Variables, Data Types & Mutability

What is a variable?

A variable is a name that refers to a value stored in memory. Think of it as a label stuck on a box. We create a variable by assigning a value with the = operator — no declaration is needed.

name = 'Ananya'        # str
age = 16               # int
height = 5.4           # float
is_monitor = True      # bool
print(name, age, height, is_monitor)
age = age + 1          # value of a variable can change
print('Next year', name, 'will be', age)

Assignment, multiple assignment and swap

a, b, c = 5, 'six', 7.0     # many values at once
print(a, b, c)
p = q = r = 0               # same value to many
print(p, q, r)
x, y = 3, 9
x, y = y, x                 # swap in one line
print(x, y)

Swapping with x, y = y, x needs no third variable — a favourite viva question.

Everything is an object: id() and type()

In Python every value — a number, a string, a list, even a function — is an object. Every object has an identity (id()), a type (type()) and a value.

x = 10
y = x
print(id(x) == id(y))     # both names point to the same object
x = x + 5                 # a NEW object 15 is created for x
print(x, y, id(x) == id(y))

Data types at a glance

Python data 📁 types/
├── 📁 Numbers/
│   ├── int        25, -7, 10**20
│   ├── float      3.14, -0.5, 2.0e3
│   ├── complex    3+4j
│   └── bool       True, False
├── 📁 Sequences/
│   ├── str        'Hello', "नमस्ते"
│   ├── list       [10, 'a', 2.5]
│   └── tuple      (10, 20, 30)
├── 📁 Set/
│   └── set        {1, 2, 3}
├── 📁 Mapping/
│   └── dict       {'roll': 1, 'name': 'Riya'}
└── None           None (no value)

print(type(25), type(3.5), type(2 + 3j))
print(type(True), type('Hi'), type(None))
print(type([1, 2]), type((1, 2)), type({1, 2}), type({'a': 1}))

Numbers: int, float, complex, bool

print(7 / 2, 7 // 2)          # float division, integer division
print(2 ** 100)               # int has no size limit
print(0.1 + 0.2)              # floats are approximate
z = 3 + 4j
print(z.real, z.imag, abs(z))
print(True + True, int(False))
print(bool(0), bool(''), bool('0'), bool([]))

Note: 0.1 + 0.2 is not exactly 0.3 because floats are stored in binary approximately. Use round(x, 2) to display money values. Empty values (0, '', []) are False; almost everything else is True — even the string '0'.

Sequences, sets, mappings and None

• str — an ordered sequence of characters in quotes: 'Python'.
• list — an ordered, changeable collection in square brackets: [90, 85, 77].
• tuple — an ordered, unchangeable collection in round brackets: (28.6, 77.2) for a location.
• set — an unordered collection of unique items: {'red', 'green'}.
• dict — key: value pairs: {'name': 'Riya', 'class': 11}.
• None — a special value meaning "nothing / no value yet".

Mutable and immutable types

L = [1, 2, 3]
M = L                # M is another name for the SAME list
M.append(4)          # list changes in place
print(L, M, L is M)

s = 'Hello'
s[0] = 'J'

The list example shows that M = L does not copy the list; both names share one list. You will learn proper copying in the list lesson.

Summary & practice

• Variable = name referring to an object; created by assignment.
• Every object has id, type and value.
• Types: int, float, complex, bool, str, list, tuple, set, dict, None.
• Mutable: list, dict, set. Immutable: numbers, str, tuple.