Test if string ONLY contains given characters

Question

What\'s the easiest way to check if a string only contains certain specified characters in Python? (Without using RegEx or anything, of course)

Specifically, I have

Answer 1

Below is the code:

a = ['aba', 'acba', 'caz']
needle = 'abc'

def onlyNeedle(word):
    for letter in word:
        if letter not in needle:
            return False

    return True

a = filter(onlyNeedle, a)

print a