Literals & Variables

What is a literal?

name = "Ava"       # "Ava" is a string literal
age = 15           # 15 is an integer literal
gpa = 3.8          # 3.8 is a float literal
on_team = True     # True is a boolean literal
age = age + 1      # no literal on the left side

A literal can never be changed

The word constant in the definition means the value is fixed. 5 always means five. You can change what a variable holds, but you can never change a literal:

age = 15     # fine: the variable age now holds 15
age = 16     # fine: age now holds 16 instead
5 = 6        # error: you cannot store into a literal

Python answers that last line with SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?

Literal or expression?

The four literal types in this unit

print(type(42))      # <class 'int'>
print(type(3.14))    # <class 'float'>
print(type("42"))    # <class 'str'>
print(type(True))    # <class 'bool'>

Why does the type matter?

The type decides what an operator does. The same symbol can do very different jobs:

So before you predict what a line will do, always ask: "What type is each value?"

The type() function

type(42)        # returns <class 'int'>
type("hello")   # returns <class 'str'>

Changing a value's type

Each type name is also a function that tries to convert a value to that type. This is how you fix the input() problem above:

Converting does not change the original variable unless you store the result back, as in age = int(age).

Integer literals

How big can an int be?

In many languages, integers have a maximum size and "overflow" when they get too large. Python does not. 2 ** 100 gives the exact answer 1267650600228229401496703205376. Python just uses more memory as the number grows.

Where do negative numbers come from?

Technically, -7 is the minus sign (an operator) applied to the literal 7. You do not need to think about it that way when you write code, but it explains why there is a space allowed in - 7.

Float literals

Reading scientific notation

The e means "times 10 to the power of." Move the decimal point right for a positive power and left for a negative one:

• 1e3: move 3 places right, so 1000.0.
• 2.5e-3: move 3 places left, so 0.0025.

Scientists use this for very large or very small numbers, like the distance to the Sun (1.496e11 meters).

Living with rounding differences

Because 0.1 + 0.2 is 0.30000000000000004, the comparison 0.1 + 0.2 == 0.3 is actually False! Two habits help:

• Round before showing results: round(0.1 + 0.2, 2) gives 0.3.
• For money, many programmers count in whole cents with ints (1999 instead of 19.99) so nothing gets rounded.

String literals

Special characters: escape sequences

A backslash \ inside a string starts an escape sequence, a code for a character that is hard to type:

Each escape counts as one character. len("a\nb") is 3: a, a new line, and b.

Boolean literals

Every value has a "truth" value

The bool() function turns any value into True or False. The rule: empty or zero is False; everything else is True.

That last one surprises everyone: "False" is a non-empty string, so it counts as true. This is one more reason to keep booleans out of quotes.

Literal sorter

Choose the type of each literal, then click Check my answers. The answers come from running type() on each one. This is practice only; nothing is recorded.

Tricky ones to watch for

• "3.14" looks like a float, but the quotes win: it is a str.
• 1e3 has no decimal point, but the e makes it a float.
• 0xFF has letters in it, but it is an int written in hexadecimal.
• True without quotes is a bool; "True" with quotes is a str.

When you get one wrong, try it at a >>> prompt with type() to see the proof.

Four ways to write the same number

What "carrying" really means

In decimal, after 9 you run out of digits, so you write a 0 and carry a 1 into the next place: 9, then 10. Binary works exactly the same way, it just runs out sooner. After 1 there are no more digits, so binary goes 0, 1, then 10 (which means two).

That leads to an old programmer joke: "There are 10 kinds of people in the world: those who understand binary and those who don't." (Read 10 in binary.)

Reading binary: place values

  1    1    0    1      (binary 1101)
  8    4    2    1      (place values)
  8 +  4 +  0 +  1 = 13

Going the other way: decimal to binary

To write 45 in binary, take the largest place value that fits, subtract it, and repeat:

1. The places are 32, 16, 8, 4, 2, 1. The biggest that fits in 45 is 32. 45 − 32 = 13.
2. 16 does not fit in 13, so that place gets a 0. 8 fits: 13 − 8 = 5.
3. 4 fits: 5 − 4 = 1. 2 does not fit (0). 1 fits: 1 − 1 = 0. Done.

32  16   8   4   2   1
 1   0   1   1   0   1     45 = 101101 in binary

Biggest value for a number of bits: with n bits, the largest number is 2ⁿ − 1. Four bits go up to 15, and eight bits (one byte) go up to 255.

Octal and hexadecimal

The shortcut: group the bits

Because one hex digit is exactly 4 bits, you can convert binary to hex without doing any big math. Split the binary number into groups of 4 from the right, then convert each group:

1011 0110
  B    6        binary 10110110 = hex B6 = 182

Octal works the same way with groups of 3 bits.

What 755 means in file permissions

Each octal digit is 3 bits, and each bit is a yes/no switch for read, write, and execute. 7 = 111 means all three; 5 = 101 means read and execute, but not write. So 755 means "the owner can do everything; everyone else can read and run it."

Writing and converting bases in Python

What these tools really give you

• A prefix literal like 0xFF is just another way to type an int. Once stored, Python forgets how you wrote it: print(0xFF) shows 255.
• bin(), oct(), and hex() go from an int to text for showing people. You cannot do math with their result: hex(10) + 1 is a TypeError.
• int(text, base) goes from text back to an int. The text must not have a space and must only use digits that exist in that base. int("1010", 2) is 10; uppercase and lowercase hex both work.

Declaring variables

player_name = "Ava"

How Python reads score = score + 10

This line looks impossible in math, but it makes sense in Python because the right side runs first:

1. Python looks up the current value of score (say 0).
2. It calculates 0 + 10, which is 10.
3. Only then does it store 10 into score, replacing the old 0.

Two handy shortcuts

x = y = 0          # both x and y get 0
width, height = 4, 3   # width gets 4, height gets 3

A variable is a label, not a box. Think of the name as a sticky note attached to a value. Assigning again moves the sticky note to a new value.

Naming variables: rules and PEP 8 style

Words you cannot use as names

Python has 35 keywords that are part of its grammar, such as if, else, for, while, def, return, import, and, or, not, True, False, and None. To see the whole list, run import keyword then print(keyword.kwlist).

Good names answer "what is this?" count is OK, student_count is better. A name can be long; clarity matters more than saving a few keystrokes.

Common mistakes

Error messages are from Python 3.13. Older versions word some messages a little differently.

The sneakiest bugs give no error

Two mistakes in this table run without any message: x = 1,000 and using text where you meant a number (like keeping input() as a string and then comparing it). For those, use print(x, type(x)) to see what the variable really holds.

Check yourself

Choose an answer, then click Check. This is practice only. Nothing is graded or recorded.

Cheat sheet

0b1010   # binary, base 2
0o12     # octal, base 8
10       # decimal, base 10
0xA      # hex, base 16

bin(10)  # '0b1010'
oct(10)  # '0o12'
hex(10)  # '0xa'
int("ff", 16)  # 255

high_score = 0     # snake_case
player_name = "Ava"
MAX_LIVES = 3      # constant

Before you run, check:

• No commas or leading zeros in numbers
• True and False are capitalized; numbers you want to do math with are not in quotes
• Binary uses only 0-1, octal only 0-7, hex only 0-9 and A-F
• Every variable has a descriptive snake_case name and is assigned before it is used

Key words to know

literal (a value typed in code) · variable (a name for a value) · int / float / str / bool · type() · base (how many digits a number system uses) · bit and byte · prefix (0b, 0o, 0x) · snake_case · PEP 8