Text becomes bytes by a rule, bytes become numbers by a layout, and both steps have to agree on both sides. Tracing one word through all of it makes the whole picture concrete.
About 2 min of reading
Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.
Build and read a self-describing record.
Encode a word to bytes.
Before you run it, say out loud what it will print. Then open the answer.
Show the byte numbers.
Before you run it, say out loud what it will print. Then open the answer.
Put a two-byte length in front.
Before you run it, say out loud what it will print. Then open the answer.
Read the length and decode the rest.
Before you run it, say out loud what it will print. Then open the answer.
Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
The encoding, the byte order, and the length width are all choices. A format writes them down once so a reader can undo them exactly.
Every text file, JSON body, and database string goes through this. When one side picks a different rule, characters change or records line up wrong.
print("owlww".encode("utf-8"))Five letters become five bytes, one each, because they are all plain. The rule is utf-8 and the result is what a file holds.
print(list("owlww".encode("utf-8")))Each byte is a number from 0 to 255. Those numbers are what a hex editor shows you, and what a checksum runs over.
b = "owlww".encode("utf-8")
print(len(b).to_bytes(2, "big") + b)A two-byte length in front makes a self-describing record. The reader knows where the next record starts.
b = "owlww".encode("utf-8")
rec = len(b).to_bytes(2, "big") + b
n = int.from_bytes(rec[:2], "big")
print(rec[2:2 + n].decode("utf-8"))The reader takes the length first, then that many bytes. Same rule both sides, so the word comes back.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.