File Compare with difflib.context_diff

[JohnV]

I have a txt file that gets appended with data over a time event.  The
data comes from an RFID reader and is dumped to the file by the RFID
software.  I want to poll that file several times over the time period
of the event to capture the current data in the RFID reader.

When I read the data I want to be able to compare the current data to
the date from the last time I read it and only process the data
appended since the last time it was read.

The first time I read the data, it might look like this:
AU08JEDD011485H14472402210
AU08JEDD020163C14472502210
AU08JEDD005029C14480102210
AU08JEDD004923H14482002210
AU08AWOL000799H14483902210

The next time it might look like this (with data appended to it)
AU08JEDD011485H14472402210
AU08JEDD020163C14472502210
AU08JEDD005029C14480102210
AU08JEDD004923H14482002210
AU08AWOL000799H14483902210
AU08AWOL000120H14495902210
AU08ARPU050241H14511702210
IF08DRTO008074H14520202210
IF08DRTO008089H14521102210
IF08DRTO008077H14553602210
IF08CHES000023H14594902210

What I want to do is compare the old data (lets day it is saved to a
file called 'lastdata.txt') with the new data (lets day it is saved to
a file called 'currentdata.txt') and save the new appended data to a
variable which I HTTP POST to a website where I process the data for
display to interested parties.  In the example below I am trying to
save the new appended data to a file called "out.txt"

I have looked at difflib.context_diff but I cannot get the syntax
correct.  This is what I have taken from an example from this page
http://docs.python.org/library/difflib.html.  One thing I do not
understand is what do I do with: fromfile='before.py',
tofile='after.py' in the example code.

**********

import sys
import difflib

sys.stdout = open("out.txt","w")


f1 = open(r'C:\Users\Owner\Desktop\lastdata.txt', 'r')
read_data1 = f1.read()
f1.close()

f2 = open(r'C:\Users\Owner\Desktop\currentdata.txt', 'r')
read_data2 = f2.read()
f2.close()

for line in context_diff(read_data1, read_data2, fromfile='before.py',
tofile='after.py'):
sys.stdout.write(line)


***************

for line in context_diff(read_data1, read_data2, fromfile='before.py',
tofile='after.py'): is the line that causes the syntax error.

I would hope that when the script worked that "out.txt" would have the
appended data.  I would then copy currentdata.txt to lastdata.txt.  No
need to clear out the data in currentdata.txt as the next dump will
overwrite that data.

Any help or insights appreciated, thanks...