Python String isalnum() Method

The isalnum() method checks whether all characters in a string are alphanumeric. This means the string can contain letters (a-z, A-Z) and numbers, but it cannot contain any spaces or special characters.

Syntax

string.isalnum()

Parameters

No parameters.

Return Value

Returns True if all characters in the string are alphanumeric (letters or numbers).

Returns False if the string contains any non-alphanumeric characters (such as spaces, punctuation marks, or special symbols) or if the string is empty.

Examples

Check if All Characters in a String are Alphanumeric

print("hello".isalnum())     # True
print("123".isalnum())       # True
print("hello22".isalnum())   # True
print("hello bro".isalnum()) # False (contains space)
print("hello!".isalnum())    # False (contains special character)
print("".isalnum())          # False (empty string)

Practical Usage

Let’s ask the user for a username input and check if it contains only letters and numbers (no spaces or special characters). If the username is valid (alphanumeric), it prints a greeting. Otherwise, it shows an "Invalid Username" message.

username = input("Enter your username: ")

if username.isalnum():
    print(f"Hello, {username}!")
else:
    print("Invalid Username")

Output:

For username: james22

Hello, james22!

For username: james@22

Invalid Username