Right to the left "Elementary"

I don't understant what's wrong in my code. It delivers what is requested by replacing the right by left and compile all as one single string ????

Please anyone review:

def left_join(phrases):

    if len(phrases) > 0 and len(phrases) < 42:
        phrases = [x.replace("right", "left") for x in phrases]

    str = ",".join(phrases)

    print(str)

if __name__ == '__main__':
    print('Example:')
    print(left_join(("left", "right", "left", "stop")))


    #These "asserts" using only for self-checking and not necessary for auto-testing
    assert left_join(("left", "right", "left", "stop")) == "left,left,left,stop", "All to left"
    assert left_join(("bright aright", "ok")) == "bleft aleft,ok", "Bright Left"
    assert left_join(("brightness wright",)) == "bleftness wleft", "One phrase"
    assert left_join(("enough", "jokes")) == "enough,jokes", "Nothing to replace"
    print("Coding complete? Click 'Check' to review your tests and earn cool rewards!")