two instances dog1 = Dog("Buddy")
dog2 = Dog("Max")
# Access class variable print(dog1.species) # Output: Canis lupus
print(dog2.species) # Output: Canis lupus
print(Dog.species) # Output: Canis lupus? All instances share
species, which are class-level variables.
? Modify class variables
You can modify class variables through classes, which affects all instances (unless the instance overrides it itself):
Dog.species = "Canis lupus familiaris" print(dog1.species) # Output: Canis lupus familiaris print(dog2.species) # Output: Canis lupus familiaris
?? But note: If an instance first modifies its own species , it becomes an instance variable:
dog1.species = "Modified species" # This will create an instance variable of dog1 and mask the class variable print(dog1.species) # Output: Modified species (instance variable) print(dog2.species) # Output: Canis lupus familiaris (still class variable) print(Dog.species) # Output: Canis lupus familiaris
? This shows that you need to be careful when modifying class variables through instances . It is best to use class names to modify class variables.
? Practical use example: Counter
A common purpose is to track how many instances are created:
class Person:
count = 0 # Class variable, record number of instances def __init__(self, name):
self.name = name
Person.count = 1 # Every time an instance is created, the count is increased by 1
def get_count(self):
return Person.count
# Use p1 = Person("Alice")
p2 = Person("Bob")
print(Person.count) # Output: 2
print(p1.get_count()) # Output: 2? Summary of the key points
- Class variables are defined in a class or outside a method.
- All instances share class variables.
- The safest thing to access or modify with
ClassName.variable. - Modifying the instance
self.class_varwill create an instance variable with the same name and will no longer be shared.
Basically all is it, not complicated but it is easy to ignore details.
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