String validation checks in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the str.isalpha() method check in a string?

It checks if all characters in the string are letters (a-z or A-Z) and the string is not empty.

intermediate

How does str.isdigit() differ from str.isnumeric()?

str.isdigit() checks if all characters are digits (0-9), while str.isnumeric() also returns True for numeric characters like fractions or superscripts.

beginner

What does str.isspace() check for in a string?

It returns True if the string contains only whitespace characters like spaces, tabs, or newlines, and is not empty.

beginner

Explain the use of str.isalnum().

It checks if all characters in the string are letters or numbers (no spaces or symbols) and the string is not empty.

beginner

What will 'Hello123'.isalpha() return and why?

It will return False because the string contains numbers along with letters, and isalpha() requires only letters.

Which method checks if a string contains only letters and is not empty?

A isalpha()

B isdigit()

C isspace()

D isalnum()

isalpha() returns True only if all characters are letters and the string is not empty.

What does str.isdigit() return for the string '12345'?

A True

B False

C Error

D Depends on Python version

Since '12345' contains only digits, isdigit() returns True.

Which method would return True for a string containing only spaces and tabs?

A isalnum()

B isnumeric()

C isalpha()

D isspace()

isspace() returns True if the string contains only whitespace characters.

What will 'abc123'.isalnum() return?

A Error

B True

C False

D None

isalnum() returns True if all characters are letters or numbers, which is true here.

Which method is best to check if a string contains only numeric characters including fractions or superscripts?

A isalpha()

B isdigit()

C isnumeric()

D isspace()

isnumeric() covers a wider range of numeric characters than isdigit().

Describe how you can check if a string contains only letters or numbers in Python.

Think about a method that allows both letters and digits but no spaces.

Explain the difference between isdigit() and isnumeric() methods.

Consider what kinds of numeric characters each method accepts.

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