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How to Take User Input with Loops in Python

This guide explores how to take user input within loops in Python, using both for and while loops. We'll cover how to:

  • Collect a fixed number of inputs.
  • Collect input until a condition is met.
  • Validate user input (specifically for integers).
  • Use list comprehensions.

Taking a Fixed Number of Inputs with a for Loop​

Use a for loop with range() when you know exactly how many inputs you need.

Collecting Strings​

my_list = []

for _ in range(3):  # Loop 3 times
    country = input('Enter a country: ')
    my_list.append(country)

print(my_list) # Example: ['USA', 'Canada', 'Mexico']
  • range(3): Generates a sequence of numbers from 0 to 2. We use _ as the loop variable because we don't actually need the number itself, just the iteration.
  • input('Enter a country: '): Prompts the user for input and returns the entered string.
  • my_list.append(country): Adds the entered string to the my_list.

Collecting Integers with Validation​

To collect integers, use int() to convert the input, and always include error handling:

my_list = []

for _ in range(3):
    while True:  # Keep looping until valid input is received
        try:
            num_str = input('Enter a number: ')
            num = int(num_str)  # Try to convert to an integer
            my_list.append(num)
            break  # Exit the inner while loop if successful
        except ValueError:
            print('Invalid number. Please enter an integer.')

print(my_list) # Example input/output: [2, 3, 4]
  • while True:: This creates an infinite loop that we'll break out of only when valid input is received. This is a very common and useful pattern.
  • try...except ValueError:: This handles the potential ValueError that occurs if the user enters something that can't be converted to an integer (e.g., "abc").
  • break: If the conversion to an integer is successful, the break statement exits the inner while loop, and the outer for loop continues.

Using list comprehension​

List comprehension can also be used to take multiple inputs.

my_list = [input('Enter a color: ') for _ in range(3)]
print(my_list) # Output (will depend on user input): ['red', 'blue', 'green']
  • However it is important to know that it is not possible to use a try/except block to handle exceptions within a list comprehension, which makes it less suitable for scenarios where validation is required.

Taking Input Until a Condition is Met with a while Loop​

Use a while loop when you don't know in advance how many inputs you'll need, but you have a condition that determines when to stop.

String Input with Validation​

password = ''

while True:  # Or: while len(password) < 4:
    password = input('Enter your password: ')
    if len(password) < 4:
        print('Password too short')
        continue # Go to the next iteration.
    else:
        print(f'You entered {password}')
        break # The condition has been met.

print(password)
  • The while True: loop keeps asking for a password until it has the required minimum length.
  • The continue statement will skip the rest of the loop's code when the input is invalid.
  • The break statement exits the loop when a valid password is entered.

Numeric Input with Validation​

num = 0  # Initialize num
while True:
    try:
        num = int(input("Enter an integer 1-5: "))
        if 1 <= num <= 5:  # More concise range check
            print(f'You entered: {num}')
            break  # Exit the loop if input is valid
        else:
            print('The integer must be in the range 1-5')
    except ValueError:
        print("Please enter a valid integer 1-5")
  • A while True loop and a try/except statement will handle invalid input, preventing the program from crashing.
  • The conditional check verifies that the integer is withing the specified range.

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