[Python] Use Vim Retab to solve TabError: inconsistent use of tabs and spaces in indentation?

Apologize for the newbie question, but I have read the manual, this question, and tried several times without results I expected.

So I was using vim to edit a file (attached). But when running, I got the TabError: inconsistent use of tabs and spaces in indentation error.

Here is what I have tried:

• Open the file with Vim. type :retab, and :x. Run the file again. Still got the TabError message.
• Open the file again and type :retab! and :x. Run the file again. Still got the TabError message.
• Open the file again and type :retab! 4 and :x. Run the file again. This time it works but I have no idea why? Plus, in the files indentation seems excessively long. (I read here that the editor might display 8 spaces for a tab)

My questions are:

• 
  What does :retab, :retab!, and :retab! 4 mean?
  
  
• 
  Why doesn't :retab work on my file?
  
  #!/usr/bin/env python
  #Reduce function for computing matrix multiply A*B
  #Input arguments:
  #variable n should be set to the inner dimension of the matrix product (i.e., the number of columns of A/rows of B)
  import sys
  import string
  import numpy
  
  #number of columns of A/rows of B
  n = int(sys.argv[1])
  
  #Create data structures to hold the current row/column values (if needed; your code goes here)
  
  currentkey = None
  alist = [] # list for elelents in A
  blist = [] # list for elements in B
  # input comes from STDIN (stream data that goes to the program)
  for line in sys.stdin:
  #Remove leading and trailing whitespace
  line = line.strip()
  #Get key/value
  key, value = line.split('\t',1)
  print(key, value)
  #Parse key/value input (your code goes here)
  key = (key.split(',', 1)[0], key.split(',',1)[1])
  value = (value.split(',', 1)[0], value.split(',',1)[1], value.split(',',1)[2])
  #If we are still on the same key...
  if key==currentkey:
  #Process key/value pair (your code goes here)
  # store all values in a lisl
  if value[0]=='A':
  alist.append([value[1], value[2]])
  else:
  blist.append([value[1], value[2]])
  #Otherwise, if this is a new key...
  else:
  #If this is a new key and not the first key we've seen, i.e. currentkey!=None
  if currentkey:
  #compute/output result to STDOUT (your code goes here)
  alist = sorted(alist)
  blist = sorted(blist)
  newlist = [a[1]*b[1] for a,b in zip(alist, blist)]
  res = newlist.sum()
  print(currentkey, res)
  currentkey = key
  #Process input for new key (your code goes here)

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