Learning Python

Class Gotchas

Most class issues can usually be boiled down to namespace issues (which makes sense, given that classes are just namespaces with a few extra tricks up their sleeves).

Changing Class Attributes Can Have Side Effects

Theoretically speaking, classes (and class instances) are all mutable objects. Just as with built-in lists and dictionaries, they can be changed in place, by assigning to their attributes. As with lists and dictionaries, this also means that changing a class or instance object may impact multiple references to it.

That’s usually what we want (and is how objects change their state in general), but this becomes especially critical to know when changing class attributes. Because all instances generated from a class share the class’s namespace, any changes at the class level are reflected in all instances, unless they have their own versions of changed class attributes.

Since classes, modules, and instances are all just objects with attribute namespaces, you can normally change their attributes at runtime by assignments. Consider the following class; inside the class body, the assignment to name a generates an attribute X.a, which lives in the class object at runtime and will be inherited by all of X’s instances:

>>> class X:
...     a = 1        # class attribute
...
>>> I = X()
>>> I.a              # inherited by instance
1
>>> X.a
1

So far so good. But notice what happens when we change the class attribute dynamically: it also changes it in every object which inherits from the class. ...