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Lesson 4: Type Casting

Back to the main roadmap | Previous lesson: Conditionals

What is Type Casting?

Sometimes a variable contains one type of data, but you need it in another type.

Type casting is the process of converting one data type into another.

Real-Life Example

Imagine you ask someone their age on a form.

They type:

25

Although it looks like a number, Python treats it as text (a string).

If you try to do math with it, Python will raise an error.

Example

age = "25"

print(age + 5)

Output

TypeError: can only concatenate str (not "int") to str

Why?

  • "25" is a string
  • 5 is an integer
  • Python cannot add text and numbers together.

Casting Functions

Python provides built-in functions to convert one data type into another.

Function Converts To Example
int() Integer int("25") → 25
float() Float float("4.5") → 4.5
str() String str(25) → "25"
bool() Boolean bool(1) → True

Converting a String to an Integer

Example

age = "25"

age = int(age)

print(age + 5)

Output

30

Now Python knows that 25 is a number, so addition works correctly.

Converting to Float

Example

price = "19.99"

price = float(price)

print(price * 2)

Output

39.98

The string "19.99" is converted into the floating-point number 19.99.

Converting to String

Sometimes you need to convert a number into text.

Example

number = 100

text = str(number)

print("The number is " + text)

Output

The number is 100

Without str(), Python would raise an error because you cannot join a string and an integer using +.

Converting to Boolean

The bool() function converts values into either True or False.

Example

print(bool(1))
print(bool(0))
print(bool(""))
print(bool("Python"))

Output

True
False
False
True

Generally:

  • 0 → False
  • Empty string ("") → False
  • Non-zero numbers → True
  • Non-empty strings → True

input() Always Returns a String

One of the most common beginner mistakes is forgetting that input() always returns text, even if the user types a number.

Example

age = input("How old are you? ")

print(type(age))

Sample Input

21

Output

<class 'str'>

Even though you typed 21, Python stores it as a string.

To perform calculations, convert it to an integer.

age = int(age)

print(age + 1)

Output

22

Back to the main roadmap | Previous lesson: Conditionals

Practice Program

Create a file named lesson4.py and write the following code.

# Try this WITHOUT casting first
# (Comment out the int() line to see the error.)

age = input("Enter your age: ")

print(type(age))

age = int(age)

print(type(age))
print("Next year you will be:", age + 1)

# Float example
price = "19.99"
price = float(price)

print(price * 2)

# String example
number = 100
text = str(number)

print("The number is " + text)

Sample Input

18

Output

<class 'str'>
<class 'int'>
Next year you will be: 19
39.98
The number is 100

Common Beginner Mistake

age = input("Enter your age: ")

print(age + 1)

Output

TypeError

Why?

Because input() returns a string, not an integer.

The correct way is:

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

print(age + 1)

Key Takeaways

  • Type casting converts one data type into another.
  • int() converts to integers.
  • float() converts to decimal numbers.
  • str() converts values into text.
  • bool() converts values into True or False.
  • input() always returns a string.
  • Convert user input before performing mathematical operations.

Next Lesson: Exceptions (Handling Errors)