How it works
An attribute set on the class is shared by every instance; one set in the initialiser belongs to that instance alone.
Put per-object data in the initialiser and shared constants on the class.
Common error
A mutable class attribute being modified through one instance and appearing changed for all of them.
Worked example
Python
class Bag:
def __init__(self):
self.items = []
a = Bag()
b = Bag()
a.items.append("book")
print(a.items)
print(b.items)Output
['book']
[]Creating the list inside __init__ gives each instance a separate list. A mutable class-level list would be shared unless replaced on an instance.
Exercises
4.2.1 · Trace
Determine the exact output and explain the state changes.
Python
class Item:
category = "tool"
a = Item()
b = Item()
a.category = "book"
print(a.category, b.category)Show solution
Output
book toolThe instance assignment shadows the class attribute for a only.
4.2.2 · Explain
Why can items = [] in the class body cause unexpected sharing between instances?
Show solution
The list is stored on the class and may be reached by multiple instances. Mutating it does not create an independent list for each instance.