Python Type Conversion

In Python, different values have different data types.

For example:

name = "Raj"       # string
age = 25           # integer
height = 5.8       # float
active = True      # boolean

Sometimes we need to convert a value from one data type to another.

This process is called Type Conversion or Type Casting.

For example:

age = "25"

age = int(age)

print(age)
print(type(age))

Output:

25
<class 'int'>

The string "25" has been converted into the integer 25.

1. What is Type Conversion?

Type conversion is the process of changing a value from one data type into another data type.

Common conversions include:

str → int
str → float
int → float
float → int
int → str
float → str
int → bool
list → tuple
tuple → list
list → set
set → list

Python provides built-in functions to perform these conversions.

Some of the most commonly used functions are:

int()
float()
str()
bool()
list()
tuple()
set()
dict()

2. Why Do We Need Type Conversion?

One common situation is receiving information from a user.

Consider:

age = input("Enter your age: ")

print(type(age))

If the user enters:

25

Python receives it as:

<class 'str'>

Even though the user entered a number, input() returns a string.

If we want to perform mathematical calculations, we need to convert it.

age = int(input("Enter your age: "))

print(age + 5)

If the user enters:

25

Output:

30

3. int() - Convert to Integer

The int() function converts a compatible value into an integer.

Example:

x = "25"

y = int(x)

print(y)
print(type(y))

Output:

25
<class 'int'>

4. String to Integer

age = "25"

age = int(age)

print(age + 5)

Output:

30

Before conversion:

"25"

After conversion:

25

The first is a string and the second is an integer.

5. Float to Integer

A floating-point number can also be converted to an integer.

price = 99.99

price = int(price)

print(price)

Output:

99

Important

int() does not round the number to the nearest integer.

It removes the fractional part.

For example:

print(int(9.8))
print(int(9.2))
print(int(-9.8))

Output:

9
9
-9

For negative numbers, this moves toward zero.

6. Integer to Integer

If the value is already an integer:

x = 50

y = int(x)

print(y)

The value remains an integer.

7. String Containing a Decimal

This will cause an error:

price = "99.99"

price = int(price)

Why?

Because "99.99" is not a valid integer representation.

Instead:

price = "99.99"

price = float(price)

print(price)

Output:

99.99

8. float() — Convert to Floating-Point Number

The float() function converts a compatible value into a floating-point number.

Example:

x = "25.5"

y = float(x)

print(y)
print(type(y))

Output:

25.5
<class 'float'>

9. Integer to Float

x = 25

y = float(x)

print(y)
print(type(y))

Output:

25.0
<class 'float'>

Python adds .0 because the result is now a floating-point value.

10. String to Float

price = "1500.75"

price = float(price)

print(price)

Output:

1500.75

This is particularly useful when working with prices, measurements, percentages, and calculations.

11. Float to Float

x = 10.5

y = float(x)

print(y)

The value remains a float.

12. str() - Convert to String

The str() function converts a value into a string.

Example:

age = 25

age_text = str(age)

print(age_text)
print(type(age_text))

Output:

25
<class 'str'>

13. Why Convert to String?

Suppose:

age = 25

message = "My age is " + age

This produces an error because Python cannot directly concatenate a string and an integer using +.

Instead:

age = 25

message = "My age is " + str(age)

print(message)

Output:

My age is 25

14. Combining Different Types

You can also use str() when constructing text.

name = "Raj"
age = 25

print("My name is " + name + " and my age is " + str(age))

Output:

My name is Raj and my age is 25

Later, we will learn cleaner approaches using f-strings.

15. bool() - Convert to Boolean

The bool() function converts a value into either:

True

or:

False

Example:

x = 10

print(bool(x))

Output:

True

16. Boolean Conversion of Numbers

Generally:

0 → False
non-zero → True

Examples:

print(bool(0))
print(bool(1))
print(bool(10))
print(bool(-5))

Output:

False
True
True
True

17. Boolean Conversion of Strings

An important rule:

An empty string is False; a non-empty string is True.

print(bool(""))
print(bool("Hello"))
print(bool("0"))
print(bool("False"))

Output:

False
True
True
True

Notice:

bool("False")

is:

True

because "False" is a non-empty string.

Python does not interpret the text "False" as the Boolean value False.

18. Boolean Conversion of Lists

An empty collection is generally False.

print(bool([]))
print(bool([1, 2, 3]))

Output:

False
True

Similarly:

print(bool(()))
print(bool((1, 2)))

Output:

False
True

And:

print(bool(set()))
print(bool({1, 2}))

Output:

False
True

19. The Main False Values

Students should remember these common cases:

bool(0)
bool(0.0)
bool("")
bool([])
bool(())
bool(set())
bool({})
bool(None)

All produce:

False

Most other values are considered truthy.

This concept becomes extremely important when we learn if statements.

20. Converting a List to a Tuple

Use:

tuple()

Example:

fruits = ["apple", "mango", "banana"]

fruits_tuple = tuple(fruits)

print(fruits_tuple)
print(type(fruits_tuple))

Output:

('apple', 'mango', 'banana')
<class 'tuple'>

21. Converting a Tuple to a List

Use:

list()

Example:

fruits = ("apple", "mango", "banana")

fruits_list = list(fruits)

print(fruits_list)
print(type(fruits_list))

Output:

['apple', 'mango', 'banana']
<class 'list'>

This is useful when you need to modify a tuple's contents.

22. Converting a List to a Set

Use:

set()

Example:

numbers = [1, 2, 2, 3, 4, 4, 5]

unique_numbers = set(numbers)

print(unique_numbers)

The duplicate values are removed.

Possible output:

{1, 2, 3, 4, 5}

Important

Sets are unordered collections, so you should not depend on their displayed order.

23. Converting a Set to a List

numbers = {10, 20, 30}

numbers_list = list(numbers)

print(numbers_list)

The result is a list.

Remember that the ordering of a set should not be relied upon when converting it to another collection.

24. Converting a String to a List

Using:

list()

on a string creates a list containing its individual characters.

word = "Python"

letters = list(word)

print(letters)

Output:

['P', 'y', 't', 'h', 'o', 'n']

This is different from splitting a sentence into words.

For that, we use:

sentence = "Python is easy"

words = sentence.split()

print(words)

Output:

['Python', 'is', 'easy']

25. Converting a String to a Set

word = "banana"

letters = set(word)

print(letters)

The set contains unique characters.

For example, it may contain:

{'b', 'a', 'n'}

The order can vary because sets are unordered.

26. Converting a List of Pairs to a Dictionary

A dictionary can be created from an iterable containing key-value pairs.

Example:

data = [
    ("name", "Raj"),
    ("age", 25),
    ("city", "Kathmandu")
]

student = dict(data)

print(student)

Output:

{'name': 'Raj', 'age': 25, 'city': 'Kathmandu'}

This is a useful conversion when working with structured data.

27. Dictionary Keys and Values

We can convert dictionary views into lists or other collections.

student = {
    "name": "Raj",
    "age": 25,
    "city": "Kathmandu"
}

keys = list(student.keys())
values = list(student.values())

print(keys)
print(values)

Output:

['name', 'age', 'city']
['Raj', 25, 'Kathmandu']

28. Converting Dictionary Items

student = {
    "name": "Raj",
    "age": 25
}

items = list(student.items())

print(items)

Output:

[('name', 'Raj'), ('age', 25)]

Each item becomes a tuple containing:

(key, value)

29. Converting Between Numeric Types

A common conversion flow is:

string
   ↓
float
   ↓
int

Example:

price = "99.99"

price = float(price)
price = int(price)

print(price)

Output:

99

The original string was first converted into a float and then into an integer.

30. Type Conversion vs Type Checking

These are different operations.

Type checking

We use:

type()

Example:

x = "25"

print(type(x))

Type conversion

We use:

int()

Example:

x = "25"

x = int(x)

print(type(x))

So:

type() → tells us what type something is

int(), float(), str(), etc. → convert something to another type

31. Explicit Type Conversion

When the programmer manually converts a value, it is called explicit type conversion.

Example:

age = "25"

age = int(age)

We explicitly told Python:

Convert this value to an integer.

32. Implicit Type Conversion

Python can sometimes automatically convert one numeric type to another during an operation.

For example:

x = 10
y = 2.5

result = x + y

print(result)
print(type(result))

Output:

12.5
<class 'float'>

Python automatically promotes the integer to a compatible numeric type for the calculation.

This is called implicit type conversion.

33. Explicit vs Implicit Conversion

34. Invalid Conversions

Not every value can be converted successfully.

For example:

x = "hello"

print(int(x))

This causes a:

ValueError

because "hello" does not represent an integer.

Another example:

x = "Python"

print(float(x))

This also produces a ValueError.

The value must have a compatible representation.

35. Converting an Empty String

This also fails:

x = ""

print(int(x))

An empty string does not represent an integer.

But:

print(bool(""))

works and produces:

False

Different conversion functions have different rules.

36. Converting None

Be careful with None.

For example:

x = None

print(str(x))

Output:

None

But:

print(int(None))

will raise a TypeError.

Likewise:

print(float(None))

is invalid.

However:

print(bool(None))

produces:

False

37. Converting User Input

This is one of the most important practical uses of type conversion.

Without conversion

age = input("Enter your age: ")

print(age + "5")

If the user enters:

25

the result is:

255

because both values are strings.

With conversion

age = int(input("Enter your age: "))

print(age + 5)

Now the result is:

30

38. Multiple User Inputs

name = input("Enter your name: ")
age = int(input("Enter your age: "))
height = float(input("Enter your height: "))

print("Name:", name)
print("Age:", age)
print("Height:", height)

This is a very common pattern in Python programs.

39. Practical Example — Shopping

price = float(input("Enter product price: "))
quantity = int(input("Enter quantity: "))

total = price * quantity

print("Total:", total)

Example:

Enter product price: 250.50
Enter quantity: 3

Output:

Total: 751.5

40. Practical Example - Student Marks

name = input("Enter student name: ")

python_marks = float(input("Enter Python marks: "))
database_marks = float(input("Enter Database marks: "))
web_marks = float(input("Enter Web marks: "))

total = python_marks + database_marks + web_marks
average = total / 3

print("Student:", name)
print("Total:", total)
print("Average:", average)

Here:

• input() receives strings.
• float() converts marks to numbers.
• Arithmetic is then performed on the numeric values.

41. Practical Example - Collection Conversion

fruits = ["apple", "mango", "apple", "banana"]

print("Original:", fruits)

unique_fruits = set(fruits)

print("Unique:", unique_fruits)

fruit_tuple = tuple(unique_fruits)

print("Tuple:", fruit_tuple)

This demonstrates:

List
 ↓
Set
 ↓
Tuple

42. Important Difference: int() vs round()

Students often confuse these two.

x = 9.7

print(int(x))
print(round(x))

Output:

9
10

int() removes the fractional portion.

round() performs rounding according to Python's rounding rules.

So:

int(9.7)   → 9
round(9.7) → 10

43. Important Difference: str() vs repr()

For beginner-level programming, str() is normally used when we want a readable text representation.

Example:

age = 25

print(str(age))

Output:

25

repr() has a different purpose and is more useful when debugging or representing objects precisely.

We can introduce repr() later when discussing objects and debugging.

44. Conversion Cheat Sheet

45. Copy Code

# ==========================================
# Python Type Conversion
# ==========================================

# String to integer

age = "25"
age = int(age)

print(age)
print(type(age))


# String to float

price = "99.50"
price = float(price)

print(price)
print(type(price))


# Integer to float

number = 10
number = float(number)

print(number)
print(type(number))


# Float to integer

number = 10.75
number = int(number)

print(number)
print(type(number))


# Integer to string

age = 25
age_text = str(age)

print(age_text)
print(type(age_text))


# Number to boolean

print(bool(0))
print(bool(1))
print(bool(100))


# String to boolean

print(bool(""))
print(bool("Python"))
print(bool("False"))


# List to tuple

fruits = ["apple", "mango", "banana"]

fruits_tuple = tuple(fruits)

print(fruits_tuple)


# Tuple to list

numbers = (10, 20, 30)

numbers_list = list(numbers)

print(numbers_list)


# List to set

values = [1, 2, 2, 3, 4, 4, 5]

unique_values = set(values)

print(unique_values)


# Set to list

numbers = {10, 20, 30}

numbers_list = list(numbers)

print(numbers_list)


# String to list

word = "Python"

letters = list(word)

print(letters)


# String to set

word = "banana"

unique_letters = set(word)

print(unique_letters)


# List of pairs to dictionary

data = [
    ("name", "Raj"),
    ("age", 25),
    ("city", "Kathmandu")
]

student = dict(data)

print(student)


# Dictionary keys and values

keys = list(student.keys())
values = list(student.values())

print(keys)
print(values)


# User input conversion

name = input("Enter your name: ")
age = int(input("Enter your age: "))

print("Name:", name)
print("Age:", age)


# Calculation using converted values

price = float(input("Enter product price: "))
quantity = int(input("Enter quantity: "))

total = price * quantity

print("Total:", total)

46. Practice Exercise 1 :  Age Calculator

Ask the user for their birth year:

birth_year = input("Enter your birth year: ")

Convert the input to an integer and calculate their approximate age using the current year.

Example:

Enter your birth year: 2000

Your approximate age is: 26

47. Practice Exercise 2 : Shopping Calculator

Ask the user for:

• Product name
• Price
• Quantity

Convert the price and quantity into appropriate numeric types.

Calculate the total.

Example:

Product: Mouse
Price: 800
Quantity: 3

Total: 2400

48. Practice Exercise 3 : Marks Calculator

Ask the user for marks in:

• Python
• Database
• Web Development

Convert the entered marks into numbers.

Calculate:

Total
Average

49. Practice Exercise 4 : Remove Duplicates

Given:

numbers = [10, 20, 20, 30, 40, 40, 50]

Convert the list into a set to remove duplicate values.

Then convert it back into a list.

Expected concept:

List
 ↓
Set
 ↓
List

50. Practice Exercise 5 : String Conversion

Given:

name = "Raj"
age = 25
course = "Python"

Create one sentence using string conversion:

My name is Raj, I am 25 years old, and I am learning Python.

51. Challenge : Student Registration

Create a program that asks for:

Student Name
Age
Course Fee
Number of Courses

Remember:

• Name → str
• Age → int
• Course Fee → float
• Number of Courses → int

Then display the information and its data types.

For example:

Student: Raj
Age: 25
Course Fee: 15000.0
Courses: 2