Python Key Error=0 - Can't find Dict error in code

Question

basically I have been racking my brains for a good while now as to why my code is not working, I have tested parts separately and have look throughout the web to see if it can help, to no avail. I am getting an error that the traceback is:

Traceback (most recent call last): File "yes2.py", line 62, in <module> g.add_edge(row_index,col_index, b) File "yes2.py", line 27, in add_edge self.adj[u].append(edge)  KeyError: 0 

The two parts with errors are

    def add_edge(self, u, v, w=0):      if u == v:          raise ValueError("u == v")      edge = Edge(u,v,w)      redge = Edge(v,u,0)      edge.redge = redge      redge.redge = edge      self.adj[u].append(edge) #### LINE 27 ####     self.adj[v].append(redge)      self.flow[edge] = 0      self.flow[redge] = 0  

and

g = FlowNetwork() map(g.add_vertex, ['0','1','2','3','4','5','6']) with open('network.txt', "r") as file: for row_index, row in enumerate(file):     for col_index, value in enumerate(row.split(",")):         b = int(value)         if b != 0:             g.add_edge(row_index,col_index, b) ### LINE 62 #### 

And here is the completed code, as without this it may be difficult to see what is happening

class Edge(object): def __init__(self, u, v, w):     self.source = u     self.sink = v      self.capacity = w  def __repr__(self):      return "%s->%s:%s" % (self.source, self.sink, self.capacity)   class FlowNetwork(object):  def __init__(self):      self.adj = {}      self.flow = {}   def add_vertex(self, vertex):      self.adj[vertex] = []   def get_edges(self, v):      return self.adj[v]   def add_edge(self, u, v, w=0):      if u == v:          raise ValueError("u == v")      edge = Edge(u,v,w)      redge = Edge(v,u,0)      edge.redge = redge      redge.redge = edge      self.adj[u].append(edge)      self.adj[v].append(redge)      self.flow[edge] = 0      self.flow[redge] = 0   def find_path(self, source, sink, path):      if source == sink:          return path      for edge in self.get_edges(source):          residual = edge.capacity - self.flow[edge]          if residual > 0 and not (edge,residual) in path:              result = self.find_path( edge.sink, sink, path + [(edge,residual)] )              if result != None:                  return result   def max_flow(self, source, sink):      path = self.find_path(source, sink, [])      while path != None:          flow = min(res for edge,res in path)          for edge,res in path:              self.flow[edge] += flow              self.flow[edge.redge] -= flow          path = self.find_path(source, sink, [])      return sum(self.flow[edge] for edge in self.get_edges(source))   g = FlowNetwork() map(g.add_vertex, ['0','1','2','3','4','5','6']) with open('network.txt', "r") as file: # enumerate allows you to iterate through the list with an index and an object for row_index, row in enumerate(file):     # split allows you to break a string apart with a string key     for col_index, value in enumerate(row.split(",")):         #convert value from string to int         b = int(value)         if b != 0:             g.add_edge(row_index,col_index, b)  print g.max_flow('1','6') 

Many thanks for your time, much appreciated.

Answer 1

The error you're getting is that self.adj doesn't already have a key 0. You're trying to append to a list that doesn't exist yet.

Consider using a defaultdict instead, replacing this line (in __init__):

self.adj = {} 

with this:

self.adj = defaultdict(list) 

You'll need to import at the top:

from collections import defaultdict 

Now rather than raise a KeyError, self.adj[0].append(edge) will create a list automatically to append to.

Answer 2

The defaultdict solution is better. But for completeness you could also check and create empty list before the append. Add the + lines:

+ if not u in self.adj.keys(): +     self.adj[u] = []   self.adj[u].append(edge) . .