gh-118150: difflib: expose autojunk flag from SequenceMatcher to public methods and functions

[faithlesstomas]

Until now difflib methods and functions took SequenceMatcher class kwarg autojunk by default which is set up to be True in this class.. This PR aims to expose this flag to the public methods and functions of this module, in order that user can choose behavior of this option in SequenceMatcher.

Issue: #118150
Related PR: #153892

• Issue: Difflib creates unreasonably large diffs #118150

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[faithlesstomas]

Quick check if this works for the original issue test:

 #!/usr/bin/python3                                                                                                                                                          
                                                                                                                                                                              
  import difflib                                                                                                                                                              
                                                                                                                                                                              
  def get_lines(filename):                                                                                                                                                    
      with open(filename, 'r', encoding='utf8') as fd:                                                                                                                        
          return fd.readlines()                                                                                                                                               
                                                                                                                                                                              
  for autojunk in (True, False):                                                                                                                                              
                                                                                                                                                                              
      old_new = list(difflib.unified_diff(                                                                                                                                    
          get_lines('small.external.old.json'),                                                                                                                               
          get_lines('small.external.new.json'),                                                                                                                               
          autojunk=autojunk                                                                                                                                                   
      ))                                                                                                                                                                      
      new_old = list(difflib.unified_diff(                                                                                                                                    
          get_lines('small.external.new.json'),                                                                                                                               
          get_lines('small.external.old.json'),                                                                                                                               
          autojunk=autojunk                                                                                                                                                   
      ))                                                                                                                                                                      
                                                                                                                                                                              
      print('Autojunk flag:', autojunk)                                                                                                                                       
      print('diff external.old external.new.json | wc -l')                                                                                                                    
      print(len(old_new))                                                                                                                                                     
      print('diff external.new external.old.json | wc -l')                                                                                                                    
      print(len(new_old))                                                                                                                                                     

Autojunk flag: True
diff external.old external.new.json | wc -l
90
diff -u external.new external.old.json | wc -l
25
Autojunk flag: False
diff external.old external.new.json | wc -l
25
diff -u external.new external.old.json | wc -l
25

#118150 (comment)

[Lenormju]

I think the Documentation (Doc/library/difflib.rst) should get updated too to include this new parameter.

[hugovk]

Please could you add tests to Lib/test/test_difflib.py?

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34035122 | 📁 Comparing 05ee0fa against main (dd5a285)

  🔍 Preview build  

19 files changed · ± 19 modified

± Modified

• index.html
• c-api/arg.html
• c-api/capsule.html
• c-api/frame.html
• library/argparse.html
• library/ast.html
• library/asyncio-task.html
• library/concurrent.futures.html
• library/difflib.html
• library/inspect.html
• and 9 more...

[encukou]

The change looks good; let's make the docs great :)

[encukou]

There's now a section for the heuristic in the description; it would be good to link there:

Suggested change

	Optional *autojunk* flag sets on/off automatic junk heuristic of :class:`SequenceMatcher`.
	Setting the optional *autojunk* argument to ``False`` will turn
	:ref:`automatic junk heuristic <difflib-junk>` off.

Please use the same wording for all of these (unless there's a difference that needs to be pointed out).

[encukou]

This is still relevant:

Please use the same wording for all of these (unless there's a difference that needs to be pointed out).

[faithlesstomas]

Done!