Python 中的有向图中的指针错误

Question

I'm trying to implement a Directed graph in python by creating a class Vertex . A vertex has the name and the adjacent as a list containing another instance of class Vertex . below is the code.

The problem is that when I assign node b as the adjacent node from node a . the pointer of node b is assigned to both adjacent of a and b which is not my intention.

I also have the picture debug mode below the code showing that the pointer of node b is also assigned to itself.

how to fix it?

 class Vertex: def __init__(self, name, adjacent=[]): self.name = name self.adjacent = adjacent def add_adjacent(self, vertex): self.adjacent.append(vertex) class Graph: # directed graph def __init__(self, edge_list): vertices = {} for o, d in edge_list: # print(o, d) if o not in vertices: v = Vertex(o) vertices[o] = v else: v = vertices[o] if d not in vertices: u = Vertex(d) vertices[d] = u else: u = vertices[d] if u not in v.adjacent: print(v.name, ' adds ', u.name) v.add_adjacent(u) self.vertices = vertices def get_vertex_names(self): return list(self.vertices.keys()) def get_adjacent(self, vertex): return self.vertices[vertex].adjacent # test Vertex edges = [ ['a', 'b'], ['a', 'c'], ['a', 'd'], ['b', 'c'], ['c', 'b'], ] g = Graph(edges) # g.vertices

error can be seen in debug mode

Answer 1

You wrote this:

 def __init__(self, name, adjacent=[]):

There is a subtle "gotcha", summarized by a simple rule,

Avoid using mutable container as a default arg.

You want this:

 def __init__(self, name, adjacent=None): self.name = name self.adjacent = adjacent or []

In the original, there is a single list for all vertices (since caller always lets it default). That list was created at def time. In contrast, the or [] clause constructs a new list at execution time each time through, rather than once during definition.

https://dollardhingra.com/blog/python-mutable-default-arguments/

https://towardsdatascience.com/python-pitfall-mutable-default-arguments-9385e8265422