String validation checks in Python - Mini Project: Build & Apply

String validation checks

📖 Scenario: You are building a simple program to check different properties of a user's input text. This helps in many real-life cases like validating passwords, usernames, or form inputs.

🎯 Goal: Build a program that checks if a given string contains only letters, only digits, or is alphanumeric, and then prints the results.

📋 What You'll Learn

Create a variable with a specific string value

Create a variable to store a check result

Use string methods to check if the string is alphabetic, numeric, or alphanumeric

Print the results clearly

💡 Why This Matters

🌍 Real World

String validation is used in forms, passwords, and user input to ensure data is correct and safe.

💼 Career

Many programming jobs require validating user input to prevent errors and security issues.

1

Create the input string

Create a variable called user_input and set it to the string "Hello123".

Python

# Create the variable user_input with the value "Hello123"
# Your code here

Hint

Use the equals sign = to assign the string to the variable.

2

Create variables for validation results

Create three variables called is_alpha, is_digit, and is_alnum. Set each one to False initially.

Python

user_input = "Hello123"
# Create variables is_alpha, is_digit, and is_alnum and set them to False
# Your code here

Hint

Each variable should be set to the boolean value False.

3

Check string properties using string methods

Set is_alpha to user_input.isalpha(), is_digit to user_input.isdigit(), and is_alnum to user_input.isalnum().

Python

user_input = "Hello123"
is_alpha = False
is_digit = False
is_alnum = False
# Use string methods to update is_alpha, is_digit, and is_alnum
# Your code here

Hint

Use the string methods isalpha(), isdigit(), and isalnum() on user_input.

4

Print the validation results

Print the values of is_alpha, is_digit, and is_alnum each on a new line with labels exactly as shown: "Is alphabetic: ", "Is digit: ", and "Is alphanumeric: ".

Python

user_input = "Hello123"
is_alpha = user_input.isalpha()
is_digit = user_input.isdigit()
is_alnum = user_input.isalnum()
# Print the results with labels
# Your code here

Hint

Use three print() statements with the exact labels and variables.

Practice

1. Which Python string method checks if all characters in a string are letters (a-z or A-Z)?

easy

A. isdigit()

B. isalpha()

C. isalnum()

D. isspace()

Solution

1. Step 1: Understand what isalpha() does

   The isalpha() method returns True if all characters in the string are alphabet letters only.

2. Step 2: Compare with other methods

   isdigit() checks for digits only, isalnum() checks for letters or digits, and isspace() checks for whitespace characters.

3. Final Answer:

   isalpha() -> Option B

4. Quick Check:

   Check letters only = isalpha() [OK]

Hint: Letters only? Use isalpha() method [OK]

Common Mistakes:

• Confusing isalpha() with isdigit()
• Using isalnum() which allows digits too
• Thinking isspace() checks letters

2. Which of the following is the correct syntax to check if a string s contains only digits in Python?

easy

A. if s.isdigit()

B. if s.isdigit:

C. if isdigit(s)

D. if s.is_digit()

Solution

1. Step 1: Recall method call syntax in Python

   Methods require parentheses to call them, so s.isdigit() is correct.

2. Step 2: Identify incorrect options

   s.isdigit misses parentheses, isdigit(s) is not a built-in function, and s.is_digit() is not a valid method.

3. Final Answer:

   if s.isdigit() -> Option A

4. Quick Check:

   Method call needs () = isdigit() [OK]

Hint: Call string methods with parentheses () [OK]

Common Mistakes:

• Forgetting parentheses after method name
• Using wrong method names like is_digit()
• Trying to call isdigit as a function

3. What will be the output of this code?

text = "Hello123"
print(text.isalnum())
print(text.isalpha())

medium

A. True\nTrue

B. False\nFalse

C. True\nFalse

D. False\nTrue

Solution

1. Step 1: Check text.isalnum()

   The string "Hello123" contains letters and digits only, so isalnum() returns True.

2. Step 2: Check text.isalpha()

   Since the string contains digits, isalpha() returns False.

3. Final Answer:

   True\nFalse -> Option C

4. Quick Check:

   Letters+digits = True, letters only = False [OK]

Hint: isalnum() allows digits; isalpha() does not [OK]

Common Mistakes:

• Assuming isalpha() returns True with digits
• Confusing isalnum() with isalpha()
• Ignoring that digits affect isalpha()

4. Find the error in this code snippet:

user_input = " 123 "
if user_input.isdigit():
    print("Digits only")
else:
    print("Not digits only")

medium

A. The string has spaces, so isdigit() returns False

B. The method isdigit() is misspelled

C. The print statement is missing parentheses

D. The variable user_input is not defined

Solution

1. Step 1: Understand what isdigit() checks

   isdigit() returns True only if all characters are digits without spaces or other characters.

2. Step 2: Analyze the string " 123 "

   The string contains spaces, so isdigit() returns False, causing the else branch to run.

3. Final Answer:

   The string has spaces, so isdigit() returns False -> Option A

4. Quick Check:

   Spaces cause isdigit() to fail [OK]

Hint: Spaces cause isdigit() to return False [OK]

Common Mistakes:

• Ignoring spaces in the string
• Thinking isdigit() ignores whitespace
• Assuming method name is wrong

5. You want to check if a string s contains only printable characters and no spaces. Which combination of methods correctly validates this?

hard

A. s.isprintable() and s.isalnum()

B. s.isprintable() and not s.isspace()

C. s.isprintable() and not s.isalpha()

D. s.isprintable() and not ' ' in s

Solution

1. Step 1: Understand isprintable()

   This method returns True if all characters are printable (letters, digits, punctuation, etc.) including spaces.

2. Step 2: Exclude spaces explicitly

   Since spaces are printable, to ensure no spaces, check that the space character ' ' is not in the string.

3. Step 3: Evaluate options

   s.isprintable() and not ' ' in s uses not ' ' in s to exclude spaces, combined with isprintable(), which is correct. s.isprintable() and not s.isspace() fails because not s.isspace() only checks if the whole string is not spaces, but allows spaces mixed with other chars. Options C and D do not correctly exclude spaces.

4. Final Answer:

   s.isprintable() and not ' ' in s -> Option D

5. Quick Check:

   Exclude spaces by checking ' ' not in string [OK]

Hint: Check printable and exclude spaces with ' ' not in s [OK]

Common Mistakes:

• Using isspace() to exclude spaces incorrectly
• Assuming isprintable() excludes spaces
• Using isalnum() which excludes punctuation