def html_email(email, title=None):
    """
    >>> html_email('username@example.com')
    '<a href="mailto:username@example.com">username@example.com</a>'
    """

    if not title:
        title = email

    return '<a href="mailto:{email}">{title}</a>'.format(email=email, title=title)
>>> html_email('username@example.com')
    '<a href="mailto:username@example.com">username@example.com</a>'