Dictionary String List string Punctuation

Isalnum

Sometimes we want to ensure a Python string has no punctuation or other whitespace. We can see if it contains just letters and digits.

With isalnum, we have a method that loops over the characters in the string. If any whitespace, punctuation, or other characters are found, it returns false.

Example

Here we see a program that uses isalnum. We can call isalnum() on a string instance, or use str.isanum and pass the string as the argument.

Detail We have 7 strings in our tests list. Some have spaces, punctuation, and one is an empty string.

Result The only strings that cause isalnum to return true are the ones with no whitespace or punctuation (and at least 1 character).

tests = []
tests.append("Dot Net Perls")
tests.append("DotNetPerls")
tests.append("Dot_Net_Perls")
tests.append("Dot0123")
tests.append("dotnetperls")
tests.append("123")
tests.append("")

for test in tests:
    # Test each string for alphanumeric status with isalnum.
    if test.isalnum():
        print("isalnum: [", test, "]")
    else:
        print("false: [", test, "]")false: [ Dot Net Perls ]
isalnum: [ DotNetPerls ]
false: [ Dot_Net_Perls ]
isalnum: [ Dot0123 ]
isalnum: [ dotnetperls ]
isalnum: [ 123 ]
false: [  ]

Useful method

Often we want to validate strings before putting them into a dictionary or list. Isalnum is a good method to remember—it is easier than trying to write a similar method.

When possible, methods like isalnum() from the standard library should be reused. They do not need testing—they are convenient and easy to understand.