ds.py

#!/usr/bin/env python2
# coding: utf-8

def read(filename):
    """
    Read a graph from a file. Returns a dictionary.
    The file must be in the format:

    SRC DST DST ...
    SRC DST
    ...

    where SRC and DST are names of nodes.
    Node names must not contain whitespace.
    """
    g = {}
    with open(filename) as handle:
        for line in handle:
            line = line.strip()
            if line == "":
                continue
            parts = line.split()
            src = parts[0]
            targets = parts[1:]
            if src not in g:
                g[src] = set()
            for target in targets:
                g[src].add(target)
    return g

def write(filename, g):
    """ Write dictionary `g` to file. """
    with open(filename, "w") as handle:
        for src, targets in g.iteritems():
            handle.write("%s %s\n" % (src, " ".join(targets)))

if __name__ == '__main__':
    g = {
        "A": ["B", "C"],
        "B": ["D"],
        "C": ["B"],
    }
    write("test.g", g)
    g = read("test.g")
    print(g)
    # {'A': set(['C', 'B']), 'C': set(['B']), 'B': set(['D'])}