gistfile1.py

#Input data:
calls = [ 
                ( 'n1', 'n2', 'success' ),
                ( 'n1', 'n3', 'failed' ),
                ( 'n1', 'n2', 'failed' ),
                ( 'n2', 'n1', 'failed' )
]

def connectivity(calls):
  """given a list of calls in the form of [(caller, callee, 'success' or 'failed'),...]
     return a string telling how many percent of calls succeeded
  """
  #1. init:
  connections = list(set([(a,b) for a,b,c in calls])) #who called whom
  successes = [0 for i in connections] #how many times 'success'
  call_ctr = [0 for i in connections] #number of calls
  #2. count successes
  for a,b,c in calls:
    i = connections.index((a,b))
    if c == 'success': successes[i] += 1
    call_ctr[i] += 1
  #3. create result and return
  return '\n'.join(["{0} => {1}: {2}%".format(
            connections[i][0], #caller
            connections[i][1], #callee
            successes[i]*100.0/call_ctr[i] #percentage
          ) 
          for i in range(len(connections))
        ])

print connectivity(calls)