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  lt module gt  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- gt  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  gt  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

User · Accepted Answer

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

User · Answer

Try this   class Flonetwork Object       def   init   self adj      flow              self adj   adj         self flow   flow

User · Answer

It only comes when your list or dictionary not available in the local function

User · Answer

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

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

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