As the OS filesystem (and network) works in bytes, we need to represent the values of an object's instance variables as a serialized stream of bytes. Often, we'll use a two-step transformation to bytes; we'll represent the state of an object as a string and rely on the Python string to provide bytes in a standard encoding. Python's built-in features for encoding a string into bytes neatly solves this part of the problem.

When we look at our OS filesystems, we see two broad classes of devices: block-mode devices and character-mode devices. Block-mode devices can also be called seekable because the OS supports a seek operation that can access any byte in the file in an arbitrary order. Character-mode devices are not seekable; they are interfaces where bytes are transmitted serially. Seeking would involve travelling backwards in time.

This distinction between character and block mode can have an impact on how we represent the state of a complex object or...