Please Critique

[Michael Fraley]

Hello,

This is my first real-world Python script, I'd like to hear any
pointers on how to improve it or do things better. 

I used it to organize screen access to one of our production systems. 
First I dumped the screen access into a text file. The lead four bytes 
are the operator number in question, followed by four bytes for each 
screen (or wild card ~) and the access. Example -

   0007D102A204I~~~
   
   This means for operator '0007', deny screen 102, allow screen 204,
   and inquire only on everything else ('~~~')
   
My python script sorts on screen number and writes out a neatly sorted
list. The input is all jumbled up, from years of user updates:

sample scracc.txt input
-----------------------
0006D900DWEBA~~~A023
0361D307D023A100A001DWEBI023A~~~
0733D006A~~~I004

Here's how it should look when straightened out

sample scrnew.txt output
-----------------------------------------------
0006A023D900DWEBA~~~
0361A001I023A100D307DWEBA~~~
0733I004D006A~~~

Here's my script. I'd really appreciate any pointers on how to 
improve it.

scracc.py
-----------------------------------------------------------
import string
screenfile = open('scracc.txt')
newfile = open('scrnew.txt', "w")
for line in screenfile.readlines():
    mydict = {}
    trimline = line[4:-1]
    start = 0
    length = len(trimline)
    while start < length:
	end = start + 4
	startkey = start + 1
	mydict[trimline[startkey:end]] = trimline[start:end]
	start = end
    sortkeys = mydict.keys()
    sortkeys.sort()
    mylist = []
    for key in sortkeys:
	mylist.append(mydict[key])
    newline = line[0:4] + string.joinfields(mylist, "") + "\n"
    newfile.write(newline)
screenfile.close()
newfile.close()


-- 

  Michael Fraley
  fraley at usfca.edu
  http://www.usfca.edu/~fraley