User Input and Type Conversion

By the end of this lesson you'll be able to:

• Use input() to ask the user a question and read their answer
• Convert between data types using int(), float(), str(), and bool()
• Check a value's type with the type() function
• Validate user input so your program doesn't crash on bad data

Until now your programs have run the same way every time. With input(), your program can have a conversation with the user — asking questions and responding to their answers.

Welcome to Chapter 13!

Today your programs come alive! Instead of running the same way every time, they'll ask the user questions and respond to the answers. Get ready to build your first truly interactive programs! Let's code it together!

The input() Function

input() stops the program, prints a message (called a prompt), and waits for the user to type something and press Enter. Whatever the user types comes back to your program as a string.

The key word to remember: input() always returns a string, even if the user types a number.

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

▶ Run ↻ Reset

Scratch Bridge

In Scratch, the "Ask ___ and wait" block pauses the program and stores the answer in a special variable called answer. In Python, input("your question") does exactly the same thing — it pauses and returns the answer as a string.

Type Conversion

Since input() always returns a string, you need to convert it before doing math.

Before we dive in, let's define three terms: - Integer (int): a whole number with no decimal point — 5, -12, 0 - Float: a number with a decimal point — 3.14, -0.5, 100.0 - Type conversion: changing a value from one data type to another

int() — Convert to Integer

int() converts a string (or float) to a whole number. If the string contains letters, int() raises a ValueError — so always convert only after checking the input looks like a number.

age_text = input("How old are you? ")
age = int(age_text)          # convert string to integer
print("Next year you'll be", age + 1)

▶ Run ↻ Reset

float() — Convert to Decimal Number

float() converts a string to a number with a decimal point. Use it when you need precision beyond whole numbers — for prices, measurements, or temperatures.

price = float(input("Enter price: $"))
tax = price * 0.08
print("Total with tax: $" + str(round(price + tax, 2)))

str() — Convert to String

str() converts a number (or other value) into a string. You need this when you want to join a number with text using +.

score = 95
print("Your score: " + str(score) + "/100")   # must convert to string first

bool() — Convert to True or False

bool() converts a value to True or False. The rules for what becomes False are worth memorizing:

Value	bool() result
0	False
"" (empty string)	False
None	False
Anything else	True

print(bool(0))       # False
print(bool(""))      # False
print(bool("hi"))    # True
print(bool(42))      # True

The type() Function

type() tells you what data type a value is. It's incredibly useful for debugging — when you're not sure whether you're working with a string or a number, just ask Python.

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If you type a number like 42, you'll see <class 'str'> first, then <class 'int'> after the conversion.

String Methods: lower(), upper(), strip()

Before you convert or validate input, it's a good idea to clean it first.

• s.lower() — converts all letters to lowercase ("Hello" → "hello")
• s.upper() — converts all letters to uppercase ("hello" → "HELLO")
• s.strip() — removes leading and trailing spaces (" hi " → "hi")

These three methods are very useful for input handling:

answer = input("Do you want to continue? (yes/no) ")
answer = answer.strip().lower()   # clean the input first
if answer == "yes":
    print("Great, let's keep going!")
else:
    print("OK, stopping here.")

Input Validation Basics

Validation means checking that the user's input is safe to use before using it.

A simple approach is to ask str.isdigit() — a string method that returns True if all characters are digits.

Before calling int(), you can use isdigit() to avoid a crash:

raw = input("Enter a whole number: ")
raw = raw.strip()

if raw.isdigit():
    n = int(raw)
    print("Double is:", n * 2)
else:
    print("That doesn't look like a number!")

▶ Run ↻ Reset

Try it twice: once with a number like 7, and once with letters like hello.

print() Tips

print() is more powerful than it looks. A few useful tricks:

Feature	Example	Output
Multiple values	print("a", "b", "c")	a b c
Custom separator	print("a", "b", sep="-")	a-b
No newline at end	print("Hello", end=" ")	stays on same line
Empty line	print()	blank line

Learning Check

Spot the Bug!

The program below is supposed to add two numbers entered by the user. But it has a bug — it concatenates strings instead of adding numbers! Fix it so the math actually works.

▶ Run ↻ Reset

If you enter 3 and 4, the buggy version prints 34 instead of 7. Fix it by converting a and b to int() before adding them.

Experiments

Try these changes. Predict what will happen first, then run it to check!

1. Change the greeting lab to ask for the user's favorite color. Print "My favorite color is [color] too!". You'll know it worked when the program echoes back whatever color you typed.

2. Ask the user for a temperature in Celsius. Convert it: fahrenheit = (celsius * 9/5) + 32. Print the result. You'll know it worked when entering 100 prints 212.0.

3. Ask the user for their name, then use .upper() to print it in capitals. You'll know it worked when typing monty prints MONTY.

4. In the validation demo, try typing a negative number like -5. Does isdigit() accept it? You'll know it worked when you see that -5 fails the isdigit() check — and you understand why!

5. Use type() to print the types of 3.14, True, and None. You'll know it worked when you see float, bool, and NoneType.

Amazing Progress!

Your programs can now have real conversations with users! You've learned input(), type conversion, validation, and string cleaning. These skills appear in nearly every real Python program. Let's keep coding together!

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