Modernize Code In IDLELib Editor Backspace

(and potentially deprecate handling of tabs in IDLE, though likely separate issue)

Lines 1365 to 1385 in 561212a

    
           # Ick.  It may require *inserting* spaces if we back up over a 
        
           # tab character!  This is written to be clear, not fast. 
        
           tabwidth = self.tabwidth 
        
           have = len(chars.expandtabs(tabwidth)) 
        
           assert have > 0 
        
           want = ((have - 1) // self.indentwidth) * self.indentwidth 
        
           # Debug prompt is multilined.... 
        
           ncharsdeleted = 0 
        
           while True: 
        
               chars = chars[:-1] 
        
               ncharsdeleted = ncharsdeleted + 1 
        
               have = len(chars.expandtabs(tabwidth)) 
        
               if have <= want or chars[-1] not in " \t": 
        
                   break 
        
           text.undo_block_start() 
        
           text.delete("insert-%dc" % ncharsdeleted, "insert") 
        
           if have < want: 
        
               text.insert("insert", ' ' * (want - have), 
        
                           self.user_input_insert_tags) 
        
           text.undo_block_stop() 
        
           return "break"

is mentioned as an area of code with quadratic complexity, which may lead to performance issues; though this is unlikely to be of large impact in practice because attackers with the ability to get a huge file loaded and/or modify configuration files to change the tab width can do much worse things than getting IDLE to hang on hitting backspace.

However, the complexity there comes from a loop which handles situations (files with tabs) that either cannot or should not occur; in the current default IDLE, you can't put tabs, and a space after a tab should just be deleted in a file coming from elsewhere as in indicated in #134874 (comment). Therefore, simplifying the function specification and/or deprecating IDLE to handle tabs (which we should do) will allow elimination of this piece of obsolete code, as indicated in #136061 (comment).

Can this not just go under #136061?

@terryjreedy

I am rejecting the refactoring of the handing of space and tab in smart_backspace_event(). Any such project must begin with a separate issue and an detailed analysis of current versus desired behavior, and it must consider the entire block of relevant code. This is lines 1365-1385, which begin with the comment

#134874 (comment) by @terryjreedy

I have not analyzed the current vs desired behaviro yet. Maybe someone else can tackle this. Also maybe make it type-refactor not type-feature.

Pleae update the title accordingly as I didn't read the resolved conservations before changing the labels (modernizing code means refactoring to me)

The type_xyz label does not matter since the PR will be backported regardless.

Any deprecation should be a separate issue; it would require discussion involving other coredevs, and I have not yet decided what, if anything, I want to ask for.

I did some of the needed analysis in the second part of the referenced comment and more since while doing some live experiments with IDLE. I concluded that tabs should be treated as normal characters whether used for indenting (where mixing with spaces is now illegal) or otherwise (where spaces also need no special handling). Spaces need only be treated specially in syntactically significant indents, but currently are also in other indents. Consider this:

>>> l = [1,
...      2,
...      3,
...

The smart indent is 5 spaces so that '2', etc, lines up with the '1'. Backspace now removes 1 and then 4 spaces. One might decide that in a style indent, it should always be one, but deciding the indent type could require backing up multiple lines. My initial goal would be to leave the current situation alone. I have not seen any requests for changes.

I will try to submit a PR soon.

Can IDLE be used to edit other types of files? For example, in Makefile tabs are mandatory.

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Modernize Code In IDLELib Editor Backspace #136556

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	# Ick. It may require inserting spaces if we back up over a
	# tab character! This is written to be clear, not fast.
	tabwidth = self.tabwidth
	have = len(chars.expandtabs(tabwidth))
	assert have > 0
	want = ((have - 1) // self.indentwidth) * self.indentwidth
	# Debug prompt is multilined....
	ncharsdeleted = 0
	while True:
	chars = chars[:-1]
	ncharsdeleted = ncharsdeleted + 1
	have = len(chars.expandtabs(tabwidth))
	if have <= want or chars[-1] not in " \t":
	break
	text.undo_block_start()
	text.delete("insert-%dc" % ncharsdeleted, "insert")
	if have < want:
	text.insert("insert", ' ' * (want - have),
	self.user_input_insert_tags)
	text.undo_block_stop()
	return "break"

Modernize Code In IDLELib Editor Backspace #136556

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StanFromIreland commented on Jul 11, 2025

johnzhou721 commented on Jul 11, 2025

StanFromIreland commented on Jul 11, 2025

johnzhou721 commented on Jul 11, 2025

StanFromIreland commented on Jul 11, 2025

johnzhou721 commented on Jul 11, 2025

picnixz commented on Jul 11, 2025

terryjreedy commented on Jul 12, 2025

serhiy-storchaka commented on Jul 12, 2025

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