Python: Essential Reference, Third Edition

Reference Counting and Garbage Collection

All objects are reference-counted. An object’s reference count is increased whenever it’s assigned to a new name or placed in a container such as a list, tuple, or dictionary, as shown here:

a = 3.4      # Creates an object '3.4'
b = a        # Increases reference count on '3.4';
c = []
c.append(b)  # Increases reference count on '3.4'

This example creates a single object containing the value 3.4. a is merely a name that refers to the newly created object. When b is assigned a, b becomes a new name for the same object, and the object’s reference count increases. Likewise, when you place b into a list, the object’s reference count increases again. Throughout the example, only one object contains 3.4. All other operations ...