JustLearn
Python Course
Beginner2 hours

Lesson 6: Type Conversion

Course: Data Types & Variables | Duration: 2 hours | Level: Beginner

Learning Objectives

  • Convert between int, float, str, and bool using built-in functions
  • Distinguish between implicit and explicit type conversion
  • Handle conversion errors gracefully
  • Apply type conversion in real-world data processing
  • Use isinstance() to check types before conversion

Prerequisites

  • Lessons 1-5 of this course

Lesson Outline

Part 1: Explicit Type Conversion (30 minutes)

Explanation

Type conversion (also called type casting) converts a value from one type to another.

Why we need it: input() always returns a string. To do math, you must convert.

python
# Explicit conversions using built-in functions:
 
# int() - converts to integer
print(int("42"))       # 42
print(int(3.7))        # 3  (truncates decimal - doesn't round!)
print(int(3.2))        # 3
print(int(True))       # 1
print(int(False))      # 0
# print(int("hello"))  # ValueError! Can't convert text to int
 
# float() - converts to float
print(float("3.14"))   # 3.14
print(float("42"))     # 42.0
print(float(5))        # 5.0
print(float(True))     # 1.0
# print(float("abc")) # ValueError!
 
# str() - converts to string (everything can be converted to string)
print(str(42))         # "42"
print(str(3.14))       # "3.14"
print(str(True))       # "True"
print(str(None))       # "None"
 
# bool() - converts to boolean
print(bool(0))         # False
print(bool(1))         # True
print(bool(""))        # False
print(bool("hello"))   # True
print(bool(None))      # False

Examples

python
# Real-world: processing user input
user_age = input("Enter your age: ")
print(type(user_age))   # <class 'str'>
 
age_as_int = int(user_age)
print(type(age_as_int)) # <class 'int'>
 
birth_year = 2024 - age_as_int
print(f"You were born around {birth_year}")
 
# One-liner (common pattern):
age = int(input("Enter your age: "))

Practice

Write a program that:

  1. Reads 3 values as strings (they happen to be numbers)
  2. Converts them to appropriate types
  3. Performs a calculation
  4. Converts the result back to a formatted string for display

Part 2: Implicit Conversion (Type Promotion) (30 minutes)

Explanation

Python sometimes converts types automatically (implicitly) in mixed-type operations:

python
# int + float → float (automatic promotion)
result = 5 + 3.0
print(result)        # 8.0
print(type(result))  # float
 
# int + bool → int
result = 10 + True
print(result)        # 11  (True is 1)
 
# Comparison: Python does NOT implicitly convert str to int
# "5" + 5 → TypeError: must be str, not int
# "5" + str(5) → "55"  (string concatenation)
# int("5") + 5 → 10    (explicit conversion)

The == comparison with type mismatch:

python
print(5 == 5.0)       # True  (numeric equality across types)
print(5 == "5")       # False (Python doesn't implicitly compare int to str)
print(True == 1)      # True  (bool is subtype of int)
print(True == "True") # False

Examples

python
# Counting using booleans as integers
data = [1, -5, 3, -2, 8, -1, 4]
# Count positive numbers
positive_count = sum(x > 0 for x in data)   # Each comparison is True (1) or False (0)
print(f"Positive numbers: {positive_count}")

Practice

Create 5 expressions with mixed types and predict the result type before running them.

Part 3: Safe Conversion Patterns (30 minutes)

Explanation

Raw int() and float() crash on invalid input. Production code needs safer conversion:

python
# UNSAFE:
age = int(input("Age: "))   # Crashes if user types "twenty-five"
 
# SAFE with exception handling (preview of Course 7):
try:
    age = int(input("Age: "))
    print(f"Age: {age}")
except ValueError:
    print("Please enter a valid number!")
 
# Check before converting:
text = input("Enter a number: ")
if text.isdigit():
    number = int(text)
    print(f"Doubled: {number * 2}")
else:
    print("That's not a positive integer!")
 
# For floats, isdigit() doesn't work (won't handle decimals or negatives)
# Use try/except instead

Using isinstance() to check type:

python
value = 42
 
print(isinstance(value, int))         # True
print(isinstance(value, float))       # False
print(isinstance(value, (int, float))) # True (check multiple types)
print(isinstance(value, bool))        # False (42 is int, not bool)
 
# Note: bool is a subtype of int!
flag = True
print(isinstance(flag, bool))   # True
print(isinstance(flag, int))    # True  (bool IS an int)
print(type(flag) == bool)       # True  (exact type check)
print(type(flag) == int)        # False

Examples

python
# Robust input handler
def get_number(prompt, data_type=float):
    """Keep asking until user provides valid number."""
    while True:
        try:
            return data_type(input(prompt))
        except ValueError:
            print(f"  Invalid! Please enter a valid {data_type.__name__}.")
 
# Usage:
age = get_number("Your age: ", int)
price = get_number("Item price: $", float)
print(f"In 5 years: {age + 5}, plus tax: ${price * 1.15:.2f}")

Practice

Build a "type-safe input collector" that asks for 3 different types of input (integer, float, string with minimum length) and validates each before proceeding.

Part 4: Hands-on Practice (30 minutes)

Exercise 1: Data Type Converter

Build a program that:

  1. Asks the user for a value
  2. Attempts to convert it to int, float, bool
  3. Shows which conversions succeed and their results
  4. Shows what type the original input was

Exercise 2: Contact Importer

Simulate importing contact data from CSV text:

python
# Raw CSV data (as strings)
raw_data = [
    "Alice Johnson, 28, 95000.50, True",
    "Bob Smith, 35, 72000.00, False",
    "Carol Davis, 42, 120000.75, True",
]
 
# Parse and convert each field to appropriate types:
# name (str), age (int), salary (float), is_manager (bool)
# Display in formatted table

Bonus Challenge

Safe calculator: Build a calculator that:

  1. Gets two values from user (they could enter text or numbers)
  2. Validates both are valid numbers
  3. Gets an operator (+, -, *, /)
  4. Validates division by zero
  5. Computes and displays result with proper type handling

Key Takeaways

  • int(), float(), str(), bool() - explicit type conversion functions
  • int(3.7) → 3 (truncates, doesn't round)
  • int("hello") → ValueError (strings that aren't numbers can't be converted)
  • Implicit conversion: int + float → float (Python promotes automatically)
  • isinstance(value, type) - check type without crashing
  • Use try/except for safe conversion of user input (Course 7 will cover this deeply)

Common Mistakes to Avoid

  • int("3.14"): fails! Must float("3.14") then int() if needed
  • Confusing type() == vs isinstance(): isinstance(True, int) is True (bool inherits from int)
  • Not converting input: forgetting int() around input() then doing math

Homework

  1. Build a data normalizer: takes a list of mixed-type strings and converts each to the most appropriate type (int, float, or keep as string)
  2. Explore: What does int("0x1A", 16) do? What about int("1111", 2)?

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Concept Map

Practice Exercises

Complete these exercises to practice what you learned. Run your code and check if the output matches.