Alternative Constructors in Python

Question

I'm playing with graphs and coded a mixin module for creating graphs. I want to have in it some alternative constructors. This is what I have:

class Graph(GraphDegree, GraphDegreePlot, GraphGeneration, object):
    def __init__(self):
        self.nodes = set([])
        self.edges = {}
    def get_nodes(self):
        """
        get nodes in graph
        """
        return self.nodes
    def get_number_of_nodes(self):
        """
        get number of nodes in the graph
        """
        return len(self.nodes)
    def get_edges(self):
        """
        get edges in graph
        """
        return self.edges
    def get_heads_in_edges(self):
        """
        get heads in edges present on the graph
        """
        return self.edges.values()
    def add_node(self, node):
        """
        add new node to graph
        """
        if node in self.get_nodes():
            raise ValueError('Duplicate Node')
        else:
            self.nodes.add(node)
            self.edges[node] = []
    def add_connection(self, edge):
        """
        adds edge to graph
        """
        origin = edge.get_origin()
        destination = edge.get_destination()
        if origin not in self.get_nodes() or destination not in self.get_nodes():
            raise ValueError('Nodes need to be in the graph')
        self.get_edges()[origin].append(destination)
        self.get_edges()[destination].append(origin)
    def get_children(self, node):
        """
        Returns the list of nodes node node is connected to
        """
        return self.get_edges()[node]

class GraphGeneration(object):
    @classmethod
    def gen_graph_from_text(cls, file):
        '''
        Generate a graph from a txt. Each line of the txt begins with the source node and then the destination nodes follow
        '''
        cls.__init__()
        file = open(file, 'r')
        for line in file:
            origin = line[0]
            destinations = line[1:-1]
            cls.add_node(origin)
            for destination in destinations:
                cls.add_node(destination)
                edge = Edge(origin, destination)
                cls.add_connection(edge)



graph = Graph.gen_graph_from_text(file)

I want that to return a graph where nodes and edges are generated from the file. The method I wrote doesn't work, I don't know if it even makes sense. What I want to do is inside that method to use the __init__ method of Graph, but then add edges and nodes from the file. I could just write an instance level method to do this, but I have other altertive initializers in mind.

Thanks !

Answer 1

Inside of your alternate constructors, use cls to create the new instance of the class. Then, just use self like you normally would and return it at the end.

NOTE: cls is a reference to the class itself, not the instance like you're expecting. Replacing all occurrences of cls with self except for the instantiation should give you the result you want. E.g.,

@classmethod
def gen_graph_from_text(cls, file):
    self = cls()
    file = open(file, 'r')
    for line in file:
        origin = line[0]
        destinations = line[1:-1]
        self.add_node(origin)
        for destination in destinations:
            self.add_node(destination)
            edge = Edge(origin, destination)
            self.add_connection(edge)
    return self