Variables, Data Types and Operators

Variables: labelled boxes for your data

A variable is just a name that points to a value. You create one with an equals sign — the name on the left, the value on the right:

age = 19
city = "Ahmedabad"
price = 49.99
is_student = True

Read = as "is set to", not "equals" in the math sense. age = 19 means "make a box called age and put 19 in it". You can change what's in the box any time, and you can use the box's name anywhere you'd use the value:

age = 19
print(age)        # shows 19
age = age + 1     # take what's in age, add 1, put it back
print(age)        # shows 20

Good variable names are lowercase and describe what they hold — total_price, user_name, num_students. Avoid x and data for anything that matters; future-you will thank present-you.

The core data types

Every value in Python has a type, and the type decides what you can do with it. The four you'll use constantly:

• int — whole numbers: 19, -4, 1000000
• float — numbers with a decimal point: 49.99, 3.14, -0.5
• str — text, always in quotes: "Ahmedabad", 'hello'
• bool — a truth value, either True or False

You can ask Python the type of anything with type():

print(type(19))        # <class 'int'>
print(type(49.99))     # <class 'float'>
print(type("hello"))   # <class 'str'>
print(type(True))      # <class 'bool'>

Python figures out the type automatically from the value — you never have to declare it. This is called dynamic typing, and it's a big part of why Python feels so light.

Doing math with operators

Python is a calculator that never gets tired. The arithmetic operators:

print(7 + 3)    # 10   addition
print(7 - 3)    # 4    subtraction
print(7 * 3)    # 21   multiplication
print(7 / 3)    # 2.333...  division (always a float)
print(7 // 3)   # 2    floor division (drops the decimal)
print(7 % 3)    # 1    modulo (the remainder)
print(7 ** 3)   # 343  exponent (7 to the power 3)

Two of these surprise beginners. The single slash / always gives a decimal, even 6 / 2 gives 3.0. And the modulo % gives the remainder after division — 7 % 3 is 1 because 3 goes into 7 twice with 1 left over. Modulo is secretly one of the most useful operators: number % 2 == 0 is how you check if a number is even.

Type conversion — the classic beginner trap

Remember input() from the last chapter? It always gives you back text, even if the user typed a number. This bites everyone once:

age = input("Your age: ")   # user types 19
print(age + 1)              # 💥 ERROR — can't add a number to text

The fix is to convert the text into a number with int() (or float() for decimals):

age = int(input("Your age: "))   # convert text "19" into number 19
print(age + 1)                   # 20 — works!

You can convert the other way too — str(20) turns the number 20 into the text "20", which you need when gluing numbers into a sentence with +. Keep this rule in your head: input gives text; convert before you calculate.

f-strings: the clean way to build text

Gluing text with + gets ugly fast, especially with numbers you have to convert. Modern Python has a much nicer tool called the f-string. Put an f before the opening quote, then drop variables right inside curly braces:

name = "Mehul"
age = 19
print(f"Hi {name}, next year you'll be {age + 1}.")
# Hi Mehul, next year you'll be 20.

Notice you can even do math inside the braces, and you never have to call str() — Python handles the conversion. From here on, f-strings are the way we'll build all our output. They're cleaner, easier to read, and far less error-prone. Practise the pattern until it's automatic, because you will type it thousands of times.