Just an example software bug

bug.py

def append_y_words(y_words, base_list=[]):
    '''
    Purpose: Return a list of words from the base_list (if any) followed by
             words starting with 'y' from the words list.
    '''
    y_words = [word for word in y_words if word.startswith('y')]
    base_list += y_words
    return base_list
 
print append_y_words(["yoyo", "player"]) # should print ['yoyo']
print append_y_words(["yours", "puppet"], ["truly"]) # should print ['truly', 'yours']
print append_y_words(["yesterday"]) # should print ['yesterday'] , but does something else