Variables, numbers, strings, and f-strings

Why this matters

Programs are useful because they remember things: a user's name, a price, a score, a piece of text from a website. Variables are how you store and reuse values by giving them names. Once you can store a value and then use it later, you can write programs that do more than print a fixed line — you can write programs that work on different input each time they run. That is the moment Python stops being a calculator and starts being a language.

Learning objectives

After this lesson you will be able to:

1. Create a variable and read its value back.
2. Name variables the way Python developers expect.
3. Do simple arithmetic with numbers.
4. Work with text (strings) and combine them cleanly with f-strings.
5. Convert between numbers and text without crashing.

1. Variables — giving a value a name

A variable is a name that points to a value. You create one with =:

name = "Ada"
age = 30

Now name holds "Ada" and age holds 30. You can use them anywhere you would have typed the value:

print(name)     # Ada
print(age)      # 30

In Python, = does not mean "equals" in the math sense. It means "assign this value to this name." So age = 30 reads as "let the name age refer to the value 30."

Reading the value back

Use the name wherever you want its value:

print(name, age)

Prints Ada 30.

Reassigning

You can point a name at a new value at any time:

score = 0
score = score + 10     # take the old score, add 10, store it back
print(score)           # 10

The line score = score + 10 works right-to-left: Python first works out the right side (0 + 10), then points the name score at that result.

2. Naming rules and style

Python is relaxed but has conventions. Follow them and your code reads well to everyone.

• Use lowercase letters, numbers, and underscores: user_name, total_price.
• Start with a letter or underscore, not a digit: price2 is fine, 2price is not.
• Do not use Python's reserved words (like if, for, while, class) as a variable name.
• Use snake_case (words separated by underscores) for variables and functions.
• Use meaningful names. user_name is better than n or x.

3. Numbers

Python has integers and floats.

count = 5          # int — a whole number
pi = 3.14159       # float — a number with a decimal

Basic arithmetic:

a = 10
b = 3

a + b        # 13    addition
a - b        # 7     subtraction
a * b        # 30    multiplication
a / b        # 3.333...   true division (always a float)
a // b       # 3     floor division (drops the remainder)
a % b        # 1     remainder (modulo)
a ** b       # 1000  power

The two that confuse beginners are / and //:

• 10 / 3 gives 3.3333... (a float).
• 10 // 3 gives 3 (the whole-number part).
• 10 % 3 gives 1 (the remainder, because 3 goes into 10 three times with 1 left over).

You can do arithmetic with variables too:

price = 20
quantity = 3
total = price * quantity
print(total)        # 60

4. Strings — text

A string is text inside quotes. Strings can use single or double quotes — both work; pick one and be consistent.

greeting = "hello"
name = 'Ada'

Combine strings with +

first = "Ada"
last = "Lovelace"
full = first + " " + last
print(full)     # Ada Lovelace

+ glues strings together. Note you must add the space yourself: " ".

Common string actions

word = "python"

word.upper()          # 'PYTHON'
word.lower()          # 'python'
word.capitalize()     # 'Python'
len(word)             # 6   (length — how many characters)
word[0]               # 'p'  (first character; counting starts at 0)
word[-1]              # 'n'  (last character)
word[0:3]             # 'pyt' (slice: characters 0,1,2)

Do not worry about slicing yet — just know it exists. We cover it properly later.

5. f-strings — the clean way to build text

Building strings with + gets messy fast, especially with numbers:

age = 30
message = "I am " + str(age) + " years old"   # clunky

The modern, clean way is an f-string. Put an f before the opening quote and braces around the value:

age = 30
name = "Ada"
print(f"I am {age} years old")          # I am 30 years old
print(f"{name} is {age}")               # Ada is 30

You can even put calculations inside the braces:

price = 20
quantity = 3
print(f"Total: {price * quantity}")

f-strings are the way to build text in Python. Always prefer them over + string concatenation.

6. Converting between text and numbers

Sometimes a number is stored as a string (for example, text a user typed in a box), and you need to do math with it. Python will not let you mix them.

# '5' is a string; 2 is an int. These two cannot be added.
"5" + 2        # ERROR: can only concatenate str, not int

You convert explicitly:

int("5")       # 5     string -> int
float("3.5")   # 3.5   string -> float
str(42)        # "42"  number  -> string

If the text is not a valid number, Python raises an error:

int("hello")   # ERROR: invalid literal for int()

That error is fine — it tells you the input was not a number, and later you will learn how to handle that gracefully.

Common mistakes

1. Forgetting quotes around text. name = Ada → NameError. Use name = "Ada".
2. Mixing types. "5" + 2 → TypeError. Convert first.
3. Reusing = like math. x = x + 1 is not "x equals x plus 1"; it is "update x." It is fine and extremely common.
4. Spelling a variable name differently. user_name and username are different names. Be consistent.
5. Off-by-one with indexing. word[0] is the first character, not word[1]. Python counts from 0.

Hands-on lab (45 minutes)

1. Create a variable name with your name and print a greeting using an f-string.
2. Create item_price = 120 and tax = 0.18. Print the total (item_price * tax).
3. Compute 17 // 5 and 17 % 5. Explain in one sentence what each gives.
4. Print "Hello" + " " + "world" then rewrite it as an f-string.
5. Convert "2026" to an int, add 4, and print "2030".
6. Challenge: a rectangle is width = 8, height = 5. Print its area and its perimeter as "The perimeter is X".

Self-check

1. What does = do in Python?
2. What is the difference between / and //?
3. Why use f-strings instead of +?
4. What does int("5") + 2 give? What does "5" + "2" give?
5. How do you get the last character of a string?

References

• The Python Tutorial — Using Python as a Calculator
• The Python Tutorial — Strings
• Real Python — Python Strings
• Python f-strings (PEP 498)