[Tutor] questions when define a class

[Qianyun Guo]

Hi all, I am trying to get a suffix tree from a string. I use three
classes, Node, Edge, SuffixTree. I have two questions when implementing:

【1】

>>> a = Edge(1,2,3,4)

>>> a.length

1
if I remove  '@property' in my code, it returns as below:

>>> a = Edge(1,2,3,4)

>>> a.length

<bound method Edge.length>

>>> a.length()

1



I don't really understand the differences w/ @property, and the differences
of a.length and a.length(), could you explain?

【2】

In SuffixTree, I define two functions, _get_str_from_edge,
_get_str_from_node, the latter depend on the first one (please see my
code).  Then I have this problem:

>>> a = SuffixTree('abcd')

>>> a._get_str_from_edge(a.edges[(0,1)])

'abcd$'

>>> a._get_str_from_node(0,1)

TypeError: _get_str_from_edge() takes exactly 2 arguments (3 given)

Could you tell me what's wrong here?




below is my code, __repr__ are removed for convenience.
###################################


class Node:

    def __init__(self, parent_node):
        self.suffix_node = -1
        self.parent = parent_node
        self.children = []

    def add_child(self, child_node_index):
        self.children.append(child_node_index)

class Edge:

    def __init__(self, first_char_index, last_char_index,\
            source_node_index, dest_node_index):
        self.first_char_index = first_char_index
        self.last_char_index = last_char_index
        self.source_node_index = source_node_index
        self.dest_node_index = dest_node_index

    @property
    def length(self):
        return self.last_char_index - self.first_char_index

class SuffixTree:

    def __init__(self, string):
        self.string = string+'$'
        self.N = len(self.string)
        self.nodes = []
        self.num_nodes = 0
        self.edges = {}

        #initialize two node tree

        root = Node(-1)
        root.suffix_node = -1
        self._add_node(root)

        leaf = Node(0)
        leaf.suffix_node = 0
        self._add_node(leaf)
        self.nodes[0].add_child(1)

        edge = Edge(0, self.N, 0, 1)
        self._add_edge(edge)

    def _get_str_from_edge(self, edge):
        return self.string[edge.first_char_index : edge.last_char_index]

    def _get_str_from_node(self, source_node_index, dest_node_index):
        return self._get_str_from_edge(self, self.edges[(source_node_index,
\
                dest_node_index)])


    def _add_node(self, node):
        self.nodes.append(node)
        self.num_nodes += 1

    def _add_edge(self, edge):
        self.edges[(edge.source_node_index, edge.dest_node_index)] = edge
        self.nodes[edge.source_node_index].add_child(edge.dest_node_index)
        self.nodes[edge.dest_node_index].parent = edge.source_node_index
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