def format_thousands(number, separator=','):
    """
    Return the number with separators (1,000,000)
    
    >>> format_thousands(1)
    '1'
    >>> format_thousands(1000)
    '1,000'
    >>> format_thousands(1000000)
    '1,000,000'
    """
    string = str(number).split('.')
    l = []
    for i, character in enumerate(reversed(string[0])):
        if i and not i % 3:
            l.insert(0, separator)
        l.insert(0, character)
    string[0] = ''.join(l)
    return '.'.join(string)
