String representation of an object in Python

Question

So in the book(Problem Solving with Algorithms and Data Structres) that I'm currently using as a reference, this is how a graph is implemented. What I don't understand here is how exactly the __str__ function in the class Vertexworks here and why we need it. Can anyone please explain? Thank you!

 class Vertex:
     def __init__(self,key):
         self.id = key
         self.connectedTo = {}

     def addNeighbor(self,nbr,weight=0):
         self.connectedTo[nbr] = weight

     def __str__(self):
         return str(self.id) + ' connectedTo: ' + str([x.id for x in self.connectedTo])

     def getConnections(self):
         return self.connectedTo.keys()

     def getId(self):
         return self.id

     def getWeight(self,nbr):
         return self.connectedTo[nbr]




 class Graph:
     def __init__(self):
         self.vertList = {}
         self.numVertices = 0

     def addVertex(self,key):
         self.numVertices = self.numVertices + 1
         newVertex = Vertex(key)
         self.vertList[key] = newVertex
         return newVertex

     def getVertex(self,n):
         if n in self.vertList:
             return self.vertList[n]
         else:
             return None

     def __contains__(self,n):
         return n in self.vertList

     def addEdge(self,f,t,cost=0):
         if f not in self.vertList:
             nv = self.addVertex(f)
         if t not in self.vertList:
             nv = self.addVertex(t)
         self.vertList[f].addNeighbor(self.vertList[t], cost)

     def getVertices(self):
         return self.vertList.keys()

     def __iter__(self):
         return iter(self.vertList.values())

score 3 · Answer 1 · answered Aug 09 '15 at 22:51

3

the __str__ function is what's called when you attempt to stringify the object, so

g = Vertex(key)
print str(g) # calls __str__

this can be implemented to make printing objects easier

answered Aug 09 '15 at 22:51

Syntactic Fructose

18,936
23
91
177

score 3 · Answer 2 · answered Aug 09 '15 at 22:52

One line of code would have sufficed to ask your question:

str(self.id) + ' connectedTo: ' + str([x.id for x in self.connectedTo])

Let's pick that apart:

self.id is the unique ID of your object. str() converts that to a string.
we concatenate connectedTo: and then the
str() conversion of a list as generated by the [...] construct.

I guess 3. is giving you problems. It's just a central Pythonic construct and selects every element in the self.connectedTo sequence, takes .id of that and returns a list of these.

After that, you have a string, that __str__ returns. Since that method is called whenever python tries to get a string form of an object, it might be called when you do something like

print Vertex

String representation of an object in Python

2 Answers2