🔢 Data Handling

A computer handles all kinds of data — your age (a whole number), your percentage (a decimal), your name (text), and whether you passed (yes/no). Python keeps each kind in its own data type and uses operators to work on them. This chapter answers the BIG questions: What types of data does Python understand? Which data can be changed and which cannot? How do operators like /, //, % and `` work? In which order does Python solve an expression? And what kinds of errors can a program have?** Let's handle some data! 📊

4.1 🏁 Introduction

Every value in Python has a type. The type decides what the value can do. You can add two numbers, but you can't divide a name by 2!

Analogy: Think of a kitchen 🍳. Milk goes in a jug, rice goes in a jar, and eggs go in a tray. Each item has the right container. In the same way, each kind of data has its right data type.

We can always check a value's type with the type() function:

print(type(25))
print(type(25.5))
print(type("25"))
print(type(True))

Output:

<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>

4.2 🗂️ Data Types in Python

graph TD
    DT["🗂️ PYTHON DATA TYPES"]
    NUM["🔢 Numbers"]
    SEQ["📚 Sequences"]
    MAP["🗺️ Mapping"]
    NONE["🚫 None"]
    INT["int\n(whole numbers)"]
    FLT["float\n(decimals)"]
    CPX["complex\n(a + bj)"]
    BOOL["bool\n(True / False)"]
    STR["str\n(text)"]
    LST["list\n[ ]"]
    TUP["tuple\n( )"]
    DICT["dict\n{ key: value }"]

    DT --> NUM
    DT --> SEQ
    DT --> MAP
    DT --> NONE
    NUM --> INT
    NUM --> FLT
    NUM --> CPX
    NUM --> BOOL
    SEQ --> STR
    SEQ --> LST
    SEQ --> TUP
    MAP --> DICT

    style NUM fill:#2196F3,color:#fff
    style SEQ fill:#4CAF50,color:#fff
    style MAP fill:#FF9800,color:#fff
    style NONE fill:#9E9E9E,color:#fff

4.2.1 Numbers 🔢

a = 2.5E3        # 2.5 × 10³
print(a)
print(True + True)
c = 3 + 4j
print(c.real, c.imag)

Output:

2500.0
2
3.0 4.0

4.2.2 Sequences 📚

A sequence is an ordered collection of items. Each item has a position number called an index, starting from 0.

subject = "Python"
marks = [85, 90, 78]
days = ("Mon", "Tue", "Wed")
print(subject[0], marks[1], days[2])

Output:

P 90 Wed

You'll study lists in detail in the List Manipulation chapter.

4.2.3 Mapping — Dictionary 🗺️

A dictionary stores data in key : value pairs inside curly brackets { }. We look up a value by its key, not by a position.

student = {"name": "Riya", "class": 11, "marks": 92}
print(student["name"])
print(student["marks"])

Output:

Riya
92

Analogy: A dictionary is like your phone's contact list 📱 — you search by the name (key) to get the number (value).

4.2.4 None 🚫

None is a special value that means "nothing" or "no value yet". Its type is NoneType.

result = None
print(result)
print(type(result))

Output:

None
<class 'NoneType'>

4.3 🔒 Mutable and Immutable Types

This is one of the most important ideas in Python!

Analogy: A mutable type is like a whiteboard 🧑‍🏫 — you can erase one word and write another on the SAME board. An immutable type is like a printed certificate 📜 — to change it, you need a brand-new certificate!

marks = [85, 90, 78]
marks[0] = 95            # allowed — list is mutable
print(marks)

Output:

[95, 90, 78]

name = "Ravi"
name[0] = "K"            # NOT allowed — string is immutable

Output:

TypeError: 'str' object does not support item assignment

x = 5     x ──► [5]
x = 6     x ──► [6]     ([5] is left unchanged)

4.4 ➕ Operators

Operators are symbols that perform an operation on values called operands.

graph TD
    OP["➕ OPERATORS"]
    A["🧮 Arithmetic\n+ - * / // % **"]
    R["⚖️ Relational\n< > <= >= == !="]
    L["🧠 Logical\nand or not"]
    AS["📥 Assignment\n= += -= *= /="]
    M["🔍 Membership\nin, not in"]

    OP --> A
    OP --> R
    OP --> L
    OP --> AS
    OP --> M

    style A fill:#2196F3,color:#fff
    style R fill:#FF9800,color:#fff
    style L fill:#9C27B0,color:#fff
    style AS fill:#4CAF50,color:#fff
    style M fill:#795548,color:#fff

4.4.1 Arithmetic Operators 🧮

The three kinds of division — the MOST confused topic!

          17 ÷ 5
             │
   ┌─────────┼──────────┐
   ▼         ▼          ▼
17 / 5    17 // 5    17 % 5
 = 3.4     = 3        = 2
(exact)  (quotient) (remainder)

print(17 / 5)
print(17 // 5)
print(17 % 5)
print(10 / 2)
print(2 ** 10)

Output:

3.4
3
2
5.0
1024

Analogy: You have 17 laddoos 🍬 to share among 5 friends. Each friend gets 3 laddoos (17 // 5), and 2 laddoos are left over (17 % 5). If you could cut them into pieces, each would get 3.4 (17 / 5)!

Handy uses of % and //:

num = 4728
print(num % 10)          # last digit
print(num // 10)         # number without the last digit
print(num % 2 == 0)      # True if even

Output:

8
472
True

+ and * also work on strings:

print("Good" + "Morning")
print("Ha" * 3)
print("-" * 20)

Output:

GoodMorning
HaHaHa
--------------------

4.4.2 Relational (Comparison) Operators ⚖️

Relational operators compare two values and always give True or False.

print("apple" < "banana")
print("A" == "a")
print(10 == "10")

Output:

True
False
False

4.4.3 Logical Operators 🧠

Logical operators join conditions.

Truth table:

Analogy: To enter a school event you need a ticket AND an ID card 🎟️🪪 — both are required (and). To pay at a shop you can use cash OR UPI 💵📱 — either one works (or)!

age = 17
marks = 82
print(age >= 16 and marks >= 75)
print(age >= 18 or marks >= 90)
print(not age >= 18)

Output:

True
False
True

4.4.4 Assignment and Augmented Assignment Operators 📥

The augmented assignment operators are shortcuts that do an operation and store the result in the same variable.

score = 50
score += 10
score *= 2
score -= 20
print(score)

Output:

100

4.4.5 Membership Operators 🔍

in and not in check whether a value is present in a sequence (string, list, tuple) or among a dictionary's keys.

print("a" in "Python")
print(20 in [10, 20, 30])
print("Mon" not in ("Mon", "Tue"))
print("name" in {"name": "Riya", "class": 11})

Output:

False
True
False
True

4.5 🧮 Expressions and Operator Precedence

An expression is a combination of values, variables and operators that gives a single value — like a + b * 2.

When an expression has many operators, Python follows a fixed order called precedence — just like BODMAS in maths!

Solving step by step:

    10 + 6 * 3 ** 2 // 4 - 1
  = 10 + 6 * 9 // 4 - 1        ← ** first
  = 10 + 54 // 4 - 1           ← * (left to right with //)
  = 10 + 13 - 1                ← //
  = 23 - 1                     ← +
  = 22                         ← -

print(10 + 6 * 3 ** 2 // 4 - 1)
print((10 + 6) * 3)
print(2 ** 3 ** 2)
print(20 / 4 * 2)
print(5 > 3 and 2 > 4 or 7 > 1)

Output:

22
48
512
10.0
True

= 15 - 6 + 8 // 3 % 2
= 15 - 6 + 2 % 2
= 15 - 6 + 0
= 9

4.6 🔁 Type Conversion

Changing a value from one data type to another is called type conversion. It happens in two ways:

4.6.1 Implicit Conversion (Automatic) 🤖

Python automatically converts a smaller type to a bigger type when needed, so no data is lost. This is also called type promotion.

a = 10          # int
b = 2.5         # float
c = a + b       # int is converted to float automatically
print(c, type(c))
print(True + 5)

Output:

12.5 <class 'float'>
6

   bool  ──►  int  ──►  float  ──►  complex
  (smaller)                       (bigger)

4.6.2 Explicit Conversion (Type Casting) ✋

We convert the type ourselves using functions like int(), float(), str(), bool().

price = "250"
qty = 3
total = int(price) * qty
print("Total = " + str(total))
print(int(7.9), int(-7.9))
print(bool(0), bool(15), bool(""))

Output:

Total = 750
7 -7
False True False

4.7 🐞 Debugging — Types of Errors

A mistake in a program is called a bug, and finding and fixing it is called debugging. Errors are of three types:

graph TD
    E["🐞 ERRORS"]
    S["✍️ Syntax Error\n(breaks Python's grammar)\nProgram does NOT start"]
    R["💥 Runtime Error\n(problem while running)\nProgram STOPS midway"]
    L["🤔 Logical Error\n(wrong logic)\nProgram runs but\nanswer is WRONG"]

    E --> S
    E --> R
    E --> L

    style S fill:#F44336,color:#fff
    style R fill:#FF9800,color:#fff
    style L fill:#9C27B0,color:#fff

Common runtime errors:

A logical error example:

a = 80
b = 90
avg = a + b / 2          # wrong! / is done before +
print("Wrong average:", avg)
avg = (a + b) / 2        # correct
print("Right average:", avg)

Output:

Wrong average: 125.0
Right average: 85.0

Analogy: Writing a letter ✉️:

• Syntax error = spelling and grammar mistakes — the reader can't even read it.
• Runtime error = the letter is fine, but the address doesn't exist — delivery fails midway.
• Logical error = the letter reaches, but you wrote the wrong date for the party — everything "works", but the result is wrong!

⚠️ Common Errors and Misconceptions

🔑 Quick Revision — Exam Ready!

• Numbers → int (25), float (3.14, 2.5E3), complex (3+4j), bool (True = 1, False = 0)
• Sequences → str "…", list [ ], tuple ( ); index starts at 0
• Mapping → dict { key: value }; None → no value (NoneType)
• Mutable → list, dict; Immutable → int, float, complex, bool, str, tuple
• Arithmetic → / float division, // floor (quotient), % remainder, ** power
• Relational → < > <= >= == != give True/False
• Logical → and (both), or (either), not (reverse)
• Augmented → += -= *= /= //= %= **=; no ++ in Python
• Membership → in, not in
• Precedence → () > ** > unary - > * / // % > + - > comparisons > not > and > or
• Same priority → left to right, except ** (right to left)
• Implicit conversion → automatic, smaller to bigger type (int + float → float)
• Explicit conversion → int(), float(), str(), bool(); int(7.9) → 7
• False values → 0, 0.0, "", [], (), {}, None
• Errors → Syntax (grammar, doesn't run), Runtime (stops midway), Logical (wrong answer, no message)

🎯 Sample Exam Questions

Q1: Very Short Answer [1 mark each]

a) What is the output of print(type(5 / 5))? Answer: <class 'float'>

b) Name two mutable data types. Answer: List and Dictionary

c) What is the result of 19 % 4? Answer: 3

d) Which operator is used for exponent in Python? Answer: **

e) What is the output of print(bool(0))? Answer: False

f) Which error occurs when you run print(10 / 0)? Answer: ZeroDivisionError (a runtime error)

Q2: Output Based [2 marks]

a = 17
b = 4
print(a / b, a // b, a % b)
print(a ** 2 - b * 10)

Answer:

4.25 4 1
249

Q3: Output Based [2 marks]

x = 10
x += 5
x //= 2
x **= 2
print(x)
print(x > 40 and not x % 2 == 0)

Answer:

49
True

Steps: x = 15 → 15 // 2 = 7 → 7 ** 2 = 49. Then 49 > 40 is True and 49 % 2 == 0 is False, so not False is True → True and True = True.

Q4: Evaluate [2 marks]

Evaluate: 4 + 18 / 3 ** 2 * 2 - 1

Answer:

= 4 + 18 / 9 * 2 - 1
= 4 + 2.0 * 2 - 1
= 4 + 4.0 - 1
= 7.0

Q5: Short Answer [2 marks]

Identify the type of error in each and correct it: (a) print("Result" , 25 (b) area = 3.14 * r + r (to find the area of a circle)

Answer: (a) Syntax error — closing bracket is missing. Correct: print("Result", 25) (b) Logical error — the formula is wrong. Correct: area = 3.14 * r * r

Q6: Program [3 marks]

Write a program to input a number of days and print it as weeks and remaining days.

Answer:

days = int(input("Enter number of days: "))
weeks = days // 7
left = days % 7
print(weeks, "weeks and", left, "days")

Sample run (the user types 45):

Enter number of days: 45
6 weeks and 3 days

✏️ Practice Problems

1. Name the data type of each: 45, 45.0, "45", [45], (45,), True, None, 4+5j
2. Why is a list called mutable and a string immutable? Show with a small program.
3. Write the output: print(29 // 5, 29 % 5, 29 / 5, -29 // 5)
4. Evaluate: 3 + 4 * 2 ** 2 // 5 % 3
5. Write the output: print("5" * 3, 5 * 3, "5" + "3")
6. Evaluate: not 5 > 3 or 4 < 2 and 6 == 6
7. If a = 12, find a after each statement runs in order: a += 3, a %= 4, a *= 10
8. What is the difference between implicit and explicit type conversion? Give one example of each.
9. Write the output: print(int(9.99), float(9), str(9) + "9", bool(""))
10. Write a program to input a three-digit number and print the sum of its digits (use % and //).
11. Write a program to input the time in seconds and print it in hours, minutes and seconds.
12. Classify each error as syntax, runtime or logical: (a) if x > 5 without a colon (b) int("12a") (c) using s = l + b for the area of a rectangle