Python Type Conversion & Casting

In Plain English

Here is a classic coding trap: If you ask Python to add "50" + "50" (strings with quotes), it gives you "5050" instead of 100! To do real math with text numbers, you must convert them using int("50") or float("3.14"). This is called type casting.

The Mental Model

"Imagine receiving a package labeled "50". Until you open the box and treat it as a real number, it is just cardboard with ink on it. Type casting opens the box so you can do math with it."

Deep Dive: How It Works

Implicit Conversion: Python automatically converts integers to floats during division (e.g. 10 / 2 becomes 5.0).

Explicit Casting: You manually convert data using Python built-in functions:

int(x): Converts text or float to an integer by chopping off decimals (e.g. int("42") is 42, int(9.99) is 9).

float(x): Converts text or integer into a decimal float (e.g. float("19.95") is 19.95, float(5) is 5.0).

str(x): Converts numbers or booleans to text strings (e.g. str(100) is "100").

Syntax Blueprint

num = int("42")       # Converts text "42" to integer 42
price = float("19.99")  # Converts to float 19.99
text = str(100)         # Converts 100 to text "100"

Wrap your variable or literal in int(), float(), or str() to convert its type.

Core Rules to Remember

int() for Whole Numbers: int("42") transforms the text "42" into a real number ready for arithmetic.

str() for Text Conversion: str(100) transforms a number into text so you can print it alongside other words.

float() for Decimals: float("9.99") transforms decimal text into floating-point numbers.

Common Beginner Traps & How to Fix Them

Mistake:Trying to int() non-numeric text: int("banana")

Why it happens: Python cannot magically turn words into numbers and throws a ValueError.

How to fix: Ensure strings contain valid digits before passing them to int() or float().

Pro Tip: When converting float strings like "9.99" to an integer, convert to float first: int(float("9.99")).

Live Interactive Example

Hit Run Code to see it live

String Math vs Number Math

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: Converting "50" to int(50) allowed actual mathematical multiplication to occur.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Convert a string to an integer and multiply

Given quantity_str = "8", convert it to an integer.
Multiply it by 5 and print the result.

Sandbox Output

Click "Run & Check" to test your solution.

Finished reading and practicing?

Mark this lesson as completed to update your course progress.