def is_pure_ascii(value):
    """
    Determines whether the given string is a pure ascii
    string
    >>> utils.is_pure_ascii(u"Cаша")
        False
    >>> utils.is_pure_ascii(u"Alice")
        True
    >>> utils.is_pure_ascii("Alice")
        True
    """
    if value is None:
        return False
    if not isinstance(value, str):
        return False
    try:
        value.encode('ascii')
    except (UnicodeEncodeError, UnicodeDecodeError):
        return False
    return True
