Python String isalnum() Method

The isalnum() method in Python is used to check whether a string contains only letters and numbers (alphanumeric characters). It’s a built-in method available for string objects.

Syntax

string.isalnum()

Parameters:

• This method does not take any parameters.

Returns:

• True – if all characters in the string are alphanumeric (letters or digits) and there is at least one character.
• False – if the string contains any special characters, spaces, or is empty.

Example 1: Alphanumeric String

text = "Python3"
print(text.isalnum())

True

Example 2: String with Spaces

text = "Hello World"
print(text.isalnum())

False

Example 3: String with Special Characters

text = "Python@123"
print(text.isalnum())

False

Example 4: Empty String

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

False

Use Case: Validating Usernames

The isalnum() method is commonly used when checking for valid usernames or codes where only letters and numbers are allowed.

username = input("Enter a username: ")
if username.isalnum():
    print("Valid username.")
else:
    print("Username must be alphanumeric.")

Common Mistakes

• Expecting isalnum() to return True for strings with spaces (it won’t).
• Using it on an empty string – it will return False.

Summary

• string.isalnum() checks if all characters are letters or digits.
• Returns True only when the string has no special characters or spaces.
• Empty strings return False.

Practice Problem

Write a program that checks whether a user’s input is a valid product code made of letters and numbers only.

product_code = input("Enter product code: ")
if product_code.isalnum():
    print("Valid code.")
else:
    print("Code should only contain letters and numbers.")