Cant use difflib with islice?

[gonksoup]

Sorry to be back so soon.... Any ideas why my code causes Differ to crash? (NB The 3,7 is just an arbitrary number for testing a range). The "for x in R1" added just for a test works fine.

import difflib
import itertools
difference = difflib.Differ()

file1 = "gonk1.txt"
file2 = "gonk2.txt"
result_file = "resultfile.txt"  

cnt1=0
with open(result_file, "w") as f3, open(file1, "r") as f1, open(file2, "r") as f2:

    r1 = itertools.islice(f1, 3,7)
    r2 = itertools.islice(f2, 3,7)

    for x in r1:
        print(x)

    for x in difference.compare(r1, r2): 
        print(x)

Error:
  File "c:\Users\gonks\Desktop\python_work\compare_v4.py", line 18, in <module>
    for x in difference.compare(r1, r2):
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\Python312\Lib\difflib.py", line 859, in compare
    cruncher = SequenceMatcher(self.linejunk, a, b)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\Python312\Lib\difflib.py", line 182, in __init__
    self.set_seqs(a, b)
  File "C:\Program Files\Python312\Lib\difflib.py", line 194, in set_seqs
    self.set_seq2(b)
  File "C:\Program Files\Python312\Lib\difflib.py", line 248, in set_seq2
    self.__chain_b()
  File "C:\Program Files\Python312\Lib\difflib.py", line 296, in __chain_b
    n = len(b)
        ^^^^^^
TypeError: object of type 'itertools.islice' has no len()

[snippsat]

differ.compare method expects sequences that support len() function,which itertools.islice dos not.
To fix this, you need to convert the islice objects into a list or another sequence type that supports len().

import difflib
import itertools

difference = difflib.Differ()
file1 = "gonk1.txt"
file2 = "gonk2.txt"
result_file = "resultfile.txt"

with open(result_file, "w") as f3, open(file1, "r") as f1, open(file2, "r") as f2:
    r1 = list(itertools.islice(f1, 3, 7))
    r2 = list(itertools.islice(f2, 3, 7))
    for x in difference.compare(r1, r2):
        print(x)
        f3.write(x)

[deanhystad]

Read the documentation. From

https://docs.python.org/3/library/diffli...er-objects

compare(a, b)
Compare two sequences of lines, and generate the delta (a sequence of lines).

Each sequence must contain individual single-line strings ending with newlines. Such sequences can be obtained from the readlines() method of file-like objects. The delta generated also consists of newline-terminated strings, ready to be printed as-is via the writelines() method of a file-like object.

So what does readlines() return?

From

https://docs.python.org/3/library/io.htm....readlines

readlines(hint=-1, /)¶
Read and return a list of lines from the stream. hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint.

So Differ().compare(a, b) needs a and b to lists. itertools.islice() makes an iterator, but not a list. You would need to do something like this:

    r1 = list(itertools.islice(f1, 3, 7))
    r2 = list(itertools.islice(f2, 3, 7))