class BTNode:
    '''Binary Tree node.'''

    def __init__(self, data, left=None, right=None):
        ''' (BTNode, object, BTNode, BTNode) -> NoneType

        Create BTNode (self) with data and children left and right.
        '''
        self.data, self.left, self.right = data, left, right

    def __repr__(self):
        ''' (BTNode) -> str

        Represent BTNode (self) as a string that can be evaluated to
        produce an equivalent BTNode.

        >>> BTNode(1, BTNode(2), BTNode(3))
        BTNode(1, BTNode(2, None, None), BTNode(3, None, None))
        '''
        return 'BTNode({}, {}, {})'.format(repr(self.data),
                                           repr(self.left),
                                           repr(self.right))

    def __str__(self, indent=''):
        ''' (BTNode) -> str

        Return a user-friendly string representing BTNode (self) inorder.
        Indent by indent.

        >>> b = BTNode(1, BTNode(2, BTNode(3)), BTNode(4))
        >>> print(b)
            4
        1
            2
                3
        <BLANKLINE>
        '''
        right_tree = self.right.__str__(indent + '    ') if self.right else ''
        left_tree = self.left.__str__(indent + '    ') if self.left else ''
        return right_tree + '{}{}\n'.format(indent, str(self.data)) + left_tree


if __name__ == '__main__':
    import doctest
    doctest.testmod()