Unit 11.1A · Term 1

Data Types & Type Conversion

Every value in Python has a type that determines what operations can be performed on it. Understanding data types and knowing how to convert between them is fundamental to writing correct programs.

Learning Objectives

  • 11.1.1.3 Distinguish between data types in Python
  • 11.1.1.4 Convert data types of variables

Lesson Presentation

11.1A-lesson-02-data-types.pdf · Slides for classroom use

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Conceptual Anchor

The Container Analogy

Think of data types as different types of containers. A glass holds liquid (numbers), an envelope holds letters (strings), and a light switch is either on or off (boolean). You can pour liquid from a glass into a measuring cup (type conversion), but you can't pour a letter into a glass — the types must be compatible.

Rules & Theory

Python Data Types

Type Description Examples Check with
int Whole numbers (no decimals) 42, -7, 0 type(42) → <class 'int'>
float Decimal numbers 3.14, -0.5, 2.0 type(3.14) → <class 'float'>
str Text (sequence of characters) "Hello", 'A', "123" type("Hi") → <class 'str'>
bool Logical value: True or False True, False type(True) → <class 'bool'>
# Checking types x = 10 y = 3.14 name = "Ali" is_student = True print(type(x)) # <class 'int'> print(type(y)) # <class 'float'> print(type(name)) # <class 'str'> print(type(is_student)) # <class 'bool'>

Type Conversion Functions

Function Converts to Example Result
int() Integer int("42") 42
int() Integer (truncates) int(3.99) 3 (not rounded!)
float() Float float("3.14") 3.14
float() Float float(5) 5.0
str() String str(42) "42"
bool() Boolean bool(0) False
bool() Boolean bool(1) True

Important: int() truncates, not rounds!

int(3.99) gives 3, not 4. It simply removes the decimal part. To round, use round(3.99) which gives 4.

Worked Examples

1 Distinguishing Types

# What type is each value? a = 100 # int — whole number b = 100.0 # float — has decimal point c = "100" # str — in quotes d = True # bool — True/False keyword # Note: "100" is NOT the same as 100! print(type("100")) # <class 'str'> print(type(100)) # <class 'int'>

2 Converting User Input

# input() always returns a string! age_str = input("Enter your age: ") # e.g., "16" print(type(age_str)) # <class 'str'> # Convert to int for calculations age = int(age_str) print(type(age)) # <class 'int'> print("Next year you'll be:", age + 1) # Next year you'll be: 17 # One-line version age = int(input("Enter your age: "))

3 Mixing Types in Expressions

# int + float → float (automatic promotion) result = 5 + 2.0 print(result) # 7.0 print(type(result)) # <class 'float'> # int / int → float (in Python 3) result = 10 / 3 print(result) # 3.3333333333333335 # String + number → ERROR! # print("Age: " + 16) # TypeError! print("Age: " + str(16)) # Age: 16 ✓

4 Boolean Conversion Rules

# What values are "truthy" vs "falsy"? print(bool(0)) # False — zero is falsy print(bool(42)) # True — any non-zero is truthy print(bool("")) # False — empty string is falsy print(bool("Hi")) # True — non-empty string is truthy print(bool(0.0)) # False print(bool(-1)) # True — even negative numbers!

Pitfalls & Common Errors

Converting Invalid Strings

int("hello") or int("3.14") will crash with a ValueError. You can only convert strings that look like valid integers: int("42") ✓. For decimals, use float("3.14") first, then int() if needed.

Confusing "100" and 100

The string "100" and the integer 100 look similar but behave very differently. "100" + "200" gives "100200" (concatenation), while 100 + 200 gives 300 (addition).

Forgetting input() Returns String

x = input("Number: ") gives a string, even if the user types 5. You must convert: x = int(input("Number: ")).

Pro-Tips for Exams

Type Questions Strategy

  • If it has a decimal point, it's a float (even 5.0)
  • If it's in quotes, it's a str (even "123")
  • If it's True or False (capitalized), it's bool
  • int() truncates (cuts off decimal), it does NOT round
  • Python 3: 10 / 3 = 3.333... (float), 10 // 3 = 3 (int)

Graded Tasks

Remember

Name the four basic data types in Python and give two examples of each.

Understand

Explain why int(7.9) returns 7 and not 8.

Apply

Write a program that asks the user for their birth year (as a string), converts it to an integer, and calculates their age.

Analyze

Predict the output and type of each: "5" + "3", 5 + 3, 5.0 + 3, int("5") + 3

Create

Write a temperature converter: input Celsius as a string, convert to float, calculate Fahrenheit (F = C × 9/5 + 32), and output the result.

Self-Check Quiz

1. What type does input() always return?
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2. What is the result of int(4.7)?
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3. What is type(True)?
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4. What is bool("")?
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5. What is the output of print(type(10 / 2))?
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