adjacency_list_directed.py

from collections import defaultdict

class graph(object):

    def __init__(self, nodes, edge_list = None):
        self.N = nodes
        self.A = defaultdict(list)
        self.in_degree = [0] * (self.N+1)
        if edge_list != None: # let's optionally give this class a list of tuples, consisting of the edges, as a little Python exercise.
            for x in edge_list:
                self.add_edge(x)

    def is_valid_tuple(self, x):
        return isinstance(x, tuple) and len(x) == 2 and all((a > 0 and a <= self.N) for a in x)
        # this function checks whether an element is a tuple meant to describe a graph edge.
        # the conditions are as follows: the tuple is of length 2, and both its elements are between 1 and N.

    def add_edge(self, e):
        if self.is_valid_tuple(e):
            if e[0] not in self.A[e[1]]:
                self.A[e[0]].append(e[1])
                self.in_degree[e[1]] += 1
        else: print("element", e, "is not formatted correctly!")

    def remove_edge(self, e):
        if self.is_valid_tuple(e):
            if e[0] in self.A[e[1]]:
                self.A[e[0]].remove(e[1])
                self.in_degree[e[1]] -= 1
        else: print("element", e, "is not formatted correctly!")