HOWTO · Python
How to Define a Class Global Variable in Python
Learn to define a Python class attribute in the class body, access it with ClassName.attribute, and distinguish it from a module-level name declared with global.
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A Python class variable is an attribute assigned in the class body. Define it once on the class, then read it with ClassName.attribute, such as Tesla.speed. A global name is separate: it belongs to the module namespace and does not become a class attribute.
Define a Class Variable in Python
Assign the value in the class body, then read it with the class name. The example also shows a method and a second class reading the same attribute.
class Tesla:
speed = 60
def show_speed(self):
return Tesla.speed
class Dashboard:
def show_tesla_speed(self):
return Tesla.speed
print(Tesla.speed)
print(Tesla().show_speed())
print(Dashboard().show_tesla_speed())
60
60
60
Here, speed is stored on Tesla, so Tesla.speed reads the shared class attribute. The method in Tesla and the method in Dashboard both use the same class-qualified lookup.
Access a Class Variable Outside Its Class
Use ClassName.attribute wherever the class is available. For example, Tesla.speed works in module-level code and in another class method. Creating an instance is not required to read a class attribute. A bare name such as speed outside the class does not refer to Tesla.speed; it is looked up in the current scope and may be undefined.
An instance can also define an attribute with the same name. For example, car.speed = 80 sets an attribute on that particular car, while Tesla.speed still refers to the class attribute. Use the class-qualified form when you specifically need the shared value.
Why global Does Not Define a Class Variable
The global statement tells Python that an assignment to a bare name in a code block should bind that name in the module namespace. In a class body, global speed followed by speed = 60 creates or updates the module-level name speed; it does not assign Tesla.speed. To define a class variable, write an assignment such as speed = 60 in the class body without global, and access it as Tesla.speed outside the class.
A method that needs the class attribute can also use Tesla.speed. If the method should work with subclasses, use type(self).speed or self.speed according to the intended lookup behavior; self.speed can resolve an instance attribute that shadows the class attribute.
