What is the single type in a dynamic typing language?
The &quot;single type&quot; for Python is called &quot;object&quot; and described in https://docs.python.org/3/reference/datamodel.html:

Objects are Python’s abstraction for data. All data in a Python program is represented by objects or by relations between objects. (In a sense, and in conformance to Von Neumann’s model of a “stored program computer”, code is also represented by objects.)
Every object has an identity, a type and a value. An object’s identity never changes once it has been created; you may think of it as the object’s address in memory... An object’s type determines the operations that the object supports (e.g., “does it have a length?”) and also defines the possible values for objects of that type... The value of some objects can change. Objects whose value can change are said to be mutable; objects whose value is unchangeable once they are created are called immutable.

Of course, note that &quot;type&quot; and &quot;value&quot; are not used here in the same way as in Harper's book: Python's &quot;type&quot; is what Harper calls &quot;class&quot;.