Comparing two text files in python

Question

I need to compare two files and redirect the different lines to third file. I know using diff command i can get the difference . But, is there any way of doing it in python ? An

Answer 1

Comparing two text files in python?

Sure, difflib makes it easy.

Let's set up a demo:

f1path = 'file1'
f2path = 'file2'
text1 = '\n'.join(['a', 'b', 'c', 'd', ''])
text2 = '\n'.join(['a', 'ba', 'bb', 'c', 'def', ''])
for path, text in ((f1path, text1), (f2path, text2)):
    with open(path, 'w') as f:
        f.write(text)

Now to inspect a diff. The lines that use os and time are merely used to provide a decent timestamp for the last time your files were modified, and are completely optional, and are optional arguments to difflib.unified_diff:

# optional imports:
import os
import time
# necessary import:
import difflib

Now we just open the files, and pass a list of their lines (from f.readlines) to difflib.unified_diff, and join the list output with an empty string, printing the results:

with open(f1path, 'rU') as f1:
    with open(f2path, 'rU') as f2:
        readable_last_modified_time1 = time.ctime(os.path.getmtime(f1path)) # not required
        readable_last_modified_time2 = time.ctime(os.path.getmtime(f2path)) # not required
        print(''.join(difflib.unified_diff(
          f1.readlines(), f2.readlines(), fromfile=f1path, tofile=f2path, 
          fromfiledate=readable_last_modified_time1, # not required
          tofiledate=readable_last_modified_time2, # not required
          )))

which prints:

--- file1       Mon Jul 27 08:38:02 2015
+++ file2       Mon Jul 27 08:38:02 2015
@@ -1,4 +1,5 @@
 a
-b
+ba
+bb
 c
-d
+def

Again, you can remove all the lines that are declared optional/not required and get the otherwise same results without the timestamp.

redirect the different lines to a third file

instead of printing, open a third file to write the lines:

        difftext = ''.join(difflib.unified_diff(
          f1.readlines(), f2.readlines(), fromfile=f1path, tofile=f2path, 
          fromfiledate=readable_last_modified_time1, # not required
          tofiledate=readable_last_modified_time2, # not required
          ))
        with open('diffon1and2', 'w') as diff_file:
            diff_file.write(difftext)

and:

$ cat diffon1and2
--- file1       Mon Jul 27 11:38:02 2015
+++ file2       Mon Jul 27 11:38:02 2015
@@ -1,4 +1,5 @@
 a
-b
+ba
+bb
 c
-d
+def