>>> is_email('username@example.com')
    True
    >>> is_email('example.com')
    False
    >>> is_email('firstname.lastname@domain.co.uk')
    True

def is_email(string):
    """
    >>> is_email('username@example.com')
    True
    >>> is_email('example.com')
    False
    >>> is_email('firstname.lastname@domain.co.uk')
    True
    """
    email_regex = r'^[A-Za-z0-9\.\+_-]+@[A-Za-z0-9\._-]+\.[a-zA-Z]*$'

    if isinstance(string, str) and not re.match(email_regex, string):
        return False
    else:
        return True