For example, when you put Serial in a program, you use an object someone else wrote. Python — and things like Micropython — have the same kind of division. Python started as a scripting ...

but it is possible to defeat it with objects that create a reference to each other creating a “circular dependency” – see the example on page 19 of the book “Python Essential Reference ...

The example we see here is called a dictionary ... as you’ll see later on. Any mutable Python object doesn’t have a consistent hash value over its lifetime, and so can’t be used as a key.