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Author cben
Recipients cben
Date 2010-01-11.12:36:17
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IDLE defaults to indenting with 4 spaces in editor windows, but hard TABs in the Python Shell window.  This is inconsistent with PEP 8; what's worse, it's makes copy-paste code between the shell and editor windows confusing and dangerous!

Recently I gave a 3-day Python course to 6 people.  I spent some time explaining the dangers of TABs with Python's indentation-sensitive syntax and telling them "just use 4 spaces and all will be OK".  A few hours later one of them asked me why IDLE is refusing to run his code.  Turns out he copy-pasted code he tried out in the shell, edited it and tried running it, which resulted in a mix of tabs and spaces (which triggered IDLE's tabnanny check) and a mixture of 4 and 8 indents (which is ugly).

I had to explain IDLE's confusing behavior, introduce them to Untabify, and apologize for the inconvenience.  All this TABs stuff (and Untabify) are things people should eventually learn - but they shouldn't bite them on their first day!  (This is what Ubuntu calls a "papercut".)

The rationale for the current behavior seems to be making indentation clear despite the prompt offsetting the first line by 4:

>>> for i in range(3):
        print i

There are 3 alternative behaviors that would be better:

 (1) Use 8 spaces instead of a TAB.
 (2) Use 4 spaces.
 (3) Use 4 spaces, but add a GUI left margin of 4 to continuation lines.

(3) would be ideal, making copy-paste work cleanly while looking good.  I'm not sure if it can be implemented easily, but I'll look into it.

If it's hard, (2) would still be a big improvement IMHO.  I think correct behaviour is more important than looking good after ">>> " - but others may disagree on this.

If there is doubt, (1) is still strictly an improvement.  It can lead to mixed 8/4 spaces - but at least there won't be invisible problems that the user doesn't understand.

[Configurability concerns: all I said above refers to the *default* behavior of IDLE, which should follow 4-spaces.  The user should be able to configure it to use another width, or TABs.  This option already exists - but it's ignored outright by the shell window.  Solutions (2) and (3) would make the shell window respect it.]
Date User Action Args
2010-01-11 12:36:21cbensetrecipients: + cben
2010-01-11 12:36:21cbensetmessageid: <>
2010-01-11 12:36:19cbenlinkissue7676 messages
2010-01-11 12:36:17cbencreate