python how to find difference between two values

[hare]

Need help in giving me an idea on how to get the output

I am new to python.
My input is like this:

Output:
							M <01/14/2019 08:07:01> Count:0 
						Total:50
						Free: 20
						A
						B
						   M <01/14/2019 08:07:04> Count:1
						Total:5
						Free:10
						A
						B
						   M <01/14/2019 08:07:07> Count:2
						Total:5
						Free:3
						A
						B

I am trying to make a output like where it prints the free and then the difference between the current free and previous free

Output:
						M <01/14/2019 08:07:01> Count:0  Free: 20
						M <01/14/2019 08:07:04> Count:1  Free: 10  absolute difference between time and prev time is -10
						M <01/14/2019 08:07:07> Count:2  Free: 3   absolute difference between time and prev time is -7
						.

And then later on i need to determine the time when we had the most negative free value.


I tried a code like this
Which printed

							 with open("summ4.txt") as f:
							# get first line/number
							nxt = int(next(f))
							for n in f:
								print("absolute difference between {} and {} = {}"
									  .format(n.rstrip(), nxt, abs(int(nxt) - int(n))))
								# set nxt equal to the next number
								nxt = int(next(f,0))
							   a=open('summ1.txt','r').readlines()
							   b=open('summ3.txt','r').readlines()
								with open('summ.txt','w') as out:
								  for i in range(0,365): 
						 print>>out,a[i].rstrip(),b[i]

.
i hit

Error:
Traceback (most recent call last):
								File "3.py", line 39, in <module>
							   .format(n.rstrip(), nxt, abs(int(nxt) - int(n))))
								ValueError: zero length field name in format 

.
I guess my input file has a tab in the start and not able to get a difference rightly.
.
Any pointers on how to achieve the desired result?

Thanks,
Hare

[Gribouillis]

It seems that you are using an old version of python, isn't that the problem?