05-input-output.md

Input and Output

Interacting with users through input and output is fundamental to most programs.

Getting User Input

The input() function allows you to get text input from users.

Basic Input:

# Getting user input
name = input("What's your name? ")
print(f"Hello, {name}!")

# Input always returns a string
age_str = input("How old are you? ")
print(f"You entered: {age_str} (type: {type(age_str)})")

Converting Input Types:

# Converting string input to numbers
age = int(input("Enter your age: "))
height = float(input("Enter your height in feet: "))

print(f"Age: {age} (type: {type(age)})")
print(f"Height: {height} (type: {type(height)})")

# Calculating with converted input
birth_year = 2024 - age
print(f"You were born around {birth_year}")

Handling Input Errors:

# Basic error handling for input
try:
    number = int(input("Enter a number: "))
    print(f"You entered: {number}")
except ValueError:
    print("That's not a valid number!")

# More robust input handling
def get_integer(prompt):
    while True:
        try:
            return int(input(prompt))
        except ValueError:
            print("Please enter a valid integer.")

age = get_integer("Enter your age: ")
print(f"Your age is {age}")

Multiple Inputs:

# Getting multiple values in one line
data = input("Enter your name, age, and city (separated by commas): ")
name, age, city = data.split(", ")
print(f"Name: {name}, Age: {age}, City: {city}")

# Getting multiple numbers
numbers_str = input("Enter numbers separated by spaces: ")
numbers = [int(x) for x in numbers_str.split()]
print(f"Numbers: {numbers}")
print(f"Sum: {sum(numbers)}")

Printing Output

The print() function is used to display output to the user.

Basic Printing:

# Basic print statements
print("Hello, World!")
print("Python", "is", "awesome")  # Multiple arguments
print("Line 1")
print("Line 2")

Print Function Parameters:

# sep parameter (separator between arguments)
print("apple", "banana", "cherry", sep=", ")  # Output: apple, banana, cherry
print("2024", "12", "25", sep="-")            # Output: 2024-12-25

# end parameter (what to print at the end)
print("Hello", end=" ")
print("World")  # Output: Hello World (on same line)

print("Loading", end="")
for i in range(3):
    print(".", end="")
print(" Done!")  # Output: Loading... Done!

Formatting Output:

name = "Alice"
age = 30
salary = 75000.50

# f-strings (recommended for Python 3.6+)
print(f"Name: {name}")
print(f"Age: {age}")
print(f"Salary: ${salary:,.2f}")  # Output: Salary: $75,000.50

# .format() method
print("Name: {}, Age: {}".format(name, age))
print("Salary: ${:,.2f}".format(salary))

# % formatting (older style)
print("Name: %s, Age: %d" % (name, age))

Advanced Formatting:

# Number formatting
pi = 3.14159265359
print(f"Pi: {pi:.2f}")        # Output: Pi: 3.14
print(f"Pi: {pi:.4f}")        # Output: Pi: 3.1416

# Percentage formatting
score = 0.875
print(f"Score: {score:.1%}")  # Output: Score: 87.5%

# Padding and alignment
print(f"{'Name':<10} {'Age':>5} {'City':^15}")
print(f"{'Alice':<10} {25:>5} {'New York':^15}")
print(f"{'Bob':<10} {30:>5} {'Chicago':^15}")

# Output:
# Name            Age      City
# Alice            25    New York
# Bob              30     Chicago

Printing Data Structures:

# Printing lists, dictionaries, etc.
numbers = [1, 2, 3, 4, 5]
person = {"name": "Alice", "age": 30, "city": "New York"}

print("Numbers:", numbers)
print("Person:", person)

# Pretty printing for better readability
import pprint
complex_data = {
    "users": [
        {"name": "Alice", "age": 30, "hobbies": ["reading", "swimming"]},
        {"name": "Bob", "age": 25, "hobbies": ["gaming", "cooking"]}
    ]
}

print("Regular print:")
print(complex_data)

print("\nPretty print:")
pprint.pprint(complex_data)

Practice Exercise - Interactive Program

def user_profile_creator():
    """Create a user profile with input validation."""
    
    print("=== User Profile Creator ===")
    print("Let's create your profile!\n")
    
    # Get basic information
    name = input("Enter your full name: ").strip()
    while not name:
        print("Name cannot be empty!")
        name = input("Enter your full name: ").strip()
    
    # Get age with validation
    while True:
        try:
            age = int(input("Enter your age: "))
            if age < 0 or age > 150:
                print("Please enter a realistic age (0-150)")
                continue
            break
        except ValueError:
            print("Please enter a valid number for age")
    
    # Get email
    email = input("Enter your email: ").strip()
    while "@" not in email or "." not in email:
        print("Please enter a valid email address")
        email = input("Enter your email: ").strip()
    
    # Get hobbies
    hobbies_input = input("Enter your hobbies (separated by commas): ")
    hobbies = [hobby.strip() for hobby in hobbies_input.split(",") if hobby.strip()]
    
    # Get favorite number
    while True:
        try:
            fav_number = float(input("Enter your favorite number: "))
            break
        except ValueError:
            print("Please enter a valid number")
    
    # Display profile
    print("\n" + "="*50)
    print("YOUR PROFILE")
    print("="*50)
    print(f"Name: {name}")
    print(f"Age: {age} years old")
    print(f"Email: {email}")
    print(f"Favorite Number: {fav_number}")
    
    if hobbies:
        print("Hobbies:")
        for i, hobby in enumerate(hobbies, 1):
            print(f"  {i}. {hobby}")
    else:
        print("Hobbies: None specified")
    
    # Calculate some fun facts
    days_lived = age * 365
    print(f"\nFun Facts:")
    print(f"- You've lived approximately {days_lived:,} days!")
    print(f"- Your name has {len(name)} characters")
    print(f"- You have {len(hobbies)} hobbies listed")
    
    # Ask if they want to save
    save = input("\nWould you like to save this profile? (yes/no): ").lower()
    if save in ['yes', 'y']:
        filename = f"{name.replace(' ', '_').lower()}_profile.txt"
        try:
            with open(filename, 'w') as file:
                file.write(f"User Profile\n")
                file.write(f"============\n")
                file.write(f"Name: {name}\n")
                file.write(f"Age: {age}\n")
                file.write(f"Email: {email}\n")
                file.write(f"Favorite Number: {fav_number}\n")
                file.write(f"Hobbies: {', '.join(hobbies)}\n")
            print(f"Profile saved to {filename}")
        except Exception as e:
            print(f"Error saving file: {e}")
    
    print("\nThank you for creating your profile!")

# Run the profile creator
user_profile_creator()

Key Takeaways

• input() always returns strings that need conversion for numeric operations
• print() has many formatting options for professional output
• Always handle potential input errors in real applications
• Use f-strings for modern, readable string formatting
• Validate user input to ensure data quality
• Consider user experience when designing input/output interactions

What's Next?

Now that you can interact with users through input and output, you're ready to learn about making decisions in your programs. Let's move on to Control Structures!