bytes — Immutable Byte Sequence

Visual reference

What you will learn

• What is a byte? — the smallest practical storage unit (8 bits)
• Range 0–255 — why one byte has exactly 256 values
• bytes() constructor — build immutable binary sequences
• Indexing and slicing — read bytes like other sequences

Chapter 1 — Before Python: what is a byte?

Every image, video, PDF, password, and network message is ultimately stored as binary. Computers work with 0 and 1.

Important definitions

Unit	Size
1 bit	One binary digit (0 or 1)
1 byte	8 bits
1 KB	1024 bytes
1 MB	1024 KB
1 GB	1024 MB

Think of a byte as a locker with 8 switches — each ON (1) or OFF (0).

Chapter 2 — Why is the range 0 to 255?

A byte has 8 bits. Each bit has 2 states, so:

2⁸ = 256 combinations → values 0 through 255.

• Valid: 0, 45, 128, 255
• Invalid: -1, 256, 500 (from your notes: 256 and -34 in a list cause ValueError)

Chapter 3 — What is bytes in Python?

bytes is a pre-defined sequence type for raw binary data.

Purpose (your notes): support scenarios like end-to-end encryption and secure transmission of confidential data on networks — each stored value must be in [0, 256) i.e. 0–255.

Definition: a bytes object is an ordered collection of integers where every value is between 0 and 255.

Your notes also say there is no general symbolic notation for arbitrary byte data in programs; you usually build bytes with bytes(object). For ASCII text, Python also allows literals like b"ABC".

Real-world uses

Area	Example
Images	PNG, JPEG pixel data
Files	PDF binary storage
Networking	Packets on the internet
Encryption	Secure binary messages

Chapter 4 — Creating a bytes object

Syntax: varname = bytes(object) — commonly a list of integers:

numbers = [12, 0, 34, 255, 122, 34, 56]

b = bytes(numbers)

print(b)
print(type(b))

b'\x0c\x00"\xffz"8'
<class 'bytes'>

Why does it print strangely?

Python often displays bytes in hex escape form (\x0c, \xff). The data is the same — only the display changes.

Decimal	Hex	In repr
12	0c	\x0c
255	ff	\xff
122	7a	z (printable ASCII)

Try it in the playground

Edit the numbers = [...] list (only 0–255), run again, and watch print(b) change.

Chapter 5 — Understanding ASCII

Many byte values map to characters (ASCII).

b = bytes([65, 66, 67])
print(b)

b'ABC'

Printable ASCII often appears as letters in the output.

Chapter 6 — Iterating through bytes

b = bytes([65, 66, 67])

for value in b:
    print(value)

65
66
67

Important: iterating bytes yields integers, not one-character strings.

Object	Iteration yields
"ABC"	'A', 'B', 'C'
b"ABC"	65, 66, 67

Your notes also show reversing with b[::-1] and printing each value.

Chapter 7 — Indexing and slicing

Insertion order is preserved — index and slice like other sequences.

b = bytes([12, 0, 34, 255, 122])

print(b[0])
print(b[-1])
print(b[1:4])

12
122
b'\x00"\xff'

Operation	str	bytes
obj[0]	Character ('A')	Integer (65)
obj[1:4]	str slice	bytes slice

From your notes: b[0:4] → b'\x0c\x00"\xff'.

Chapter 8 — Bytes are immutable

bytes objects do not support item assignment.

b = bytes([65, 66, 67])
# b[0] = 90
# TypeError: 'bytes' object does not support item assignment

Think of bytes as a sealed envelope — read and copy, but do not change a byte in place.

Need mutable binary data? Use bytearray (next lesson).

Chapter 9 — Invalid byte values

# bytes([256])
# ValueError: bytes must be in range(0, 256)

# bytes([-1])
# ValueError: bytes must be in range(0, 256)

Only 256 values per byte — negatives and 256 cannot be stored.

Chapter 10 — str vs bytes

Feature	str	bytes
Stores	Characters (text)	Raw 0–255 values
Example	"ABC"	b"ABC"
obj[0]	Character	Integer
Mutable?	No	No
Typical use	UI text, messages	Files, images, network, crypto

text = "ABC"
data = b"ABC"

print(text[0])
print(data[0])

A
65

Practice exercises

1. Build bytes([65, 66, 67]) and relate values to letters.
2. Print b[0:4] for [12, 0, 34, 255, 122, 34, 56] (from your notes).
3. Try bytes([256]) once to see ValueError.

Quick knowledge check

1. Valid range per byte?
   0–255.

2. Can you run b[0] = 1?
   No — immutable.

3. Create bytes from a list?
   bytes(list_of_ints).

Summary

• A byte is 8 bits; values 0–255.
• bytes is an immutable sequence for binary data.
• Create with bytes([...]); ASCII also allows b"..." literals.
• Index → int; slice → bytes.
• Used in files, networking, encryption, images; use bytearray to mutate in place.

Next lesson: bytearray — mutable byte sequences.