compair two files and display missing result - python-3.x

Question

A list of IP addresses are downloaded to a file and rename to Old_file. As days goes by device get update with more IPs(or deleted). Therefore, I download a new list of IP addresses to another file named as New_file
I then want to run a compare these two files and see what is not matching
Old_file = [1.1.1.1,
1.1.1.2,
1.1.1.3,
1.1.1.4,
1.1.1.6,]
new_file = [1.1.1.1,
1.1.1.2,
1.1.1.3,
1.1.1.5,
1.1.1.6]
return needs to 1.1.1.4, and stops there. But never from Old_file e.g: 1.1.1.5 (we need the results only from the New_file only)
I really hope this would explain.
Thanks in advance
Tony

Answer 1

For a simple element-wise comparison, you could do
def get_first_unequal(s0, s1):
for e0, e1 in zip(s0, s1): # assumes sequences are of equal length!
if e0 != e1:
print(f"unequal elements: '{e0}' vs. '{e1}'!")
return (e0, e1)
return None # all equal
a = ['a', 'b', 'c']
b = ['a', 'b', 'd']
get_first_unequal(a, b)
# unequal elements: 'c' vs. 'd'!
# ('c', 'd')
# --> to get a list of all unequal pairs, you could also use
# [(e0, e1) for (e0, e1) in zip(s0, s1) if e0 != e1]
If you want to go more sophisticated, as mentioned in the comments, difflib might be your way to go. to run e.g. a comparison of two sequences (which are the list of strings you read from the two txt files you want to compare):
import difflib
a = ['a', 'b', 'c']
b = ['s', 'b', 'c', 'd']
delta = difflib.context_diff(a, b)
for d in delta:
print(d)
gives
*** 1,3 ****
! a
b
c
--- 1,4 ----
! s
b
c
+ d
to check the difference between two strings, you could do something like (borrowing from here):
a = 'string1'
b = 'string 2'
delta = difflib.ndiff(a, b)
print(f"a -> b: {a} -> {b}")
for i, d in enumerate(delta):
if d[0] == ' ': # no difference
continue
elif d[0] == '-':
print(f"Deleted '{d[-1]}' from position {i}")
elif d[0] == '+':
print(f"Added '{d[-1]}' to position {i-1}")
gives
a -> b: string1 -> string 2
Deleted '1' from position 6
Added ' ' to position 6
Added '2' to position 7

Answer 2

If you're assuming that both files should be exactly identical, you can just iterate over the characters of the first and compare them to the second. I.e.
# check that they're the same length first
if len(Old_file) != len(New_file):
print('not the same!')
else:
for indx, char in enumerate(Old_file):
try:
# actually compare the characters
old_char = char
new_char = New_file[indx]
assert(old_char == new_char)
except IndexError:
# the new file is shorter than the old file
print('not the same!')
break # kill the loop
except AssertionError:
# the characters do not match
print('not the same!')
break # kill the loop
It's worth noting that there are faster ways to do this. You could look into performing a checksum, though it wouldn't tell you which parts are different only that they are different. If the files are large, the performance of doing the check one character at a time will be quite bad -- in that case you can try instead to compare blocks of data at a time.
Edit: re-reading your original question, you could definitely do this with a while loop. If you did, I would suggest basically the same strategy of checking each individual character. In that case you would manually need to increment the indx of course.