import re


def substitute_word(text: str, word: str, substitute_by: str) -> str:
    """
    Substitutes in the given text the specified word by the substitute_by word. \
    It doesn't substitutes part of words. For example, substitute_word('you about u', 'u', '-') \
    returns 'you about -' and NOT 'yo- abo-t -'.

    :param text: Text where you want to substitute the word.
    :param word: Word to be substituted.
    :param substitute_by: Word that substitutes the word to be substituted.
    :return: The text with all occurrences of 'word' substituted by 'substitute_by'.
    """
    return re.sub('\\b' + word + '\\b', substitute_by, text)