Solution to https://gist.github.com/smitp/074cb86c2eacc3885639f53d83f72836

graph_execution.py

import dag


def always_fail():
    raise Exception('Manually throwing')


processes = {
    "root": lambda: print("root"),
    "a":    lambda: print("A"),
    "b":    lambda: print("B"),
    "c":    lambda: print("C"),
    #"c":     always_fail,
    "d":    lambda: print("D"),
    "e":    lambda: print("E")
}

nodes = {
  'root': ['a'],
  'a': ['b', 'e'],
  'b': ['c', 'd'],
  'd': ['e'],
  'c': [],
  'e': []
}


class Executor:

    def __init__(self, nodes):
        self.dag = dag.DAG()
        self.dag.from_dict(nodes)
        self.nodes = nodes
        self.executed = []
        self.preds = []

    def execute(self, curr_node='root'):
        preds = self.dag.predecessors(curr_node, self.dag.graph)
        for pred in preds:
            if not pred in self.executed:
                return

        results = []
        try:
            processes[curr_node]()
            self.executed.append(curr_node)
            descendents = self.dag.downstream(curr_node, self.dag.graph)
            for desc in descendents:
                results.append(self.execute(desc))

            return all(results)
        except Exception as e:
            print(e)
            return False
        return True


ex = Executor(nodes)
print(ex.execute())