pygraphviz: finding the max rank node using successors

I'm trying to find the max rank node and the depth. Here is my code.

import pygraphviz as pgv


class Test:
    def __init__(self):
        self.G = pgv.AGraph(directed=True)

        self.G.add_node('a')
        self.G.add_node('b')
        self.G.add_node('c')
        self.G.add_node('d')
        self.G.add_node('e')
        self.G.add_node('f')

        self.G.add_edge('a', 'b')
        self.G.add_edge('b', 'c')
        self.G.add_edge('b', 'd')
        self.G.add_edge('d', 'e')
        self.G.add_edge('e', 'f')
        print(self.G.string())
        self.find_max_rank_node()

    def find_max_rank_node(self):
        nodes = self.G.nodes()
        depth = 0
        for n in nodes:
            layer1 = self.G.successors(n)
            if layer1:
                depth = depth + 1
                for layer_one in layer1:
                    layer2 = self.G.successors(layer_one)
                    print(n, layer2)


if __name__ == '__main__': Test()

The output should be f and 4. I started to code it but realized that I'm not going to know how depth the branch is ... and I'm not sure how to write the loop.

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