import types

def build_tree(start_point=None):
    if start_point is not None and type(start_point) is not types.ModuleType:
        print "start_point is not None (meaning use globals) and is not a module"
    check = [start_point]
    checked = set()
    tree = {}
    while len(check) > 0:
        current = check.pop(0)
        if current is None:
            d = globals()
        else:
            print "Popping %s" % current.__name__
            d = current.__dict__
        current_key = "globals()" if current is None else current.__name__
        for k, v in d.iteritems():
            if type(v) is types.ModuleType:
                if current_key not in tree:
                    tree[current_key] = set()
                tree[current_key].add(v.__name__)
                if v not in checked:
                    check.append(v)
                    print "Appending %s" % v.__name__
        checked.add(current)
    return tree