Find substring in string but only if whole words?

Question

What is an elegant way to look for a string within another string in Python, but only if the substring is within whole words, not part of a word?

Perhaps an example

Answer 1

Here's a way to do it without a regex (as requested) assuming that you want any whitespace to serve as a word separator.

import string

def find_substring(needle, haystack):
    index = haystack.find(needle)
    if index == -1:
        return False
    if index != 0 and haystack[index-1] not in string.whitespace:
        return False
    L = index + len(needle)
    if L < len(haystack) and haystack[L] not in string.whitespace:
        return False
    return True

And here's some demo code (codepad is a great idea: Thanks to Felix Kling for reminding me)