String representation of an object in Python
So in the book ( Solving Problems with Algorithms and Data Structures ) that I am currently using as a reference, this is what the graph looks like. Here I do not understand how the function works __str__
in the class Vertex
and why we need it. Can someone please explain? Thank!
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())
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One line of code would be enough to ask your question:
str(self.id) + ' connectedTo: ' + str([x.id for x in self.connectedTo])
Throw it away separately:
-
self.id
- the unique identifier of your object.str()
converts it to string. - we concatenate
connectedTo:
and then -
str()
transformation of the list generated by the construction[...]
.
I think 3. is giving you problems. This is just a central Pythonic construct and selects each element of the sequence self.connectedTo
, takes .id
and returns a list of them.
After that, you have a string that returns __str__
. Since this method is called whenever python tries to get the lowercase form of an object, it can be called when you do something like
print Vertex
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