User Input & Type Casting

Why user input matters

So far, a program can run with values that you wrote into the code yourself. That is useful for practice, but real programs become interesting when they react to the user.

A calculator asks for numbers. A login screen asks for a username. A weather app asks for a city.

To do that in Python, you use input().

The input() function

input() pauses the program and waits for the user to type something.

python
name = input("What is your name? ")
print("Hello,", name)

The text inside input() is called the prompt. It tells the user what to enter.

When the user presses Enter, Python gives that typed value back to your program.

input() always returns text

This is one of the most important things in the whole module:

input() always returns a string.

Even if the user types 25, Python receives it as text, not as a number.

python
age = input("Age: ")
print(type(age))

The result is str.

This is why beginners often get confused when they try to do math with input.

Why type casting is needed

If Python reads "25" as text, then it cannot safely treat it as a number until you convert it.

That conversion is called type casting.

The most common casting functions in this module are:

• int() for whole numbers
• float() for decimal numbers
• str() for text

python
age = int(input("Age: "))
price = float(input("Price: "))

Now those values can be used in calculations.

int() and float()

Use int() when you expect a whole number.

python
count = int(input("How many books? "))

Use float() when decimals are possible.

python
temperature = float(input("Temperature: "))

This difference matters because some real values, such as money, height, and temperature, often include decimals.

A common beginner problem

Look at this example:

python
age = input("Age: ")
print(age + 1)

This fails because age is text and 1 is a number.

Python does not guess. It expects you to be clear.

The fix is to convert first.

python
age = int(input("Age: "))
print(age + 1)

Reading more than one value

Many programs need more than one piece of information.

python
weight = float(input("Weight in kg: "))
height = float(input("Height in meters: "))

You can call input() as many times as needed. Each call asks for one value.

This is how small tools like calculators, converters, and forms work.

Using input in formulas

Once the input is converted to the correct type, you can use it in calculations.

python
celsius = float(input("Celsius: "))
fahrenheit = (celsius * 9 / 5) + 32
print(fahrenheit)

The same idea works for BMI, tips, totals, and many other beginner projects.

The flow is usually:

1. read input
2. convert input
3. calculate
4. print the result

Formatting the final result

After calculating a value, you often want to show it clearly.

python
bill = float(input("Bill: "))
tip_percent = int(input("Tip %: "))
total = bill * (1 + tip_percent / 100)

print(f"Total: {total:.2f}")

The :.2f part means "show two digits after the decimal point." That is useful for money.

You do not need to master formatting yet, but you should recognize it when you see it.

Invalid input

If you try to convert text that is not a valid number, Python raises an error.

python
age = int("hello")

This causes a ValueError.

For now, the important thing is to understand why it happens: Python cannot turn the word hello into a numeric value.

Later, you will learn how to handle bad input more safely.

Good habits with input

A few simple habits make beginner code much better:

• write clear prompts
• convert input as soon as needed
• choose variable names that explain the meaning
• keep the steps simple: input, convert, calculate, print

These habits make programs easier to read and easier to fix.

What this lesson should give you

After this lesson, you should understand how to:

• ask the user for input
• remember that input starts as text
• convert text to integers or floats
• use converted values in formulas
• read more than one input in the same program
• print clean final results

That is the core of interactive beginner programs.