words = ['HETEAMT','TOWIMO','NNSEJN','DTOSEB','RERRHO','OISSMI','URNORI','SHIROR']
 
ctext = ''.join(words)
l = len(ctext)
print('\ntotal chars', l)
 
key = '7456321'
n = len(key)
print('\nkey length', n)
 
# here we do exactly the same as what you did to encode, except we just
# count the letters.  so chars[x] is the number of characters in column x,
# which starts at 0 and is added 1 or 2 on each line.
chars = [0] * n
remaining = l
ichar = 0
chunk = 0
while remaining > 0:
    k = min(chunk+1, remaining)
    chars[ichar] += k
    remaining -= k
    ichar = (ichar + 1) % n  # this indexes the columns 0..n-1,0..n-1,...
    chunk = (chunk + 1) % 2  # this goes 0,1,0,1,... and we +1 above
 
# now we have the number of characters in each column, so display that
print('\nkey digit and number of characters')
for i, digit in enumerate(key):
    print(digit, chars[i])
 
# but the ordering in the encrypted data is different, so we re-order to
# match that.  this uses a bit of a trick in python - if we order a list of
# pairs (a,b) then is it ordered by the first thing in the list (the key 
# digit) here.  so we order by the key digit, but also re-arrange the
# columns sizes.
digitsandchars = [(digit, chars[i]) for i, digit in enumerate(key)]
print('\nbefore sorting', digitsandchars)
digitsandchars = sorted(digitsandchars)
print('after sorting', digitsandchars)
 
# now that we have the columns sizes in the right order we can cut up the
# text into the columns
columns = [''] * n
for i in range(n):
    digit, nchars = digitsandchars[i]
    columns[i] = ctext[:nchars]
    ctext = ctext[nchars:]
    print('digit', digit, 'column', columns[i])
 
# now switch the columns back to the order they were originally
ordered = [columns[int(key[i])-1] for i in range(n)]
print('\nordered columns', ordered)
 
# and finally we can decode by doing the same process as before - we pick
# off 1 or 2 characters from each column until we have nothing left.
print('\ndecode')
icolumn = 0
remaining = l
chunk = 0
while remaining > 0:
    column = ordered[icolumn]
    k = min(chunk+1, remaining)
    print(column[:k], end='')  # print the first k characters
    remaining -= k
    ordered[icolumn] = column[k:]  # remove the printed characters
    icolumn = (icolumn + 1) % n
    chunk = (chunk + 1) % 2
print()
 
 

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