Session 2 · Variables, constants and data types#

📖 Based on the blog post Python Fundamentals: Constants, Variables and Data Types.

The big idea#

Programs remember things in variables: a name for a value, so you can use it again. Every value also has a type (number, text, list…) that decides what you can do with it: you can add numbers, join text, add items to a list.

Everyday example: labelled jars in a kitchen. The label (“sugar”) is the variable name; what’s inside is the value; and you don’t pour rice into the tea, because the kind of thing matters. (In Session 3 we’ll see the label is really a tag pointing to the jar.)

Making a variable#

age = 25                 # int
name = "John Doe"        # str
height = 5.9             # float
is_student = True        # bool
print(name, age, height, is_student)

John Doe 25 5.9 True

= means “assign”, not “is equal to”. age = 25 means “let the name age refer to 25”.

Naming rules#

✅ Valid

❌ Invalid

Why invalid

my_variable

1variable

can’t start with a digit

variable1

my-variable

- means minus

_hidden

for

for is a Python keyword

userName

my variable

no spaces

Names are case-sensitive: age, Age and AGE are three different variables. Python’s convention is snake_case: lowercase words joined by underscores.

Many at once, swapping and unpacking#

a, b, c = 5, 10, 15            # three variables in one line
print(a, b, c)

x, y = 1, 2
x, y = y, x                    # swap without a temporary variable
print(x, y)

person = ("Alice", 30, "Engineer")
name, age, profession = person     # unpack a tuple into variables
print(name, profession)

5 10 15
2 1
Alice Engineer

Dynamic typing#

You never declare a type. Python works it out from the value, and a name can later point to a different type:

my_var = 42
print(type(my_var))
my_var = "Python"
print(type(my_var))

<class 'int'>
<class 'str'>

Constants#

A constant is a value that shouldn’t change, like π or the maximum number of users. Python has no real constants; by convention we write them in UPPER_CASE so everyone knows not to change them:

PI = 3.14159
MAX_USERS = 100

Python won’t stop you writing PI = 3, but your teammates will!

The data types#

Python's built-in data types Every value has a type. The type decides what you can do with it: add numbers, join strings, change a list, look up a key in a dictionary. Numbers int 42 float 3.14 complex 2+3j Text · Bool · None str "Idly" bool True NoneType None Sequences list [1, 2] tuple (1, 2) range range(5) Mapping · Sets dict {"a": 1} set {1, 2} frozenset
Every value has a type. The type decides what you can do with it: add numbers, join strings, change a list, look up a key in a dictionary.

Type

Example

Notes

int

42, -7

whole numbers, any size

float

3.14, 2.0

decimals

complex

2 + 3j

for maths and engineering

str

"Idly"

text, in quotes

bool

True, False

yes/no

NoneType

None

“no value”

list

[1, 2, 3]

ordered, changeable (Session 7)

tuple

(1, 2, 3)

ordered, unchangeable (Session 8)

range

range(5)

a sequence of numbers 0–4

dict

{"rice": 60}

key → value (Session 9)

set

{1, 2, 3}

unique items (Session 10)

frozenset

frozenset({1, 2})

a set that can’t change

Note

Since Python 3.7, dictionaries keep the order in which you add keys. Older material (including some blog posts) still calls them “unordered”.

Converting between types#

print(int("42") + 1)       # text → number
print(str(42) + "!")       # number → text
print(float("3.5"))
print(int(3.99))           # cuts off the decimals, doesn't round
print(bool(0), bool(""), bool("0"))

43
42!
3.5
3
False False True

0, "", [], {} and None count as False; almost everything else is True, even the text "0".

Why data types matter#

The type decides which operations work:

print(5 + 3)        # numbers add
print("5" + "3")    # strings join
print("5" * 3)      # a string repeats

8
53
555

Mixing them ("5" + 3) raises a TypeError. Python refuses to guess what you meant.

Common mistakes#

  • Using = to compare. if age = 18: is a syntax error. Comparison is == (Session 4).

  • Quotes around numbers. age = "25" is text; age + 1 fails. Use age = 25.

  • Overwriting built-ins. list = [1, 2] or print = 5 hides Python’s own list/print for the rest of the program.

  • Expecting int(3.99) to round. It truncates to 3. Use round(3.99) for 4.

Hands-on exercises#

Exercise 1 · Introduce yourself. Store your name, age, city and whether you’re a student in four variables, then print one sentence using all four.

Solution 
name = "Priya"
age = 21
city = "Madurai"
is_student = True
print(f"I am {name}, {age} years old, from {city}. Student: {is_student}")

Exercise 2 · Valid or not? Which of these names are valid: total_marks, 2nd_place, _count, class, firstName, price$?

Answer

Valid: total_marks, _count, firstName. Invalid: 2nd_place (starts with a digit), class (keyword), price$ ($ isn’t allowed).

Exercise 3 · Swap. Given a = "tea" and b = "coffee", swap them in one line and print both.

Solution 
a, b = "tea", "coffee"
a, b = b, a
print(a, b)     # coffee tea

Exercise 4 · What type? Print the type of 10, 10.0, "10", True, None, [10], (10,) and {10}.

Solution 
for value in [10, 10.0, "10", True, None, [10], (10,), {10}]:
    print(repr(value), type(value))

int, float, str, bool, NoneType, list, tuple, set.

Exercise 5 · Circle area. Use a constant PI = 3.14159 and a variable radius = 7 to print the area (π r²) with two decimals.

Solution 
PI = 3.14159
radius = 7
area = PI * radius ** 2
print(f"Area: {area:.2f}")      # Area: 153.94

Exercise 6 · Fix the bug. age = "25"; print("Next year you'll be " + age + 1). Make it print Next year you'll be 26.

Solution 
age = "25"
print("Next year you'll be " + str(int(age) + 1))
# or simply: print(f"Next year you'll be {int(age) + 1}")

Convert the text to a number to add, then back to text to join.

Exercise 7 · Unpack. Unpack ("Ooty", 2024, "July") into three variables and print them on one line separated by |.

Solution 
place, year, month = ("Ooty", 2024, "July")
print(place, year, month, sep=" | ")     # Ooty | 2024 | July