def _isint(string):
    """
    >>> _isint("123")
    True
    >>> _isint("123.45")
    False
    """
    return type(string) is int or \
        (isinstance(string, _binary_type) or
         isinstance(string, string_types)) and \
        _isconvertible(int, string)
>>> _isint("123")
    True
    >>> _isint("123.45")
    False